Soft Gamma-ray burst GRB 211225A

news-1308 December 28, 2021 by Mihara

MAXI detected a gamma-ray burst GRB 211225A in the constellation Hydra at 03:22 UT on 2021 December 25 (GCN #31303). The intensity was 200 mCrab. There was no flux in the previous nor the next MAXI scan. The spectrum was soft, and probably therefore, Swift/BAT did not detect it. Such a soft X-ray transient is a suitable target for OHMAN.


X-ray flare from the flare star AU Mic

news-1307 December 23, 2021 by Mihara

MAXI detected a flare from AU Mic at 2021/12/15 22:48 UT with an intensity of 430 mCrab ( ATel #15139 ).
AU Mic is a "hot topic" flare star of 8th magnitude. It is very close (32 lt-yr), very young (24 million years old), still wears a dust disk (discovered in 2003) where planets were born, and has two planets (a Neptune size and an Earth size, just discovered in 2020 with TESS). The planets orbit around AU Mic in 8 and 18 days, respectively. AU Mic itself is a small star (80% of the sun size, M-type pre-main sequence) rotating fast in only 4.8 day. It would have two large sun spots on the surface. The star is vary active and causes flares frequently. The MAXI's X-ray flare was very large, 10^25 W, which is 1/10 of the whole emitting energy of the sun. The flare typically lasts for some thousands of seconds.


Two X-ray pulsars outbursting around Aquila

news-1306 December 16, 2021 by Mihara

MAXI detected a new outburst from 4U 1907+09 in Aquila on 2021 Nov 27 at 150 mCrab. It is an X-ray binary pulsar with the spin period of 438 s. Also MAXI detected a new outburst from XTE J1829-098 in Scutum (south neighbor of Aquila) on 2021 Dec 13 at 50 mCrab. It is also an X-ray binary pulsar of 7.8 s.
Both pulsars locate on the Galactic plane (galactic latitude is 0.5 deg and 0.3 deg, respectively) and both pulsars have a high-mass companion star (O-type supergiant, and B-type star, respectively). The companion star of XTE J1829-098 has not yet identified, but would be a Be star as MAXI has reported periodical activities probably due to the orbital motion. The activity of XTE J1829-098 were also reported on MAXI news #1261 2020 Aug 13 and MAXI news #1284 2021 Apr 16.
Stars are born on the Galactic plane. High-mass stars are young and have not traveled so much. They are on the plane. The pulsar in 4U 1907+09 would still maintain a strong magnetic fields, and the spin would have slowed down to 438 s.


Both 4U 1907+09 and XTE J1829-098 pulsars are on the Galactic plane. Both have a high-mass companion, and "just born". Other X-ray binaries in the figure (Ser X-1, H 1822-000, 4U 1957+115, Aql X-1) have a low-mass companion and not on the Galactic plane.

Long X-ray burst again from 4U 1820-30

news-1305 December 2, 2021 by Mihara

MAXI detected a long X-Ray Burst at 2021 November 25 23:24 UT from 4U 1820-30 ( ATel #15071 ). The intensity was 1.1 Crab. In the next scan, the intensity was 0.4 Crab and the burst was still lasting. The e-folding decay time is 1.1 hour, indicating a long burst. MAXI had just reported a long burst from this source in August, 2021 ( ATel #14871 ). It is in the globular cluster NGC 6624 in the constellation Sagittarius.
The globular cluster is an old system born in the early age of the universe. Heavier stars than the sun have already burnt out and have become a neutron star (NS) or a white dwarf (WD). Many WD are seen in the globular cluster with the Hubble telescope. Also, there are 6 radio pulsars (NS) and one X-ray binary 4U 1820-30 (NS-WD binary) in NGC 6624 ( Revnivtsev et al. 2002 ).

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. 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 separate the stars in the globular cluster, it can tell which star is the MAXI's outburst by detecting the pulsation. We asked for NuSTAR observation and it was approved.

NuSTAR detected 2.2 ms pulsation and confirmed it SAX J1748 ( ATel #15048 , ATel #15095 ).


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 60 Crab (200 Crab was a miscalculation), 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 it, an 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 1543 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 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)
https://ui.adsabs.harvard.edu/abs/2017symm.conf..121I/abstract

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.
http://integral.esac.esa.int/bexrbmonitor
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 the time in sec. The red line indicates the burst duration, which 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. The detector area was about 9 cm2. Those yields about 200 c/s/cm2.

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.