MAXI News

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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.

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 et.al (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.

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.

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.

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.

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.

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.

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

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 http://www.astronomerstelegram.org

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.

MAXI J1820+070 on 2018 3 23.

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 P_orb=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.

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