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 MiharaMAXI 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 MiharaMAXI 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 MiharaMAXI 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 http://maxi.riken.jp/star_data/J1911+005/J1911+005.html 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 locating just 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.
http://www.ioffe.ru/LEA/GRBs/GRB190305_T47118/
MAXI's error circle and localization with IPN, and Swift/XRT.
Taken from GCN #23936 and the link in it
http://www.ioffe.ru/LEA/GRBs/GRB190305_T47118/IPN/
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 MiharaThe 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 MiharaMAXI 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 MiharaMAXI 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 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.
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 
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 http://www.astronomerstelegram.org
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.
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. 