GRB 211115A and OHMAN test-running

news-1304 November 25, 2021 by Mihara

MAXI detected a Gamma-Ray Burst at 2021 November 15 16:31:19 UT in the constellation Puppis (GCN #31082). The GRB 211115A lasted only 12 s (figure). The intensity was 330 mCrab. CALET/GBM and Fermi/GBM also detected it.
OHMAN (MAXI-NICER) is in the running-test on ground for two months. It also detected the GRB and issued an on-going alert.

GRB 211115A detected by MAXI. S and E note the start and end time when the source is in the MAXI field of view. Taken from MAXI Flash Report of GRB 211115A. Click 16: link to LC.

Outburst from NGC 6440

news-1303 November 18, 2021 by Mihara

MAXI detected X-ray in the constellation Sagittarius on 2021 November 13 13:31 UT ( ATel #15033 ). There lies a globular cluster NGC 6440. The distance form the earth is 28,000 lt-yr, which is almost as far as the galactic center. MAXI has detected outbursts from NGC 6440 twice, in December 2009 and February 2015. There are two bright X-ray binaries in this globular cluster, SAX J1748.9-2021 and NGC 6440 X-2. Pooley et al. (2002) shows Chandra image taken in 2000 (figure). Both are milli-second pulsars as well. The spin periods are 2.2 ms (Altamirano et al. 2008) and 4.9 ms (Altamirano et al. 2010), respectively. The MAXI's twice outbursts were from SAX J1748, The star this time may be SAX J1748 again, NGC 6440 X-2 in stead, or another new star.
Since the sun is close to NGC 6440 now, Swift and NICER cannot observe it until 2022 Feb. NuSTAR is the only X-ray telescope that can observe it. Although the image resolution of NuSTAR is not enough to distinguish the stars in the globular cluster, detecting a pulsation can tell which star is the MAXI's outburst. We asked for NuSTAR observation and it was approved.

NuSTAR detected 2.3 ms pulsation and confirmed it SAX J1748 ( ATel #15048 ).

Chandra image of NGC 6440. (Pooley et al. 2002, Fig.1). 24 sources are detected. They are concentlated within 30 arcsec (4 lt-yr) in diameter. 1 is SAX J1748.9-2021 and 2 is NGC 6440 X-2. Which star is the MAXI's outburst ?

MAXI J1735-272 quickly faded

news-1302 October 21, 2021 by Mihara

MAXI found a faint X-ray transient source at 06:22 UT on 2021 October 17 in the constellation Ophiuchus ( ATel #14979 ). The intensity was 30 mCrab. As it was apart from any known sources, we named it MAXI J1735-272. The source was not detected in the next MAXI scan. Follow-up searches were made by NICER, Swift (10 h later, ATel #14983 ) and INTEGRAL (5-17h later, ATel #14985 ), but any source was detected. The source may have disappeared quickly. Such an event is a target of OHMAN.

Short burst from SGR 1935+2154

news-1301 October 7, 2021 by Mihara

MAXI detected a short burst from SGR 1935+2154 on 2021 October 7 11:57UT. The intensity was 200 Crab, but lasted only for 0.2 s. It is in the constellation Vulpecula again ( after previous MAXI news #1300). It is considered to be a neutron star with a very strong magnetic field (some 10^14 Gauss at surface). The burst is caused by an abrupt energy release of the magnetic fields. SGR 1935 is famous for the first identified galactic Fast Radio Burst, FRB 200428 in 2020 ( Bochenek et al. 2020). Also MAXI news #1251. The distance to SGR 1935 is about 30,000 lt-yr.

XTE J1946+274 makes Cygnus more lively

news-1300 September 30, 2021 by Mihara

MAXI found an appearance of XTE J1946+274 in the constellation Vulpecula on 2021-09-23 ( ATel #14936 ). The flux was 66 mCrab and is increasing. It is a binary of a Be star and a 15 second pulsar. The orbit is eccentric and the period is 172.7 day. The pulsar will pass the periastron on September 29. However, the pulsar does not always emits X-ray at periastron. This is the fourth detection of an outburst since its discovery in 1998. Cygnus has got more lively.

Cygnus has changed to lively

news-1299 September 16, 2021 by Mihara

There are three bright persistent sources in Cygnus. Cyg X-1 (BH), Cyg X-2 (LMXB) and Cyg X-3 (BH). But now, Be X-ray binary pulsar EXO 2030+375 which appeared in July is at the peak of the giant outburst. It is shining as the forth bright star in Cygnus. Additionally the white dwarf SS Cyg is staying in its maximum brightness. Two more pulsars Be pulsar GRO 2058+42 and long-period pulsar 4U 1954+319 appeared.

GRB 140814A was the first merger-triggered core-collapse supernova

news-1298 September 9, 2021 by Mihara

MAXI detected a 15s-lasting bright soft burst in 2014, and named it GRB 140814A (GCN #16686). Swift follow-up was undetection, and the nature was left unknown. We include the source in MAXI Unidentified Short Soft Transient (MUSST).
In the Science issue on 2021 Sep.3, Dong et al. reported that GRB 140814A was the first merger triggered super nova (and GRB). They found a very luminous source VLASS J121001+495647 in the VLA sky survey made in 2017. It lies in a small galaxy at 480 million lt-yr in Ursa Major. The location coincides with GRB 140814A. They understand it as a blackhole or a neutron star went into the companion star, spiraled-in for 300 years, finally merged with the stellar core and ended up as a super nova and GRB.
We have noticed MUSST a possible new kind of object, and GRB 140814A was it in fact.
The object that a neutron star goes into the companion star is called Thorne-Zytkow ojbject. GRB 140814A was such a object.

Long X-ray burst of 4U 1820-30, every 10 years ?

news-1297 August 26, 2021 by Mihara

MAXI detected a bright X-ray burst from 4U 1820-30 at 11:27 UT on 2021 August 23 ( ATel #14871 ). It is a low-mass X-ray binary in the globular cluster NGC 6624 in Sagittarius. The burst flux was 1.8 Crab, and it decayed to 0.49 Crab in the next MAXI scan at 13:01. In the third scan at 14:34 it had gone back to the usual level of 0.2 Crab (figure). The burst spectrum was shaped as a blackbody with a temperature of 24 million degree K. Considering the distance of 25000 lt-yr, the X-ray luminosity was 2.8 x 10^31 W, which is 74 % of the Eddington luminosity of the He accretion.
Such a long X-ray burst from 4U 1820-30 was first observed in 1999 ( Strohmayer & Brown 2002 ), and the second one was in 2010 ( ATel #3625 ). It is a rare phenomenon. MAXI detection suggests that it might occur in every 10 years.
4U 1820-30 has a short orbital period (11 minutes). The binary separation is only 120,000 km (1/3 of earth-moon distance). The companion star is considered to be a white dwarf. The rapid binary rotation emits gravitational wave radiation. A calculation tells that the two stars are getting closer by 0.4 m/day. That is, the two stars will collide after 1 million years.

RS Ophiuchus eruption after 15 years

news-1296 August 12, 2021 by Mihara

MAXI detected an X-ray emergence from the recurrent nova RS Oph at 2021 Aug. 10 18:28 UT ( ATel #14846 ). The intensity was 36 mCrab. By looking back the data, the brightening had started around Aug. 9 12h UT. Even half a day before, optical amateur astronomer had reported the optical brightening of Aug. 8 22h UT ( vsnet-alert 26131 ) at 5 magnitude with 10cm binoculars and digital camera. (It is usually 10 mag.) Surprisingly, almost at the same time, Fermi/LAT had detected GeV gamma-ray at Aug. 8 18-24 UT ( ATel #14834 ).
Optical spectroscopy by Seimei telescope in Okayama Observatory shows the ejecta is expanding at the high speed of 2600 km/s at Aug. 9 10 UT ( ATel #14838 ). So it is a nova explosion. Swift and NICER and radio observatory followed ( ATel #14848 , #14849 , #14850 ).
The last nova from RS Oph was in 2006 ( O'Brien et al. 2006 ). Current one is several times bigger than that. Recurrent nova is a white dwarf binary and repeats nova in some tens of years. RS Oph is a bright and famous one.

EXO 2030+375 giant outburst after 15 years

news-1295 July 29, 2021 by Mihara

MAXI detected the onset of a possible giant outburst from Be/X-ray binary pulsar EXO 2030+375 in Cygnus on 2021 July 20 ( ATel #14809 ). The last giant outburst was in 2006. EXO 2030+375 made the orbital phase shift of the normal outburst in 2016 ( Laplace et al. 2017 ).
Since then it has been showing the normal outburst at every periastron passage of 46 days orbit. However, after the last periastron passage, the flux stayed high at 20 mCrab. And then, the flux started to increase with the rate of 10 mCrab/day on July 20. The light curve traces remarkably well the previous giant outburst in 2006 (figure). If it grows up to a giant outburst, it will be the first one for MAXI. The peak will be about 800 mCrab around September 17.

GRS 1915+105 and absorption lines with NICER

news-1294 July 21, 2021 by Mihara

MAXI detected a flux increase from the black hole GRS 1915+105 on July 9, 2021. It dimmed in 2018 after 26 years of activity ( ATel #11828 ). Upon the MAXI GCN alert, NICER pointed GRS 1915+105 ( ATel #14792 ). GRS 1915 showed a transition from ~25 cts/s to 250 cts/s. The spectra, linked from the ATel, are remarkable to have over a dozen strong absorption lines from Si, S, Ar, Ca, Fe, and Ni, as well as an Fe emission line.

GRB 210709B / MAXI J1849-237 or Ross 154

news-1293 July 15, 2021 by Mihara

MAXI detected an X-ray of 110 mCrab in the constellation Sagittarius at 22:12 UT on 2021 July 9 ( ATel #14772 ). There was no flux in the scans 1.5h before and 3h after. We reported the source as either a gamma-ray burst or a new star, GRB 210709B / MAXI J1849-237. Actually there are many stars in the error box. Ross 154 is a famous one of them. It is the 7th closest star system at only 10 light-year away. It is a single small red star with 10th visual magnitude. But, it is known as a flare star. If the MAXI event (0.1 counts/s/cm2) came from Ross 154, the X-ray luminosity would be 3 x 10^23 W, which is 100 times stronger than the usual X-ray intensity. However, it is possible, as such a strong X-ray flare was once observed with Chandra in 2002 ( Wargelin et al. 2008 ). It lasted for about 1000s. It is consistent with the MAXI detection of only one scan.

MAXI J0903-531 redetection suggesting orbital period 57 day

news-1292 July 1, 2021 by Mihara

On 2021 June 22, MAXI detected X-ray (10 mCrab) again from MAXI J0903-531. It is a Be X-ray binary pulsar which was discovered with MAXI on 2021 April 14 ( ATel #14555 ). It had been shining at 10 mCrab for about 10 days in April and then went out below the MAXI detection. The reappearance in June was also reported from Fermi/GBM and Swift/BAT ( ATel #14730 ). Look-back analysis of Fermi/GBM detected positive fluxes around 2020 Dec 22 and 2021 Feb 12. Then MAXI J0903-531 repeats an outburst every about 57 days. It is a typical value for an orbital period of a Be binary.
Monitoring is important to understand the new system.

A 0535+262 started an outburst while sun protection

news-1291 June 24, 2021 by Mihara

Since the sun is a very bright X-ray emitter (~1000 Crab in quiescence), we shutdown MAXI detectors for protection in closer than 5 degrees from it. The sun is in the Taurus in June, and the Be/X-ray binary pulsar A 0535+262, located just next to the Crab nebula in the Taurus, was not observable from June 10 to 19, 2021. There was no emission (<20 mCrab) on June 9. In fact it had been off for 200 days. However, when the pulsar came out from the protection region on June 20, it had brightened up to 380 mCrab ( ATel #14173 ). (figure)
The Be/X-ray binary pulsar occurs an outburst at the periastron passage of the binary orbit, but not always. It does only when the activity of the Be star is high and much gas is expelled. The orbital period of A 0535+262 binary is 111.07 days.

4U1543-47, brightest 10 Crab X-ray nova

news-1290 June 17, 2021 by Mihara

MAXI found a bright star in the constellation Lupus (wolf) at 12:00 on June 11, 2021 ( ATel #14701 ). The position was consistent with the black hole binary 4U 1543-47. The X-ray flux rapidly rose up and reached about 10 Crab on June 14. It has become the brightest X-ray nova that MAXI ever detected in 11 years. X-ray luminosity reached in fact the maximum one that an X-ray source can emit, so-called the Eddington luminosity ( ATel #14708 ). Previous outbursts were in 1971, 1983, 1992, and 2002. Thus 4U 1743 usually makes an outburst in every 10 years, but the one in 2012 was absent. The flux was usually 3 - 5 Crab at peak, but this time it is doubled to 10 Crab. The X-ray spectrum is that from an accretion disk around the black hole. The temperature of the innermost part is 1 keV, = 10 million K. The inner radius of the accretion disk is 75 km, suggesting a cavity around the black hole.

4U1730-22 came again after 49 years

news-1289 June 10, 2021 by Mihara

MAXI detected a new X-ray star with 59 mCrab in constellation Ophiuchus at 19:43 UT on 2021 June 7 (figure) ( ATel #14683 ). Either in the previous nor in the next MAXI scan it was not detected (<20 mCrab). Swift made follow-up observations at 8:53 UT and 23:16 UT on June 8 and identified the source as 4U 1730-22 and CXOU J173357.5-220156 ( ATel #14688 ).
4U 1730-22 was discovered in 1972. The outburst lasted for 200 days and the peak flux was 120 mCrab. CXOU J1733 was a faint (0.3 mCrab) star in the error box of 4U 1730 detected with Chandra in 2004. The nature is a neutron star ( Tomsick et al. 2007 ). MAXI detected an outburst from 4U 1730-22 after 49 years, for the first time since 1972.

4U 1705-32 first time with MAXI

news-1288 June 3, 2021 by Mihara

MAXI detected a new X-ray source in the constellation Scorpius on May 25, 2021, with 25 mCrab intensity ( ATel #14663 ). NICER confirmed the source was 4U 1705-32 with 4 mCrab ( ATel #14669 ). It was the first detection with MAXI in the 11 years. The upper limit of the period was < 1 mCrab. Looking back MAXI data in the long term, 4U 1705-32 had brightened to 2 mCrab in December 2020. On May 24, 2021, the X-ray flux suddenly increased to 20 mCrab. However, the flux dropped on May 25, and even below the MAXI detection limit (5 mCrab) on May 28 ( ATel #14670 ). The intensity of 25 mCrab was the brightest since the discovery with UHURU in 1971. An X-ray burst was detected by Beppo-SAX in 2000 implying the nature is a neutron star (1RXS J170854.4-321857, in't Zand et al. 2005 ).

NS-LMXB SAX J1810.8-2609 in Outburst

news-1287 May 27, 2021 by Mihara

MAXI detected a new X-ray outburst from the neutron-star low-mass X-ray binary SAX J1810.8-2609 in the constellation Sagittarius on 2021 May 13 ( ATel #14649 ). The X-ray flux was about 40 mCrab and increasing slowly. This is the fourth outburst from SAX J1810.8-2609 detected by MAXI. The first one was 2012 May 12, the second one on 2015 Jan 20, and the third one on 2018 Apr 27 ( ATel #11593 ). However, the current outburst looks quite different with previous ones. The outburst is lasting long, emitting hard X-ray (Swift/BAT), and radio ( ATel #14659 ).

Discovery of MAXI J1803-298, a black hole binary

news-1286 May 6, 2021 by Mihara

MAXI discovered an X-ray nova (about 50 mCrab) on 2021 May 1 19:50 UT at the "bow and arrow" of the constellation Sagittarius ( ATel #14587 ). NICER performed multi-pointing observations at May 2 03:36 after seeing off Crew-1 departure from ISS, and identified a new source within the MAXI error circle ( ATel #14588 ). The source kept brightening, and the BAT, XRT and UVOT onboard Swift automatically detected the source on May 2 17:39 ( ATel #14591 ). Almost at the same time 17:56, the followup optical observation with MITSuME at Okayama observatory detected a 15 magnitude star in R (red) and I (infrared) bands ( ATel #14594 ). Southern African Large Telescope obtained the optical spectrum from May 2 22:56 ( ATel #14597 ) with Bowen fluorescence emission lines, Hα P Cyg-like emission line and Hβ double-peaked emission line. Those suggest a central UV light source ionizing the surrounding gas, outflow of the surrounding gas, and a rapidly (800 km/s) rotating accretion disk, respectively. It is the spectrum of the low-mass X-ray binaries (LMXB). INTEGRAL showed the spectrum extending to a hard X-ray (100-200 keV) ( ATel #14598 ). NICER's detailed observation showed a spectrum of a 0.3 keV disk blackbody and 0.2Hz QPO, suggesting a black hole binary ( ATel #14602 ).
It is the 31st nova that MAXI discovered and probably the 14th black hole.

Discovery of MAXI J0903-531, a Be binary pulsar

news-1285 April 20, 2021 by Mihara

MAXI found a new X-ray source at 04:33 UT on 2021 April 14 in the constellation Vela ( ATel #14555 ). We named it MAXI J0903-531. The intensity was of 16 mCrab. Swift/XRT (X-ray telescope) and UVOT (UV telescope) identified it. There is a star of 13 magnitude at 10 kpc ( ATel #14557 ). There is the Perseus arm. (figure)
Considering the distance, the optical luminosity of the star is 2000 times of the sun, suggesting a high mass star (8 solar mass). NICER detected 14 s pulsation ( ATel #14559 ). Southern African Large Telescope detected a strong H alpha emission line ( ATel #14564 ).
It is the 30th nova that MAXI discovered. Thanks to the quick followups over the world, the nature of the nova turned out to be a Be X-ray binary pulsar.

Orbital period updated for the Be X-ray pulsar XTE J1829-098

news-1284 April 16, 2021 by Mihara

MAXI detected an X-ray star (70 mCrab) in the constellation Scutum on 2021 April 12 ( ATel #14554 ). The source position is consistent with the Be X-ray pulsar XTE J1829-098. After the MAXI's report in the last summer ( ATel #11927 ), SRG reported a beginning of a new outburst on 2021 April 5 ( ATel #14521 ). The MAXI detection shows the pulsar is brightening to an outburst. Although two outburst periods (orbital period of the Be binary) were proposed, 480 days by Halpern et al. (2007) and 246 day by Markwardt et al. ( ATel #2007 ), the 246 day is turned out to be correct by the date separations of the April outburst.

X-ray burst from NS-LMXB 4U 1323-619

news-1283 March 25, 2021 by Mihara

MAXI detected an X-ray burst (90 mCrab) from 4U 1323-619 in the constellation Centaurus on 2021 March 23 04:11 UT. It is a binary of a neutron star (NS, 1.4 solar mass) and a low-mass star (0.3 solar mass) (Gambino et al. 2016). The binary period is 2.94 hour. The binary separation is 860 thousand km, which is only 2.3 times of the earth-moon distance. It is so close that the gas on the stellar surface flows onto the NS. When it accumulates to a certain amount, a sudden nuclear fusion (an X-ray burst) occurs on the surface of the NS. Such a system is called NS low-mass X-ray binary (NS-LMXB). The distance from the earth is 14 thousand light-year. It locates 1 degree east of the coal sack, but is much further than the coal sack (600 lt-yr).

Reappearance of Be X-ray binary Swift J1626.6-5156

news-1282 March 18, 2021 by Mihara

MAXI detected an appearance of X-ray star of about 40 mCrab on 2021 March 11 in the constellation Norma ( ATel #14454 ). The position was consistent with the Be X-ray binary Swift J1626.6-5156. Follow up observation with NICER on March 11 ( ATel #14457 ) found a pulsation of 15.34 s, which is the pulse period of Swift J1626. NICER detected a 9.1 keV dip in the spectrum. It is the cyclotron resonance of an electron in the magnetic field, whose strength is 8e11 Gauss. Since the source located on the course of SRG scan, SRG/ART-XC also detected it. Follow up observation with NuSTAR confirmed the cyclotron resonance and also found the second harmonic at 17 keV ( ATel #14462 ). Swift J1626 was discovered in 2005 with Swift. MAXI detected a small outburst in 2014 September ( ATel #14498 ), and this is the third detection in history.

GRB 210222A wih MAXI, CALET, and Konus-wind

news-1281 March 11, 2021 by Mihara

MAXI detected a bright (390 mCrab in 4-10 keV) transient in the constellation Sculptor at 2021/02/22 15:59:29 UT ( GCN #29543, GRB 210222A) (figure). At 116 seconds before, CALET Gamma-ray Burst Monitor (CGBM) triggered it (GCN #29553). and Konus-wind also did (GCN #29564). The peak flux was about 40 Crab (100 - 1000 keV) and the burst duration was 87 second. When MAXI detected it, the GRB had already darkened under CGBM sensitivity. MAXI detected the tail of the long GRB.
In 5 years, simultanious detection with MAXI and CALET is 18 out of total 55 MAXI detections. Those with MAXI, CALET and Konus-wind is 13.

Chandra confirmed MAXI J1848-015 is a new star

news-1280 March 4, 2021 by Mihara

Swift localized MAXI J1848-015 to a star cluster GLIMPSE-C01 with an accuracy of 2.3 arcsec ( ATel #14420 ). To find out which star in the cluster made the outburst, Chandra observed MAXI J1848 on Feb. 21 with an accuracy of 0.8 arcsec ( ATel #14424 ). They found a bright star (1 mCrab) around the boundary of Swift's error circle. In the previous observations in 2006 and 2019 with sensitivity down to 0.1 micro Crab, this star did not exist (figure). Although 17 stars had been detected, the brightest star was only 2 micro Crab. So, MAXI J1848-015 was really a new star.
NICER observed MAXI J1848 on Feb. 23 and found many iron lines ( ATel #14429 ).

Swift confirmed MAXI J1848-015

news-1279 February 25, 2021 by Mihara

MAXI nova MAXI J1848-015 which was discovered on Dec. 20 last year was observed with Swift satellite at last on Feb. 21 ( ATel #14420 ). Swift had to wait until the sun moved far from the source. It was lucky that the star was still bright and Swift found it within the NuSTAR error box. The position became 40 times more accurate, and there sits a hidden globular cluster GLIMPSE C01. It is on the Galactic plane and the optical lights are absorbed by the interstellar gas. This hidden cluster was discovered recently in 2005 with Spitzer infrared satellite. X-ray luminosity was calculated using the cluster distance (5 kpc). The low luminosity suggests the source to be a neutron star binary rather than a blackhole binary. The position was also consistent with ASCA source AX J184848-0129. The spectral changes with MAXI NuSTAR Swift and ASCA also indicate a NS binary. The nova ended in the common view as a NS binary in globular cluster.

Blackhole GX 339-4 brightening

news-1278 February 18, 2021 by Mihara

After reported on Jan. 28, X-ray flux from the blackhle binary GX 339-4 keeps increasing. Today's flux is 0.07 photon/cm2/s in 10-20 keV. From the three-times experience of MAXI, GX 339-4 made a spectral state transition at 0.12 - 0.18 ph/cm2/s from the hard spectrum to the soft one. This indicates that the accretion disk emitting the soft X-ray comes closer to the blackhole and it finally reaches the 3 times of the Schwarzschild radius. There is a variance on the threshold level. So the state change may be tomorrow or 3 weeks later, Many astronomers is keeping their eyes on the behavior of this famous blackhole.

Blackhole GX 339-4 started an outburst

news-1277 January 28, 2021 by Mihara

The recurrent black-hole transient GX 339-4 started an outburst. It was first noticed by the weekly monitoring with the MeerKAT radio telescope on 2021 Jan. 4 ( ATel #14336 ). NuSTAR and Swift/XRT made TOO observations and detected a weak X-ray on Jan. 23 ( ATel #14352 ). Swift/BAT detected a positive hard X-ray on Jan 25, and also, MAXI detected a soft X-ray on Jan. 26 ( ATel #14351 ). Looking back the data, INTEGRAL had detected X-ray on Jan. 17 and 21 ( ATel #14354 ). Now the flux is 40 mCrab and is rising. It is still in the hard spectral state (beginning of an outburst). The last outburst had ended in 2020 April. Now, a new outburst has begun.

NuSTAR confirmed MAXI J1848-015

news-1276 January 21, 2021 by Mihara

After the MAXI's discovery on 2020 Dec. 20, NuSTAR detected MAXI J1848-015 successfully on Dec. 23 ( ATel #14290 ). The location of MAXI J1848 looks slightly offset (about 71 arcsec) from the known source AX J1848.8-0129. But, it is within the error of Nu-STAR and they could not confirm it a new star. Just after it on Dec. 23 the X-ray flux dropped quickly below the MAXI detection limit ( ATel #14292 ). One explanation was that the source might have made a soft-to-hard state transition and soft X-ray decreased. To confirm it we made a NuSTAR TOO observation on Dec.31 - Jan.1 ( ATel #14327 ). The spectrum had changed to that of the hard state and our idea was confirmed. The image looks similar and identification is not certain yet.

Very soft Gamma-ray burst GRB 210102B

news-1275 January 7, 2021 by Mihara, Negoro, Serino

MAXI detected a bright uncatalogued X-ray source at 2021 January 2 09:03:48 UT in the constellation Aquarius with the 240 mCrab intensity. We reported as GRB 210102B (GCN #29223). Swift made a follow-up observation in 6 hour later with 7 pointings and found a source with 0.1 mCrab (GCN #29230). At the refined position, an optical transient was found with 22 magnitude in 28 hour later (GCN #29240). However, the second Swift follow-up in 58 h later did not detect the source ( weaker than 0.01mCrab, GCN #29271). Nor, the second optical follow-up in 76 hour later (weaker than 23 magnitude, GCN #29270). GRB 210102B was a very soft Gamma-ray burst, showed a quick decay in X-ray and optical, and there was no star or galaxy at that position.

New star MAXI J1848-015 or a cataloged star ?

news-1274 December 24, 2020 by Mihara, Negoro

MAXI found an X-ray transient source in the constellation Aquila at 05:04 UT on 2020 December 20 ( ATel #14282 ). The X-ray flux was 63 +- 10 mCrab (4.0-10.0keV). The source locates just on the galactic plane (figure). The source may be a new source as MAXI J1848-015, or may be rebrightening of a cataloged weak X-ray source AX J1848.8-0129, whose nature is unknown yet. Swift and NICER cannot observe it because of sun angle constraints. We asked the Nu-STAR team for a follow-up, and they observed it from 02:50 UT on Dec. 23.
They detected the source at 71 arcsec away from AX J1848.8-0129 ( ATel #14290 ). But it is marginally within the positional error, and they could not reach a conclusion.

Active binary star AT Microscopium

news-1273 December 17, 2020 by Mihara

MAXI detected a flare from AT Mic at 19:11 UT on 2020 December 12, with the intensity of 300 mCrab. Considering that the distance to AT Mic is 35 light-years from the earth, the X-ray power was as huge as 10^25 W, which is 1/10 of the optical solar luminosity. AT Mic consists of two dwarf M-type stars with emission lines (dMe stars). Size of each star is about 40 % of the sun.

Solar flare and Si from Crew Dragon

news-1272 December 3, 2020 by Mihara

A large solar flare occurred around 2020 November 29 13:00 UT. Reflected X-ray on the Crew Dragon around the nose cap was seen with MAXI. The spectrum showed a strong Si line. It would be the fluorescent X-ray from the Crew Dragon material.

A0535+26 in its highest flux

news-1271 November 26, 2020 by Mihara, Nakajima

The Be X-ray binary pulsar A 0535+26 has brightened since the report of two weeks ago. MAXI team reported the giant outburst ( ATel #14173 ). After the periastron passage the pulsar remains in the high level against expectation. It is twice higher than the 2009's outburst (figure). In fact, in the 10-20 keV range it is about 10 Crab. It is much brighter than the Crab nebula in the daily image on 2020.11.24 (figure).

A0535+26 in the giant outburst

news-1270 November 14, 2020 by Mihara, Nakajima

MAXI detected a slight X-ray from the Be X-ray binary pulsar A 0535+26 at 2020/11/3 20:46. It locates next to the Crab nebula (Figure), both in the constellation Taurus. The orbital phase was 0.90, whereas the orbital period of the system is 111 day. The system makes, but not always, an outburst near periastron (phase=0). Sometimes it is a normal outburst of about 200 mCrab, and rarely it is a giant outburst reaching 1 Crab. We were watching the growth of the X-ray light curve, meanwhile a group reported the outburst on Nov. 6 ( ATel #14157 ). On Nov. 12 it reached almost 1 Crab and it clearly means a giant outburst.
The light curve and hardness curve are shown in the figure ( ATel #14173 ).
Latest light curve and hardness curve .

GRB 201021B

news-1269 October 22, 2020 by Mihara

MAXI nova alert system triggered an uncatalogued bright X-ray source (210 mCrab) at 2020-10-21 16:18:55UT in the constellation Tucana. We reported it as a gamma-ray burst GRB 201021B (GCN #28726). The star was not detected in the previous (14:46 UT) nor the next (17:51 UT) MAXI scans.

Superburst from Aquila X-1 and NICER follow-up

news-1268 October 15, 2020 by Mihara

MAXI detected a superburst from the neutron-star low-mass X-ray binary Aquila X-1 on 2020 October 8, 16:59 UT ( ATel #14079 ). The luminosity was about a half of the Eddington luminosity ( = 40000 times of the sun) and the temperature was 2.0 keV ( = twenty million K). We waited for the next scan at 18:32 when the burst was still going on, and confirmed it the superburst lasting for hours, not an X-ray burst lasting for 100 s.
We issued an alert mail and also informed the NICER team at 19:55. Since Aql X-1 had been in an outburst for a month ( MAXI Light curve ) and NICER had been observing it every day, NICER could quickly turn the telescope and started the observation at 20:05, that is 10 minutes after the mail. It was the quickest record.

Short X-ray burst from the UCXB 4U 1820-303

news-1267 October 8, 2020 by Mihara

MAXI detected an X-ray burst from the neutron-star low-mass X-ray binary 4U 1820-303 in the globular cluster NGC 6624 in the constellation Sagittarius at 2020/10/6 01:32. It lasted for 10s (figure), which is categorized in the short (~15 s) X-ray burst. (see MAXI news #1256 figure 2) The shortness comes from the He burning.
The system is an ultra compact X-ray binary with a neutron star and a low-mass star. The orbital period is only 11 minutes. The separation of the two stars is only 120,000 km, which is 1/3 of earth-moon distance. The low-mass star is probably a white dwarf. The surface would consist of Helium. Then the short burst and the ultra compact orbit is related.

Flare from the active binary UX Ari

news-1266 September 17, 2020 by Mihara

MAXI detected a flare from UX Arietis at 7:59 UT on 2020 September 13. The flare lasted for 12 hours and the peak intensity was 100 mCrab (figure). The luminosity was one million times of the largest solar flare. UX Ari is a 6 magnitude star located at 6 degrees north-west of the Pleiades star cluster. The distance from the earth is 165 lt-yr by GAIA.
It is a RS CVn type variable, that is, a close binary system with a K-type subgiant star and a small G-type main-sequence star. They rotate around each other by 6.4 days orbital period. The system with this combination is very active.

Record high X-ray from dwarf nova SS Cyg

news-1265 September 11, 2020 by Mihara, Negoro

MAXI recorded the highest X-ray flux 40 mCrab from the dwarf nova SS Cyg. Increase of flux stared around 2018 October ( ATel #13744 ) and were succeedingly reported by MAXI news #1237 in 2019 July and MAXI news #1253 in 2020 May. The average X-ray flux is about 10 times higher then before. It is the highest in the 25 years since 1995 when RXTE/ASM started monitoring. The optical flux in the quiescence is also 2.5 times higher than before and may be the highest in the 110 years since 1909.

Aql X-1's 13th outburst triggered by Swift/BAT, radio, and optical

news-1264 September 3, 2020 by Mihara

Aquila X-1 is a neutron-star low-mass X-ray binary known since 1973. It repeats bright outbursts lasting for a month in roughly once in a year. According to the MAXI light curve (figure), the current outburst is the 13th one. This outburst was first suggested with the Swift/BAT on 2020 August 9 and the brightest radio from this source on August 12 ( ATel #13016 ). Optical monitoring had detected a brightening from 18.1 mag. to 16.7 mag., at least by one magnitude (= 2.5 times) (ATel #13020, 13953, 13981). ( ATel #13029 , ( ATel #13953 , ( ATel #13981 ). They expected emergence of MAXI's X-ray around 20 August. MAXI did detect X-ray on August 31 after the source came out of the un-scanned region. Extrapolating the light curve, the rise might be about August 27.

MAXI J1216-655 : Very Fast X-ray Transient

news-1263 August 27, 2020 by Mihara, Negoro

MAXI found a faint transient MAXI J1216-655 in the constellation musca on 2020 August 9 in one-day bin light curve ( ATel #13946 ). Peak flux was 30 mCrab in 4-10 keV in 00h-04h UT on Aug. 9. It went down < 10 mCrab on Aug. 10.
On Aug. 15, Swift performed a 7-point tiling observations covering the whole MAXI's error region ( ATel #13951 ) (figure). But, there was no new source. There was one supernova remnant, G299.2-02.9. MAXI J1216-655 would have faded out below < 0.1mCrab in the 5 days. It might be a very fast X-ray transient (VFXT) like MAXI J1957+032 ( ATel #7504 ).
The nature of the source is unknown.

AT2019wey : Mysterious slow transient

news-1262 August 20, 2020 by Mihara, Negoro

The optical transient AT 2019wey was discovered by ATLAS telescope on 2019 Dec. 7 in the constellation Camelopardalis. Spectrum Roentgen Gamma satellite discovered the X-ray source on 2020 Mar. 18 ( ATel #13571 ). X-ray brightening has been monitored by Swift and NICER since Apr. 12. The type of this source remains uncertain. We made a long-term MAXI light curve of the source ( ATel #13948 ).
There was no X-ray before the optical discovery. At the discovery there was a possible X-ray flare at 00:13 on 2019 Dec. 6, in only one scan. In the next observation period from Dec. 30 to 2020 Jan. 18, MAXI detected a significant X-ray (~3.4 sigma level). So, AT 2019wey had already started the X-ray emission before the SRG detection. The X-ray intensity has kept increasing exponentially since then to date. Thus, MAXI can trace back the past activities even after a later notice.

XTE J1829-098 : hot summer outburster

news-1261 August 13, 2020 by Mihara

MAXI detected a slight enhancement (~ 20 mCrab) of X-ray in the constellation Scutum on 2020 July 31, August 6, and August 10. The source positions are consistent with the Be X-ray pulsar XTE J1829-098. Since the discovery on 2004 July 31 ( ATel #317 ), XTE J1829-098 made outbursts on 2008 August 4 with RXTE ( ATel #2007 ), 2018 August 6 with MAXI ( ATel #11927 ) and this time. All of the four were around the beginning of August of the even-number year, suggesting 2 years is a multiple of the orbital period of the binary.

X-ray flares from the second youngest pulsar (and also a magnetar) AX J1846.4-0258

news-1260 August 7, 2020 by Mihara, Negoro

The MAXI/GSC nova alert system triggered a weak X-ray transient source at 07:49 UT on 2020-07-30 (59060 MJD) in the one-day accumulation ( ATel #13922 ). The source position was (R.A., Dec) = (281.47 deg, -2.63 deg) = (18 45 53, -02 37 33) (J2000) with a statistical error of 0.26 deg and a systematic error of 0.1 deg.
There were three scans (2020-07-30 01:38, 03:11, and 04:44 UT) with a significant X-ray flux (see MAXI nova page 2 : Possibly AX J1846.4-0258 , scan No. 13, 14, and 16, respectively). The averaged flux over the three scans was 50 +- 15 mCrab in 4.0-10.0keV. There was no significant excess flux in the previous transit at 00:01 UT (scan No. 12) and in the next transit at 06:17 UT (scan No. 17) with an upper limit of 20 mCrab for each.
However, there were several scans with less significant (~2 sigma) fluxes (~30 mCrab). They are previous scans at 07-28 00:03 (page 1 : No. 22) and 01:36 (page 1 : No. 23), 07-29 11:41 (No. 4) and 14:47 (No. 6), and a next scan at 07-30 21:46 (No. 28). Here the page 1 refers to here, AX J1846.4-0258
There were three known sources in the error region. On 2020-08-01 20:11 UT Swift/BAT detected a flare from AX J1846.4-0258, which is one of the three sources (GCN #28187 and ATel #13913 ).
AX J1846.4-0258 is the second youngest (700 yerars old) pulsar in our galaxy (and also a magnetar) and shows a magnetar-like activity (Gotthelf et al. 2000).
Intermittent detections with MAXI from 2020-07-28 to 07-30 are consistent with the flaring behavior of the source. Therefore, the X-ray fluxes which MAXI detected would also come from AX J1846.4-0258.

Spectrum tells the bright flare was from SFXT XTE J1739-302

news-1259 July 17, 2020 by Negoro, Mihara

MAXI detected a bright X-ray (~300 mCrab) from ~2 degrees west of the Galactic center (GC) at 9:34 UT on 2020 July 9 ( ATel #13871 ). There were no detections in the previous and the next MAXI scans. The short flare reminds us an X-ray burst, but there is no X-ray burster in the region. The spectrum can be fit by a 2 keV blackbody, typical for an X-ray burst, but an interstellar gas absorption is not needed. It is unusual for an X-ray source near the GC.
Another kind of X-ray sources with short (~1000s) flares is supergiant fast X-ray transient (SFXT). There is one SFXT in the region in fact, XTE J1739-302. MAXI detected a short flare in 2015 ( ATel #6900 ), too. The spectrum can also be fit with a power-law with the photon index of 1.5 and the galactic absorption of 5 x 10^22 cm^-2. Those are reasonable values for a SFXT and we conclude the flare was from XTE J1739-302.

Intermediate duration X-ray burst from IGR J17062-6143

news-1258 June 25, 2020 by Mihara, Iwakiri

MAXI detected a bright (~ 2 Crab) X-ray burst from IGR J17062-6143 at 2020 June 22 8:15 UT in Ara constellation ( ATel #13827 ) (figure). It was a rare "intermediate duration X-ray burst" lasting a few hours (see figure 2 of MAXI news #1256 ), and was shining when NICER turned the telescope at 3.0 hours after the MAXI detection. Such a long X-ray burst were reported in 2012 (GCN #13386) and 2015 ( ATel #8241 ) from this source. Since the discovery in 2008 ( ATel #1840 ), observations have revealed that the spin period of the neutron star (NS) is 6.1 ms, the orbital period of the binary is 38 minutes and the distance from the Earth is about 30,000 lt-years (Strohmayer et al. 2017). It is the NS low-mass X-ray binary. The separation of the two stars is only 0.5 of the solar radius. Such a small system is called "ultra compact X-ray binary (UCXB)". Somehow, the long burst tends to occur in UCXB.
(see also MAXI news #1254 )

SpaceX crew-Dragon shadows (DEMO-2)

news-1257 June 18, 2020 by Mihara

SpaceX crew-Dragon space ship (DEMO-2) docked the ISS on May 31, 2020. Since then, right ends of 25 degrees in the MAXI horizontal camera and 10 degrees in the zenithal camera are shadowed (figure). Because the X-ray in the MAXI's energy band cannot penetrate the star ship, Dragon hides X-ray stars behind it, including the now-outbursting GRO J1008-57 ( MAXI news #1252 ). The shadow moves on the sky with 70-day precession period as arrows in the figure. The shadowed part is a little bluer (harder X-ray) in the figure, since the Dragon stops the cosmic X-ray background (CXB), while the particle background is still there which is harder than the CXB.

Short X-ray burst from GRS 1747-312

news-1256 June 11, 2020 by Mihara, Negoro, Serino

MAXI detected a short spike at 5:50:13 UT on June 5, 2020 from the constellation Scorpius. The X-ray emission lasted only for 4 s (figure 1). There locates the low-mass X-ray binary GRS 1747-312, which is known to emit short X-ray bursts. An X-ray burst is a sudden thermo-nuclear burning on the surface of a neutron star. The duration distribution is spread in 10-100 s with two peaks at about 15 s and 80 s (figure 2). Those are interpreted as He fuel (15 s), and mixed (H and He) fuel (80 s), respectively. The reason that GRS 1747-312 always emits short X-ray bursts is considered as that H is slowly burning into He on the surface, and since the accumulation time in GRS 1747-312 is relatively long, all of H has turned to He when a burst ignites.

Short X-ray burst from GRS 1747-312. S and E are when the source is in the MAXI field of view. The burst occurred in the middle.

Taken from "Understanding superbursts" MAXI 7year conference proc. p121 In't Zand (2017)

Flares ! from sleeping GRS 1915+105

news-1255 June 4, 2020 by Mihara, Iwakiri

At 15:20 UT on 2020 May 26, MAXI detected a strong (~1 Crab) X-ray from GRS 1915+105 in Aquila. It is known as a violently variable blackhole binary since 1992, but recently has darkened and kept undetected with MAXI ( MAXI news #1232 , ATel #13652 ). Responding to the alert mail from MAXI, through OHMAN ground route, NICER quickly observed the source at 16:31. GRS 1915+105 was still in the bright flare and NICER recorded excellent data of rapid variations shorter than 0.1 s, "strange cycles of X-ray emission", and blowing-out iron lines. 1 hour 11 minutes has become the shortest record of OHMAN. The conditions were lucky, as MAXI's detection with zenithal cameras and NICER's observation at the next rise from the horizon.

Long X-ray burst from MAXI J0911-655

news-1254 May 28, 2020 by Mihara, Iwakiri

MAXI detected a bright (~ 1 Crab) X-ray burst from the globular cluster NGC 2808 in the constellation Carina at 17:04 on May 22, 2020 ( ATel #13754 ). It contains the low-mass X-ray binary MAXI J0911-655 discovered in 2016 ( ATel #8872 ). Since the discovery, it kept shining at about 10 mCrab in X-ray ( MAXI news #1242 ). The distance from the earth is 31000 light-year. The neutron star is an accreting ms pulsar (Pspin=2.9 ms, Porbit = 44.3 minutes, Sanna et al. 2017). This X-ray burst was long-lasting (~100 minutes), and this kind of the X-ray burst is known to occur in the ultra-compact binary like MAXI J0911-655.
NICER quickly turned the telescope by the command from the ground, and observed the source at 18:57 successfully ( ATel #13760 ). It was 1 hour 53 minutes after the MAXI detection and became the quickest record of the MAXI-NICER connection.

MAXI X-ray image at burst detection at 17:04 on May 22, 2020. The intensity was about 1 Crab. In the next MAXI scan at 18:37, it had faded to about 100 mCrab.

Dwarf nova SS Cyg getting brighter

news-1253 May 21, 2020 by Mihara, Negoro

MAXI found X-ray flux from SS Cygni has been continuously increasing since around 2018 October ( ATel #13744 ). The activity was reported in MAXI news #1237 in 2019 July. Since then the X-ray flux has kept increasing, and reached about 30 mCrab in 2020 May. It is the highest flux that MAXI has ever observed from SS Cyg since 2009 August. Comparing the MAXI data with the optical data by AAVSO, both fluxes are increasing (figure). Usually X-ray is emitted only at the beginning and the end of an optical outburst (Wheatley et al. 2003), and optical flux is low between the outbursts. So, it is very strange that X-ray and optical flux remains high even between the outbursts.

GRO J1008-57 starting a giant outburst

news-1252 May 14, 2020 by Mihara, Nakajima

MAXI is monitoring the activity of the Be X-ray binary pulsars. A normal outburst was reported in MAXI news #1206 . The data in the recent several days shows that GRO J1008-57 seems to start a giant outburst. Figure is the X-ray flux in log scale vs. orbital phase. The orbital period is 250 days and phase zero is set at the periastron passage. The light curve (pink) is tracing that of the giant outburst in 2012 (green) very well.
After a normal outburst at phase -0.05 the flux has kept in a constant level, and at phase 0.24 it dropped once and started to rise immediately after it.
The BeXRB monitor by ESAC/ESA summarizes the data from MAXI, Swift/BAT and Fermi/GBM.
Now the activity of GRO J1008-57 is ranked at the top.
( A snap shot was taken on May 28 2020 ).

Recent light curve of GRO J1008-57 in pink (updated on May 22 at the time of ATel #13750 ). It traces the light curcve of the giant outburst in the 2012 in green. Two kinds of outbursts, normal and giant ones, are shown in black and others.

SGR 1935+2154 became active

news-1251 April 30, 2020 by Mihara, Sugita

MAXI detected two bright short bursts from the Soft Gamma-ray Repeater SGR 1935+2154 on 2020-04-27 (GCN #27661). The first one was at 20:01:45.99 UT and had a double-peaked light curve with a total duration of only ~100 ms (Figure). The peak width was 5ms and the peak flux was about 75 Crab in the 2-20 keV band, for each pulse. The second burst started at 21:56:02.63 UT and had a single-peaked structure with a width of 50 ms. This peak flux was about 40 Crab in 5 ms.
The activity was first reported from Swift/BAT at 2020-04-27 18:26:20 UT (GCN #27657, ATel #13675 ). and Fermi/GBM at 2020-04-27 18:26:20.16 UT (GCN #27659). NICER, INTEGRAL, AGILE, Konus-Wind, and HXMT also detected the SGR activities.
This was the second detection of the short burst from SGR with MAXI. The first one was also from SGR 1935+2154. It was 2016 Jun 20 (GCN #19545).

Light curve of the first short burst from SGR 1935+2154 with MAXI. The X axis is time in sec. The red line indicates the burst duration was only 0.1 s. There were two spikes in the burst. The Y axis is the count rates. The bin size is 5 ms. Each step corresponds to one photon. For example, the highest peak had 9 photons in 5 ms.

GRB 200205A -> Swift J0840.7-3516

news-1250 February 13, 2020 by Mihara

MAXI detected a bright (72 mCrab) uncatalogued X-ray source at 2020 February 5 06:54 UT in the southern constellation Pyxis (GCN #26987). The position was consistent with the gamma-ray burst GRB 200205A triggered by Swift (GCN #26982) at 06:35. But it was 19 minutes ago. In the next MAXI scan at 07:16 the intensity increased to 262 mCrab in 4.0-10.0 keV. This is unusual. There is no GRB whose afterglow is brighter than the prompt emission. In the third scan at 8:27 it started to decrease down to 66 mCrab. The X-rays in the MAXI prior transit at 05:43 and in the next transit at 08:49 were not detected, < 20 mCrab for each. The light curve is very peculiar (Figure).
Swift reported a detection of 9 s pulsation and renamed it Swift J0840.7-3516 ( ATel #13452 ). However, the pulsation was not confirmed with NICER and the star faded out ( ATel #13456 ). The spectrum and the rapid time variations are similar to those of a black hole. 9 s pulsation might be a quasi periodic oscillation (QPO) seen from a BH. So, the nature of the star remains unknown.

Light curve of GRB 200205A, later renamed as Swift J0840.7-3516. Light curve is very peculiar as a GRB. It is also peculiar as a galactic transient.

Violent flares from XTE J1901+014

news-1249 December 5, 2019 by Mihara

On December 3 2019 MAXI detected a flare (80 mCrab) from XTE J1901+014 in the constellation Aquila ( ATel #13328 ). The flare lasted only for two scans separated by 90 minutes. Looking back the MAXI alerts, it was the third detection with MAXI after 2012 and 2015. The source was discovered with the former all-sky monitor RXTE/ASM in 2002 ( ATel #88 ). But in reality the first detection was in 1997. Swift/BAT detected flares in 2006 and 2010, which lasted for only from tens of seconds to hundreds of seconds ( ATel #2375 ). XMM and Chandra localized the position, but there is no optical or infrared star ( ATel #1268 ). The spectrum with ROSAT, RXTE and Integral is a simple power-law in a wide range of 0.6 - 100 keV ( Karasev et al. 2007 ). These properties are similar to the violent-type of the black hole binary such as V4641 Sgr and V404 Cyg. However, the nature of this source remains unknown.

Be X-ray binary RX J0209.6-7427 in Magellanic bridge

news-1248 November 28, 2019 by Mihara

MAXI discovered a faint (23 mCrab) source near Small Magellanic Cloud (SMC) on 2019 November 20, and named it MAXI J0206-74 ( ATel #13300 ). By the follow-up observation with Swift/XRT it was identified as RX J0209.6-7427 ( ATel #13303 ). The source was discovered in 1993 with ROSAT and was identified to a Be star ( Kahabka and Hilker 2005 ). This time , too, a strong and broad H alpha emission line was detected with SALT telescope in South Africa, which indicates an active phase of the Be star ( ATel #13307 ). IRSF telescope also detected a little brightening in infrared ( ATel #13315 ). By the following NICER observation the 9.29 s pulsation was discovered ( ATel #13309 ). The system was turned out to be a Be X-ray binary pulsar located in the Magellanic bridge, which is a gas stream connecting SMC and LMC (figure). The distance is 200,000 lt-yr from Earth. Although it looks dim apparently, it is very luminous in fact, three times as bright as the Eddington luminosity.

Location of MAXI detection. The Be X-ray binary RX J0209.6-7427 lies on the Magellanic bridge. It was born in SMC and travelling in the vast space for 200 million years.

Massive star binary Eta Carinae is active

news-1247 November 14, 2019 by Mihara

MAXI detected an X-ray increase from Eta Carinae on 2019 November 5 and 6. Usually it is 5 mCrab, while it is 10 mCrab now. It is a very massive star binary (~90 and ~30 solar masses). The X-ray outburst occurs in every 5.54 year, which is considered to be the binary period. After the peak the flux drops sharply probably due to the eclipse of the X-ray emitting shock region in the stellar wind by the primary star. The previous drop was around 2014 August 2 and the next one is expected on 2020 February 16. The flux has already started to increase towards that day.

MAXI 10 years light curve of Eta Carinae. X-ray has started to increase towards the periastron in 2020 February.

MAXI J0637-430 : New black hole was found

news-1246 November 5, 2019 by Mihara

MAXI discovered a new source MAXI J0637-430 on 2019 November 2 in constellation Columba ( ATel #13256 ). The spectrum was soft and the 2-4 keV flux was 60 mCrab. Swift performed a follow-up observation on November 3 and confirmed an uncatalogued new X-ray source ( ATel #13257 ). The spectrum was disk-blackbody of 0.9 keV temperature with a hard power-law tail, which is typical for a black hole transient. It is the 13th black hole binary that MAXI discovered. The UV telescope (UVOT) on Swift also detected a new bright star (15th magnitude). A follow-up spectroscopy of the star was carried out by a ground telescope in Chile on November 3 and they found a double-peaked strong H alpha emission line, which would come from an accretion disk ( ATel #13260 ). The dim and soft X-ray state indicates a far distance of about 10 kpc. The star would be a black hole low-mass X-ray binary wandering far in the galactic halo.

New black hole binary MAXI J0637-430 was discovered on 2019 November 2. The red color of the source indicates that it emits soft X-ray.

A Be X-ray binary pulsar became active

news-1245 October 24, 2019 by Mihara

When MAXI recovered from the power-saving operation (2019.10.18-20), the Be X-ray binary pulsar (BeXBP) XTE J1858+034 had already started an outburst ( ATel #13217 ) following the previous one in 2011 ( ATel #3766 ). The intensity was about 20 mCrab. Swift/BAT also confirmed the outburst. The BeXBP generally has an outburst at the periastron passage of the elongated orbit, but not always. This is the fifth outburst of XTE J1858+034 in history. The discovery was in 1998 ( Paul and Rao 1998 ). The pulse period of XTE J1858+034 is 221 s, while the orbital period is not known.

Nova ? No, again. Transient Swift J1845.7-0037 = XMMSL1 J194555.4-003941

news-1244 October 17, 2019 by Mihara

MAXI detected an outburst in the constellation Aquila on 2019 October 11 ( ATel #13189 ). The X-ray intensity changed violently between 20 mCrab and 190 mCrab in Oct. 12-13. The followup observation of Swift on Oct. 14 identified the source to the known Swift J1845.7-0037 ( ATel #13191 ).
It had been detected by Swift/BAT with 15-30 mCrab on 2012 May 16, and followed by Swift/XRT with 4 mCrab on 2012 May 24 ( ATel #4130 ).
Far before that, XMM-Newton observatory had discovered the source, XMMSL1 J184555.4-003941, on 2005 September 20 during the slew at 0.1 mCrab. The quiescence at 0.1 mCrab was also observed serendipitously by Swift/XRT on 2011 June 27. There is an infrared star of 15th magnitude.
However, the nature of this source still remains unknown.

Nova ? No. Neutron star transient GRS 1747-312

news-1243 October 3, 2019 by Mihara

MAXI detected an outburst from the region near (2 degrees south of) the galactic center on 2019 September 29 ( ATel #13154 ). Followup observations of Integral and Swift identified the source to GRS 1747-312 ( ATel #13155, #13157 ). It is a binary of a neutron-star (NS) and a low-mass star in the globular cluster Terzan 6. The distance is 24000 lt-years. The peak flux is 50 mCrab, corresponding to an X-ray power of 6 x 10^29 W, which is 1500 times larger than the emission of the sun. It repeats an outburst every about half a year ( in't Zand et al. 2003 ). The X-ray binary with this luminosity range repeats outbursts in every ~100 days due to the instability of the accretion disk. It is similar to Aql X-1 in the MAXI News on 2019 August 8.

Long-lasting MAXI nova MAXI J0911-655

news-1242 September 26, 2019 by Mihara

MAXI detected an X-ray increase from MAXI J0911-655 on 2019 September 24. It is a MAXI nova discovered on 2016 February 19 (ATel #8872), and also detected independently by Swift BAT (Swift J0911.9-6452). Observations with X-ray telescoples of Swift/XRT (ATel #8884) and Chandra (ATel #8971) identified MAXI J0911 as a star in the globular cluster NGC 2808, which is source 15 in Servillat et al. (2008). NGC 2808 is a large globular cluster at the distance of 31000 lt-years. MAXI J0911 is the longest-lasting MAXI source. The X-ray flux is about 10 mCrab for the three years. It brightened up by 10000 times since the first Chandra observation.
XMM-Newton and NuSTAR observations found a pulsation of 2.9 ms ( Sanna et al. 2017).
It once faded out in 2019 June (ATel #12831), and this time MAXI detected a reflaring.

(left) The globular cluster NGC 2808 taken by Hubble Space Telescope. (c) NASA, ESA, A. Sarajedini (University of Florida) and G. Piotto (University of Padua)
(right) MAXI J0911-655 was the source 15 in the Chandra image. The two circles are core radius and half-mass radius of the globular cluster. Taken from figure 1 in Servillat et al. (2008).

MAXI J1820+070 rebrightened again. It also did 100 years ago

news-1241 September 5, 2019 by Mihara

When the ISS changed the attitude back for 60S docking in late August, the black hole binary MAXI J1820+070 moved into the scan area of MAXI's good cameras. Then by chance, MAXI detected a rebrightening of MAXI J1820+070 on 2019 August 24. It was discovered with MAXI on 2018 March 11, faded out, and rebrightened to make a small outburst on 2019 March 21. This time (~30 mCrab) is the second rebrightening.
Recently optical astronomers found in the digital archive that MAXI J1820+070 had made outbursts in 1898 and 1934 (ATel #13066), suggesting that the outbursts repeat with a recurrence period of about 40 years. The astronomers who took photographs 100 years ago would never have thought that they had taken photographs of a blackhole binary.

An outburst of MAXI J1820+070 taken in photographs in 1898. The star had become as bright as this time in 2018.

"Missing-link" object, accreting ms pulsar SAX J1808.4-3658 starts an outburst

news-1240 August 15, 2019 by Mihara

MAXI detected an X-ray from the accreting ms pulsar SAX J1808.4-3658 on 2019 August 9, following the optical on July 25 (ATel #12964), Swift/XRT (ATel #12993) and NICER (ATel #13006) on August 6. X-ray flux with MAXI was about 40 mCrab, which is consistent with the latest Nu-star observation on August 11 (ATel #13022). SAX J1808.4-3658 is the first ms pulsar in a binary system ( Wijnands & van der Klis 1998). The pulsation is 2.5 ms, which is considered to result from the acceleration by the mass accretion from the companion star. It makes an outburst in about every 3 years, and this is the eighth outburst.

Recurrent transient Aql X-1 starts an outburst

news-1239 August 8, 2019 by Mihara

MAXI detected a start of an outburst from Aql X-1 on August 5, 2019. It is a neutron star binary with a low-mass star. Due to the instability of the accretion disk around the neutron star, Aql X-1 makes a bright outburst in every about 300 days (Asai et al. 2012). The previous outburst was detected by MAXI on November 18, 2018.
Aql X-1 in MAXI HP.

Blackhole binary EXO 1846-031 woke up after 34 years of quiescence

news-1238 August 2, 2019 by Mihara

MAXI detected a X-ray transient in the constellation Aquila on 2019 July 23 (ATel #12968). There are two known X-ray sources in the MAXI error circle. The transient has increased to about 100 mCrab on July 28, and Swift/BAT identified it as EXO 1846-031.
EXO 1846-031 was a black hole binary discovered in 1985 (Parmar et al. 1993). The outburst lasted for half a year, and then went into a quiescence. This outburst is since then after 34 years.
The follow-up observation with Swift/XRT was performed only 1 hour and 10 minutes after the MAXI alert, and obtained the source position as accurately as 2 arcsec (ATel #12969), which is 10 times better than before. There are still two infrared-cataloged stars within the error circle. The next day the VLA detected radio emission from the source and the position was determined with the accuracy of 0.03 arcsec (even 100 times better) (ATel #12977). Neither of the two infrared stars is EXO 1846-031.

MAXI detected X-ray outburst from SS Cyg

news-1237 July 25, 2019 by Mihara

MAXI detected an X-ray increase from SS Cyg on 2019 July 22 following the detection on 2019 February 3. The flux was about 15 mCrab for both. SS Cyg has become active in 2019.
SS Cyg is a dwarf nova at 374 light-years away. It is a non-magnetic cataclysmic variable (CV = a white dwarf binary). SS Cyg makes an optical outburst in every about 50 days. Compared to the optical light curve, the X-ray detections with MAXI were not at the peak, but almost at the end of the outbursts. It is consistent with the light curve taken with RXTE (figure, Wheatley et al. 2003). When the accreting matter becomes less, the plasma turns to optically thin and the temperature rises. Then it starts to emit X-ray.

Optical, UV, and X-ray light curves of the dwarf nova SS Cyg with RXTE taken from Wheatley et al. (2003).

Soft MAXI gamma-ray burst GRB120714A was in fact an X-ray flare of a M-type star

news-1236 July 17, 2019 by Mihara

MAXI detected a bright (230 mCrab) X-ray transient in the constellation Hercules at 13:20 UT on 2019 July 14 (GCN #25068). MAXI team reported it as GRB 190714A. Swift promptly performed followup observations in about 3 hours later, and found an X-ray star with 2 mCrab (ATel #12944). The location was consistent with an M-type star named UCAC4 525-061488 in 113 light-years away. It was also a cataloged X-ray source, 2E 1624.6+1504.
The prompt followup revealed that the MAXI detection was not a gamma-ray burst, but a flare from the nearby M-type star.

MAXI's soft gamma-ray burst GRB190708B was in fact a soft tail of the GRB

news-1235 July 15, 2019 by Mihara

MAXI triggered a bright uncatalogued soft X-ray transient at 19:49:58 UT on 2019 July 8 in the constellation Pegasus (GCN #25037). The X-ray flux was 394 +- 60 mCrab (4.0-10.0keV, 1 sigma error). There was no significant excess flux in the previous transit at UT 19:29 and in the next transit at 21:02 UT with an upper limit of 20 mCrab for each. KONUS/WIND detected a gamma-ray burst at ~240 s before the MAXI trigger (GCN #25043). So the MAXI source was the tail, or early afterglow of the GRB. There was a faint source in the Swift/XRT follow-up observations in 16 hours later (GCN #25053), and it faded in the following observation after 6 days. The swift source was the afterglow of the GRB (GCN #25073).

X-ray flare from sigma Gem, a RS CVn type variable

news-1234 June 20, 2019 by Mihara

MAXI detected a flare from sigma Gemini at 01:03 UT on 2019 June 19. The flare was about 80 mCrab and lasted for about ten hours. Sigma Gem is at 125 light-years and a 4-magnitude star next to Pollux. It is a binary star which consists of a K-type giant star and a smaller main-sequence star. This type of binary is called as the RS CVn variable. This combination is active due to the large sun spot on the K giant star. The previous flares from sigma Gem with MAXI occurred on February 4 and January 24, 2019.

MAXI J1348-630 rebrightened

news-1233 June 6, 2019 by Mihara

MAXI detected X-ray rebrightening of the black hole candidate MAXI J1348-630 (ATel #12838). It was discovered with MAXI on 2019 January 26 (58509 MJD) (ATel #12425), and had a peak in intensity on February 11. The outburst declined and once disappeared on May 11. Then usually the X-ray nova goes to a quiescent state. But MAXI J1348 reappeared on May 31 and is still getting brighter and brighter.

X-ray light curves of MAXI J1348-630 with MAXI (top) and Swift/BAT (middle). A distance 3 kpc is assumed. The bottom curve is the hardness ratio between BAT and MAXI. After once it fainted out on 2019 May 11 (58614 MJD), it started shining again on May 31 (58634 MJD). We can tell the spectral states by the hardness ratio (HR). That with HR ~ 1 is the hard state, and that with HR ~ 0.1 is the soft state.

GRS 1915+105 woke up ?

news-1232 May 16, 2019 by Mihara

Flares from the micro-quasar GRS 1915+105 were detected by MAXI/GSC on 2019 May 14 (ATel #12761). One of the peak flux reached 316 +/- 30 mCrab (1 sigma error) in 2 - 20 keV. The 15 - 50 keV flux by Swift/BAT exceeded 1000 mCrab. The source was in an unusual low-flux state (90 mCrab with MAXI) for the last one year (ATel #11828, #12742, #12743, #12755). We are watching whether GRS 1915+105 is going to the quiescence (ATel #12755), or becoming active again.

Recent X-ray light curves of GRS 1915+105. The flux went down to zero level, and suddenly some flares occurred.

Cyg X-3 in the quench state

news-1231 April 25, 2019 by Mihara

Cyg X-3 is a mysterious star. We do not know exactly what kind of a star it is. A blackhole binary has been suggested. The X-ray flux and spectrum changes violently (figure). Since February 2019, Cyg X-3 entered a quenched state (ATel #12510). It is a rare state when the radio emission drops down and X-ray spectrum changes to a pure blackboby (ultra soft state). It was the 4th during MAXI 10 years of observations. The quench state ended on April and a strong radio was emitted (ATel #12677). At the same time, a hard X-ray emerged in the MAXI spectrum as the hardness ratio went up in the figure.

Recent Cyg X-3 went into the fourth radio quench state, or X-ray ultra soft state.

LIGO/Virgo O3 started

news-1230 April 11, 2019 by Mihara

The operation 3 (O3) of LIGO/Virgo gravitational wave detectors has started. The first event S190408an occurred at 18:18:02.29 UT on 2019 April 8, which is likely a BH-BH merger (GCN #24069). MAXI/GSC was operating at the instant and scanning a small part of the GW error region. However, there was no flaring event. Within one orbital period of ISS, MAXI covered about 82% of the 90% probability region. There was no event again, whose 1-sigma upper limit was 28 mCrab in 2-20 keV (GCN #24091).
(The O3 was terminated on March 27, 2020 due to COVID-19.)

Short GRB, GRB 190503A detected

news-1229 March 6, 2019 by Mihara

MAXI detected an X-ray increase of 100 mCrab at 2019 March 5 13:07:43 UT in the constellation Phoenix (GCN #23933). Swift/XRT follow-up found 6 X-ray sources (#1-6) within MAXI error circle (GCN #23935). At about 13:05:18, which was 2 minutes before the MAXI detection, gamma-ray satellites : AGILE, CALET, Konus-Wind, INTEGRAL, Swift, and Insight-HXMT had detected a very-strong short (1.8 s) Gamma-ray burst (GCN #23930, GRB 190305A). IPN localization by Konus, INTEGRAL and BAT is consistent with the #1 source (GCN #23936). MAXI's detection would be an "extended X-ray emission" of a short GRB. If this is the case, it is the first one for MAXI. We note that a short GRB is the most promising candidate of a NS (neutron star) - NS merger.
Gamma-ray light curves are available from Konus-wind page.

MAXI's error circle and localization with IPN, and Swift/XRT. Taken from GCN #23936 and the link in it

4U 1901+03 revealed

news-1228 February 20, 2019 by Mihara

On 2019 February 8, MAXI detected X-ray brightening from the binary X-ray pulsar 4U 1901+03 (ATel #12498). This is the fourth X-ray outburst in 40 years of X-ray astronomy history, after the ones in 1970, 2003 and in 2011 (Sootome+ ATel #3829). The flux was 120 mCrab on Feb. 9 and will reach the maximum of 300 mCrab around Feb. 24 (figure is on Feb. 20). Because 4U 1901+03 locates on the milky way in the constellation Aquila, the optical star was not identified due to crowdedness and absorption. But this time, precise follow-up observations have changed the situation. The location was obtained precisely by Swift/XRT and there was an infrared star (ATel #12514). NICER observations were performed (ATel #12515) suggesting a Be star. The Chandra archive had detected 3 X-ray photons from this star (ATel #12519). The GAIA has shown the distance to this star, 2.9 kpc.

X-ray all-sky map on 2019 February 20. The marked bright source is 4U 1901+03. The color blue means a hard spectrum. X-ray binary pulsars appear in blue.

Swift J1728.9-3613 appeared

news-1227 February 6, 2019 by Mihara

On 2019 January 26, MAXI detected an X-ray increase from the constellation Scorpius. We first misidentified this source as EXO 1722-363 (ATel #12437). On Jan. 28 Swift/BAT detected it independently, and reported it as a new source Swift J1728.9-3613 (ATel #12436). It was followed up by Swift/XRT (ATel #12445) on Jan. 28 and NICER (ATel #12455) on Jan. 29. The spectrum and the quasi periodic oscillation indicate that it is a black hole binary. It reached a peak of 0.5 Crab on Feb. 2.

X-ray all-sky map on 2019 February 1. Three novae are shining brightly in the southern sky. All of them have turned out to be blackhole binaries.

MAXI J1348-630 was discovered

news-1226 January 30, 2019 by Mihara

MAXI discovered an X-ray nova MAXI J1348-630 in the constellation Centaurus at 03:16 UT on 2019 January 26 (ATel #12425). Swift/XRT obtained the precise location (ATel #12434). There is a 16 magnitude star (ATel #12430) getting brighter ( ATel #12439, #12448). NICER observation suggests that it is a black hole binary (ATel #12447). MAXI J1348-630 has reached 1.5 Crab on January 30 (figure). Now it is the second brightest star in the X-ray sky.

Discovery of MAXI J1631-479

news-1225 January 13, 2019 by Mihara

MAXI discovered a bright hard X-ray transient source at 04:33 UT on December 21, 2018 in the constellation Norma ( ATel #12320). The intensity was 209 +- 27 mCrab (4-10 keV). The Swift/BAT also detected it with flare-like behavior. There exists the 1309 second X-ray pulsar AX J1631.9-4752/IGR J16320-4751. But the MAXI intensity was about 10 times higher than the previous observations.
The Nu-STAR X-ray telescope made a follow-up observation on Dec.28 ( ATel #12340). The MAXI star was 0.21 degrees away from AX J1631.9-4752 (figure), and was found to be a new star. The name is MAXI J1631-479. The X-ray spectrum taken with the Nu-STAR indicates that MAXI J1631-479 would be a black hole binary. Then it is the fifth new black hole found with MAXI in 2018.

MAXI light curve of MAXI J1631-479. Taken from MAXI HP.
It became very bright, more than 1 Crab.

X-ray image taken with Nu-STAR X-ray telescope. White circle is the error circle which MAXI reported. AX J1631.9-4752 is in the error circle, but the new star is not AX J1631. MAXI J1631-479 is a new star.

The magnetar XTE J1810-197 woke up

news-1224 December 20, 2018 by Mihara

MAXI has detected another new faint source at (272.6, -19.6) since the end of November (see the figure in the previous news). Radio telescopes reported intense flares from the magnetar XTE J1810-197 on Dec 9 and 11 after 10 years of quiescence ( ATel #12284, #12285). Since the locations coincide, MAXI's source seems to be XTE J1810-197 ( ATel #12291).
(MAXI light curve : XTE J1810-197).
Nu-STAR TOO observations confirmed that this X-ray source is XTE J1810-197 indeed ( ATel #12297). It is a magnetar, which was discovered at the outburst in 2003. Magnetars are faint. XTE J1810-197 is the only magnetar whose emission is detected with MAXI.

Discovery of MAXI J1810-222

news-1223 December 12, 2018 by Mihara

MAXI discovered a new source, MAXI J1810-222, on December 1, 2018 in constellation Sagittarius ( ATel #12254). This is a peculiar source which emits only soft X-ray below 4 keV. The 2-4 keV intensity was 20-40 mCrab (Figure). Looking back the MAXI data, the source appeared in the beginning of November. Such long-lasting soft X-ray source is unusual. It can be a neutron star binary in a low state, a bright blackhole binary at a far distance, or a new kind of X-ray source. Since the source locates close to the Sun with the separation of about 25 deg, X-ray followup observation is impossible with Swift and NICER. It is only possible with Nu-STAR, the hard X-ray telescope.
Nu-STAR carried out 25 tiling observations on Dec. 9 and confirmed the existence of MAXI J1810-222 ( ATel #12283). The position is better determined with an accuracy of 2 arcminutes (=0.03 deg.) The spectrum is similar to that of the soft state of a black hole.

MAXI 2-4 keV X-ray image of MAXI J1810-222 and aother new source. They appeared among 5 bright X-ray stars. The new source (indicated "unknown" in the figure) was found to be a magnetar XTE J1810-197 (see the next news).

Rapid variability from Swift J1858.6-0814

news-1222 November 7, 2018 by Mihara

MAXI confirmed an X-ray emission (4 mCrab) from Swift J1858.6-0814 ( ATel #12163), which was discovered with Swift/BAT on 2018 October 25 ( ATel #12151). The source showed flaring variabilities as NICER detected an X-ray increase by a factor of a few hundred in 15 s ( ATel #12158). Such fast variability is very rare among X-ray binaries.

Magnetic white dwarf AM Hercules

news-1221 October 17, 2018 by Mihara

On October 10, 2018, MAXI detected an X-ray increase from the white dwarf (WD) binary, AM Hercures. It is a persistent source of a few mCrab, but has brightened to 10mCrab, the bightest in the 2 years. This star is categorized as "Polar", which shows a strong optical polarization, resulted from the strong magnetic field (100 M Gauss = 10^4 T) of the WD. The binary period is 3.1 hour. The star locates 286 light-years away.

Two Be X-ray binary pulsars made outbursts

news-1220 October 10, 2018 by Mihara

MAXI detected outbursts from the two Be X-ray binaries, A 0535+26 and GRO J1008-57 ( ATel #12092). They are binaries of a pulsar (neutron star) and a B-emission type star. Brightening of A 0535+26 was found on 2018 September 22, and that of GRO J1008-57 was on 2018 September 30.
(MAXI homepage : A 0535+26, GRO J1008-57)
Their binary orbits are eccentric. The binary stars become the closest in every 111 days and 250 days (orbital period). Sometimes around the periastron, but not always, the stellar gas falls onto the pulsar and an X-ray outburst starts. The Be X-ray binary pulsar is one of the major categories in X-ray binaries.

MAXI J1820+070 goes back to the hard state

news-1219 September 26, 2018 by Mihara

The black hole candidate MAXI J1820+070 showed a soft-to-hard spectral transition on September 22 ( ATel #12057). The soft state, whose emission is from the standard accretion disk, lasted from 2018 July 1 (58300 MJD) to September 22 (58383 MJD).
(MAXI homepage : MAXI J1820+070. MAXI-BAT light curve).

Light curves of MAXI J1820+070 with MAXI and Swift/BAT, and their intensity ratio.

Outburst from H 1743-322 again and again

news-1218 September 18, 2018 by Mihara

The blackhole binary, H 1743-322, in Scorpio is brightening. It reached 100 mCrab on September 18 with MAXI. This outburst was found by the INTEGRAL observatory (ESA) on September 3, 2018 at 40 mCrab ( ATel #12007). It was a 408 day after the previous outburst, and actually the 12th outburst detected by MAXI in 9 years (Figure). H 1743-322 repeats outbursts in 200 - 400 days due to the accretion-disk instability. MAXI (X-ray) and Swift/BAT (gamma-ray) have been watching the activity.

Repeated outbursts from H 1743-322 in 9 years with MAXI and Swift/BAT. The bottom is the BAT/MAXI intensity ratio. A distance of 34,000 light-year is assumed.

Stellar flares FT Psc and CF Tuc

news-1217 September 6, 2018 by Mihara

MAXI detected two stellar flares this week. One is from FT Psc at 14:31 UT on September 1, 2018 ( ATel #12016). The instensity was 160 mCrab. This detection was the second one following that on May 1, 2017. FT Psc is a 10-magnitude small star of spectral type dMe at 38 light-year, but is very active in X-ray. Another flare was from CF Tuc at 07:24 UT on September 3 2018 ( ATel #12009). The intensity was 150 mCrab. This is the second detection with MAXI following that on January 30, 2012 ( ATel #3897). It is a RS CVn type binary at 290 light-year, which is a star-pair with a very high X-ray activity.

GRB180809A with MAXI, Calet/GBM, Fermi/GBM and Swift/XRT

news-1216 August 23, 2018 by Mihara

MAXI detected a Gamma-ray burst GRB180809A on 2018 August 9 11:38:25 in the constellation Sagittarius ( GCN #23103 ). The intensity was 2 Crab. Calet/GBM, which is mounted on ISS, also detected it (GCN#23117) as well as Fermi/GBM (GCN#23118). Since both are non-imaging detector, MAXI was the only one that could localize the position. Being informed by MAXI, after 4-6 hours, the X-ray telescope Swift/XRT searched the MAXI error circle by 7 tiling and found an X-ray afterglow of 1.5 mCrab (GCN#23106). It had decayed out below 0.01 mCrab on 5 days later (GCN#23141). An optical telescope searched the region after 73 minutes from the MAXI detection, but there was no new star brighter than 20 magnitude (GCN#23107). There is no known galaxy at the position. The GRB180809A must have exploded in a very distant galaxy.

MAXI image of GRB180809A at detection.

Mysterious star Cir X-1 flared up

news-1215 July 18, 2018 by Mihara

MAXI detected Cir X-1, which flared up suddenly from under detection level (<30mCrab) to 250 mCrab on 2018 July 12 16:53 UT. In the next two scans (90 and 180 minutes later) it stayed at 50 mCrab, and then it flared up again to 150 and 300 mCrab in the following two scans. ( MAXI alert page of Cir X-1 ). The spectrum was shaped as a blackbody of 1.2 keV ( = 10 million degree K). Cir X-1 is a peculiar binary consisting of a neutron star with weak magnetic fields and a Be star. In 2013 a supernova remnant was found around it and the age is estimated to be younger than 4600 years (Heinz (2013) ApJ 779 171). The flares occur near periastron of the 16.6 day orbit, but not always. The last flare was 130 days ago. The flare lasts only for some hours to a few days. Violent behavior of the X-ray emission still remains ununderstandable.
Latest MAXI light curve of Cir X-1

MAXI X-ray light curve of Cir X-1, which started to flare up on July 12.

The Demon Star fired up in X-rays

news-1214 July 11, 2018 by Mihara

MAXI detected a large X-ray flare from Algol at 2018-07-04 05:52 (UT) ( ATel #11812). The flux was 300 mCrab. It was the largest among 14 flares which MAXI observed from Algol in 8 years. Distance to Algol is 90 light-years. Then the X-ray luminosity was 7 x 10^25 W, which is 1/6 of the optical energy of the sun ! Algol is an eclipsing variable star. The orbiting period is 2.86 day. MAXI team predicted an eclipse (in X-ray ! ) of this flare. MAXI's alert was followed by NICER, and NICER successfully detected the flare and the eclipse (Figure). From these observations, location and evolution of the huge X-ray flare will be analyzed. The overall view of the "Demon" star will be revealed.
Latest MAXI light curve of Algol

X-ray light curve of Algol which flared up on July 4, 2018.

MAXI detection was followed by NICER observations.

Gamma-ray burst GRB 180701A was detected

news-1213 July 4, 2018 by Mihara

At 06:47 UT on July 1, 2018, MAXI detected a gamma-ray burst GRB180701A ( GCN #22880) in constellation Aquarius. The intensity was 74 +- 19 mCrab, and it shone only for 10 s. It was so weak that the nova search program did not find it. Instead, a graduate student found it. Although GRB occurs one in every day, it is rather rare that it occurs in the MAXI field of view (2% of all-sky). This event was the second GRB for MAXI in 2018.

List of MAXI GRBs

X-ray image of GRB 170701A at detection.

Taken from MAXI alert page of GRB 180701A.

NS-LMXB 4U 1608-52 started an outburst

news-1212 June 27, 2018 by Mihara

On June 19 2018 MAXI detected a sudden X-ray increase from the neutron-star low-mass X-ray binary 4U 1608-52 in constellation Norma. The star is usually under MAXI detection (quiescence) and occurs an outburst intermittently about every year. It is because the accretion disk around the neutron star is unstable for this mass accretion rate. Spectral ratio (HR = Swift/BAT counts / MAXI/GSC counts) is an indicator of a spectral state (Figure). On June 20, 4U 1608-52 made a spectral state transition from hard to soft state as HR went down below 0.2. On June 22, it reached 700 mCrab and is staying in an almost flat level since then.
Latest MAXI light curve of 4U 1608-52

Rise of an outburst of NS-LMXB 4U 1608-52.

Taken from Spectral hardness monitor with MAXI and BAT.
4U 1608-52 is listed by the name of H 1608-522.

Blackhole binary MAXI J1727-203 was discovered

news-1211 June 6, 2018 by Mihara

MAXI J1727-203 was discovered by MAXI in constellation Ophiuchus at 9:41 UT on June 5, 2018 ( ATel #11683). In response to the MAXI's ATel, NICER on ISS tuened the X-ray telescope and confirmed the new source at 15:40 UT ( ATel #11689). The intensity is 150 mCrab now (June 6, 3h UT) and increasing. The spectral transition which was observed by MAXI, and the spectrum taken with NICER both indicate the source is likely a blackhole binary ( ATel #11696).
Latest MAXI light curve of MAXI J1727-203

MAXI J1727-203 at discovery.
MAXI J1727-203 appeared at a blank position (red cross). No.3 source is a bright LMXB GX 9+9. No.7 is a weak X-ray source Ophiuchus cluster of galaxies. No.1 is Kepler's SNR which is too soft to be deteced by MAXI/GSC.

Superburst from the neutron star binary SAX J1712.6-3739

news-1210 May 16, 2018 by Mihara

MAXI detected an X-ray brightening from SAX J1712.6-3739 on May 8, 2018 ( ATel #11636). The source is in the constellation Scorpio, and was discovered in 1999 by the Italian X-ray satellite BeppoSAX. The distance is 23,000 lt-years and almost as far as the Galactic center. The source is known to be a close binary of a neutron star and a low-mass star because it sometimes exhibits an X-ray burst. It is a thermo-nuclear-fusion runaway on the surface of a neutron star. The burst lasts for 10-100 seconds. Meantime, about 10^19 kg of H and He burns, and emits X-ray with 100,000 times more energy per second than the light from the Sun per second. Surprising on May-8 burst was that it had already started 8 minutes ago, as detected by Swift/BAT ( ATel #11623). It turns out a "super burst", which is a rare phenomenon known to us only recently since around 2000. It lasts for some hours, and emits 100 times more energy than a regular X-ray burst. Only about 16 superburst-sources are known.

Blackhole binary MAXI J1535-571 dimmed out

news-1209 April 24, 2018 by Mihara

The X-ray nova with a blackhole, MAXI J1535-571, decayed under MAXI detection last week (2018 April 16, MJD 58224). ( ATel #11568). It appeared on September 2, 2017 It reached 5 Crab on September 19 and shone as the brightest X-ray nova in the 21st century. MAXI had monitored the X-ray flux for 7 months after the discovery ( MAXI light curve of MAXI J1535-571). It started to decay more rapidly around March 26, 2018 when the flux went down to 1 % of the peak. Then the peak in late September is supposed to be the Eddington luminosity. The distance can be calculated about 30000 light years.

MAXI operation has been extended for another 3 years (2018.4 - 2021.3.) by JAXA. MAXI will find more and more X-ray novas.

MAXI-BAT light curves of MAXI J1535-571

Taken from
A distance of 11 kpc was assumed for flux-luminosity conversion.

IGR J17379-3747 was a ms pulsar

news-1208 April 11, 2018 by Mihara

MAXI detected a postive flux (37 mCrab) from the gamma-ray source IGR J17379-3747 on March 19 2018 ( ATel #11447).
This source was discovered at the outburst in 2004 by the INTEGRAL satellite. The system was turned out to be a binary of a neutron star and a low-mass star. It had an outburst again in 2008, when Swift/XRT determined the accurate position ( ATel #1714). At the MAXI detection of the 3rd outburst this time, VLA radio observatory poionted the position and detected a radio flux on March 22 ( ATel #11487). It means that a compact jet was emitted just like a ms pulsar.
Follow-up observation by NICER on ISS which has a good timing capability in fact detected an X-ray pulsation of 2.1 ms (468 rotations per second !) on March 29 ( ATel #11507). NICER also measured the orbital period, which is only 1.88 hours, and then the companion star weighs only 0.06 solar mass (or somewhat heavier).
Thus MAXI's detection triggers many observations, which in cooperation reveal a secret of the nature.

ATel : Astronomer's Telegram

X-ray nova MAXI J1820+070

news-1207 March 27, 2018 by Mihara

The X-ray nova MAXI J1820+070 is now the second brightest star in the X-ray sky. MAXI J1820+070 was discovered by the Japanese payload MAXI on ISS on March 11 2018 ( ATel #11399). J1820+070 means the location (alpha, delta) = (18h 20m, +7.0deg) in J2000 coordinate. It is in Ophiuchus. MAXI is monitoring the flux in every ISS orbit (~90minutes). At first it was only 30 mCrab. It gradually brightened up and reached 2 Crab on March 21. Since then in this week, it has stayed in almost the same brightness till today (March 27). Many follow up observations are carried out by Swift, NICER on ISS (USA), XMM-Newton, Integral (ESA), HXMT (China), as well as many optical and radio observatories. It is revealed to be a binary of a blackhole and a normal star.

Crab is the unit of X-ray flux. 1 Crab is the flux from the Crab nebula.

Latest MAXI light curve of MAXI J1820+070

MAXI J1820+070 on 2018 3 23.

Shift of onset orbital-phase of the normal outburst in GRO J1008-57

news-1206 January 30th, 2018 by Nakajima and Mihara

MAXI/GSC detected X-ray birghtening from the Be/X-ray binary pulsar GRO J1008-57 from 2018 January 23 (MJD 58141) ( ATel #11244 ). The onset of this outburst occurred at the orbital phase of 0.90, using the the orbital parameters, T0=54416.65 (MJD) and Porb=249.48 d (Kühnel et al. 2013).
In 2009-2015 season, MAXI/GSC observations showed that the onset phase of the normal outbursts was in a narrow range (0.92-0.93) in orbital phase. However, the last 4 normal outbursts started from the orbital phase of 0.91. The shift of the outburst onset might imply that the geometrical parameters of the circumstellar disc around the Be star have changed.

Folded light curves of outbursts in GRO J1008-57.

Latest MAXI light curve of GRO J1008-57

Brightest blackhole nova ever in MAXI history

news-1205 September 29th, 2017 by Tatehiro Mihara

MAXI discovered the X-ray nova MAXI J1535-571 in constellation Norma at 2017 September 2nd 23:55 UT (57998 MJD) ( ATel #10699 ). In 4 hour prior to it (20:00 UT) Swift/BAT discovered it independently ( GCN #21788 ). More 5 hours prior to it (14:41 UT) MAXI nova-search program had detected it automatically.
Further observation with MAXI revealed that it is a black hole binary ( ATel #10708 ). The brightness of MAXI J1535-571 kept rising and finally reached 5 Crab on September 19th (58015 MJD). It has become the brightest X-ray nova ever in MAXI history. Appearance of an X-ray nova brighter than 3 Crab was for the first time in 18 years since 1999. In other words, MAXI J1535-571 is the brightest X-ray nova in this century. In fact, it is the seventh brightest blackhole X-ray nova in history [ Top list of Blackhole nova].

MAXI all-sky map on 2017 September 19th

MAXI light curve of MAXI J1535-571

Latest MAXI light curve of MAXI J1535-571

Announcement of MAXI 7 years international conference (Dec5-7)

news-1204 September 12th, 2016 by Tatehiro Mihara

We will hold an international conference,allsky6y_ad

7 years of MAXI : Monitoring X-ray Transients

on 5-7 December, 2016

at Suzuki Umetaro Hall, RIKEN
2-1 Hirosawa, Wako, Saitama, 351-0198, Japan

It is just three months ahead !
Program is available and Registration page is open at

We call for contributed talks/posters.
The deadline of registration is 30 September 2016.
Please join us.

MAXI was honored for Innovation in Earth and Space Science in 2016 ISS R&D conference

news-1203 July 29th, 2016 by Tatehiro Mihara

On July14, 2016, MAXI instrument was awarded for b01Innovation in Earth and Space Scienceb01 in the b012016 ISS Research and Development conferenceb01 held in San Diego, USA. Organizers of the conference, which are American Astronautical Society (AAS), NASA and CASIS, select a top research in each of seven categories in the ISS R&D every year.

Commemorative shield

Commemorative shield

Award ceremony. Center is Tatehiro Mihara on behalf of the MAXI team.

Award ceremony. Center is Tatehiro Mihara on behalf of the MAXI team.

MAXI has been observing the X-ray sky for seven years since August, 2009. MAXI team issued 350 Astronomerb01s telegrams noticing variability in the X-ray celestial bodies to the world-wide astronomers. MAXI data is open from MAXI homepage in Riken, which contains light curves and spectra of blackhole binaries and X-ray binary pulsars, etc. They are used in more than 150 refereed papers in the high energy astronomy.
In February, 2016, the first detection of gravitational wave (GW) was reported by LIGO team. MAXI scanned the GW source region in 4 b01 30 minutes after the event and obtained an upper limits for an X-ray emission. Next GW event may occur in the field of view of MAXI. Combination of MAXI with new instruments will make MAXI more valuable. They are CALET (high energy electron and gamma-ray detector) on JEM and NICER (X-ray telescope on ISS coming up in early 2017).

NASA News (2016.7.15)c01 (hardcopy of MAXI part)

JAXA ISS news (2016.7.15) (in Japanese)

RIKEN topics (2016.7.22) (in Japanese)

MAXI presentation on ISS R&D conference (pdf 18MB) (2016.7.13)

Suzaku & MAXI special issue of PASJ (2016.6.1)

Orbital phase shift in a new type I outburst of the Be/X-ray binary EXO 2030+375

news-1202 July 25th, 2016 by Tatehiro Mihara

In the last four years, the Be/X-ray binary EXO 2030+375 has displayed an unusual behaviour: its X-ray activity decreased and type I outbursts were not detected at several periastron passages. At the same time, its pulse-period derivative changed from a constant spin up to almost zero (ATEL #8835). Optical observations revealed a large equivalent width of the H alpha line (ATEL #8927).

We report a shift of the outburst peak phase in the most recent (MJD 57589) type I X-ray outburst of EXO 2030+375. The peak flux of the latest outburst was ~30 mCrab on 2016 July 20 (MJD 57589) for both MAXI/GSC and Swift/BAT. This corresponds to an orbital phase of 0.015 (orbital ephemeris of Wilson et al. 2008) and represents a significant change from the almost constant value of ~0.13 for all the detected type I outbursts since 2007. The peak time changed from ~5 days after the periastron to ~0.7 days after the periastron.

In addition, the latest Fermi/GBM spin frequency measurement (2016 April 25) suggests that the source is now starting to spin down.

All these observations are strikingly similar to the events which occurred from January 1994 to October 1995. The X-ray flux decreased, leading to several undetected outbursts, followed by a slow transition to spin down. In October 1995, a sudden shift of the outburst peak time from ~5 days after the periastron to ~5 days before the periastron was observed (Wilson et al. 2002).

The first giant outburst was detected on MJD 46204 (1985 May). The peak could be 0-30 days earlier than this date. The orbital phase shift occurred at MJD 50040 (1995 October). The time between these events is of 3836 ~ 3866 days. The peak of the second giant outburst was around MJD 53960 (2006 May), which is 3920 days after the phase shift. The time between the second giant outburst and the current phase shift is of 3629 days. All these time spans are similar.

We strongly encourage optical and infra-red observations to better understand the origin of this apparently recurrent events.

The recent outburst compared to the last two brightest outbursts is shown below.

The latest evolution of the outburst can be followed at the BeXRB monitor page: which combines data from MAXI/GSC, Swift/BAT and Fermi/GBM. This news has been posted to ATEL #9263.

GX 304-1 repeating normal outbursts by shifting onset phase

news-1201 January 27th, 2016 by Tatehiro Mihara

On 2016 January 24 (MJD 57411), the MAXI/GSC nova-search system detected an X-ray emission from the recurrent Be/X-ray transient pulsar, GX 304-1 (ATel #8592). The Swift/BAT also observed the brightening from January 23. The MAXI trigger time corresponds to the orbital phase of 0.019 (adopting Porb=132.189 d and To=55425.6 (MJD) ; Sugizaki et al. 2015). Thus the present event is a normal outburst.
Comparing it with the recent four normal outbursts, we found that their onsets steadily shifted later and later (from ~0.90 orbital phase on 2014 December to 0.02 on 2016 January). Such a shift has been observed from three other Be X-ray binaries (GS 0834-430 ; Wilson et al. 1997, EXO 2030+375 ; Wilson et al. 2002, A0535+26 ; Nakajima et al. 2014). Although not all of the Be X-ray binaries show the shift, the shift of the outburst phase might be a common phenomenon in the Be X-ray binaries and it might be based on some physical mechanism.
The MAXI 2-20keV light curves of the recent 6 orbits of GX 304-1 from the top to the bottom. The horizontal axis is the orbital phase. A steady shift of the onset phase is seen in the succeeding 4 outbursts.

NS-LMXB GS 1826-238 turns into a soft state

news-1200 June 20th, 2014 by Tatehiro Mihara

The neutron-star low-mass X-ray binary GS 1826-238 turned into a soft state on June 8, 2014. This source remained in the hard state since the discovery in 1988 (Cocchi et al, 2010, Ubertini et al. 1999, Barret et al. 1996, Tanaka 1989) including the recent five years of MAXI observation.
However, on 8 June 2014, the MAXI/GSC hardness ratio (7-20keV / 2-7 keV) showed a rapid drop from ~ 0.4 (previous level) into 0.1, which meant a softening. The 2-10 keV X-ray flux accordingly increased from 50 mCrab (before May 2014) to 140 mCrab (15 June 2014) (Fig.1). On the other hand, Swift/BAT light curve shows a decline in the hard X-ray band (15-50 keV) after 2014 June 8. The GSC spectra of the hard state and the soft state are shown in Fig.2. These results indicate that the source entered into a soft state for the first time.

Fig.1. Light curves with MAXI/GSC and Swift/BAT. The MAXI flux started to increase and the BAT flux started to decrease from MJD 56816 (2014/06/08). The two jumping points in MAXI at 56793 and 56815 MJD are X-ray bursts. The BAT light curve is taken from .

Fig.2. Spectra of the hard state (black, 56770- 56800 MJD) and the soft state (red, 56820 b01 56827 MJD) of GS 1826-238 with MAXI/GSC. The fit model is wabs*(diskbb+compbb). Now the disk-blackbody is dominabt in the soft energy band.
The GSC spectrum can be obtained through MAXI On-demand page .

GRB 140219A: MAXI/GSC detection

news-1199 February 26th, 2014 by Motoko Serino

The extremely bright GRB 140219A has been detected by MAXI/GSC. The position of the source was not in the field of view of any of GSC cameras. The photons came through counter walls, shields, or collimator sheets. Part of the signals might be produced by the Compton-scattered electrons induced by the gamma-rays.


The light curve of GRB 140219A observed by GSC_3 counter. The high resolution light curve around the peak is shown in the inset.

Pulse period of X-ray Binary Pulsar 4U 1626-67

news-1198 October 2nd, 2013 by Tatehiro Mihara

The pulse period (about 7.6 s) and change rate of the pulse period (about -2.6 x 10-11 s/s) of the X-ray Binary Pulsar 4U1626-67 are obtained with MAXI/GSC data.

The pulse period (red points in the figure, plotted in frequency) was obtained in each 60-day MAXI/GSC data from MJD 55110 up to 56249. The results were superposed with those with Fermi/GBM (black points). MAXI data are consistent with the Fermi/GBM data. Each red bar with the red point represents an average change-rate of the pulse period with MAXI in the 60 days. The pulse period is getting shorter and shorter (= spinning up) with alomost a constant change rate.


The next figure shows residuals from a linear fit to the Fermi/GBM data. Red points and black points represent MAXI and Fermi/GBM data, respectively. The vertical axis is in frequency. MAXI data points have an error bar, but it is small enough as is sometimes included in the size of the red points. The data is V-shaped, and there is a break in the middle. We found that the flux after this point was brighter than that before. We interprete that amount of gas accreting to the neutron star increased at this point to cause an increase in the spin-up rate.