July 29th, 2016 by Tatehiro Mihara
On July14, 2016, MAXI instrument was awarded for “Innovation in Earth and Space Science” in the “2016 ISS Research and Development conference” held in San Diego, USA. Organizers of the conference, which are American Astronautical Society (AAS), NASA and CASIS, select a top research in each of seven categories in the ISS R&D every year.
Award ceremony. Center is Tatehiro Mihara on behalf of the MAXI team.
MAXI has been observing the X-ray sky for seven years since August, 2009. MAXI team issued 350 Astronomer’s telegrams noticing variability in the X-ray celestial bodies to the world-wide astronomers. MAXI data is open from MAXI homepage in Riken, which contains light curves and spectra of blackhole binaries and X-ray binary pulsars, etc. They are used in more than 150 refereed papers in the high energy astronomy.
In February, 2016, the first detection of gravitational wave (GW) was reported by LIGO team. MAXI scanned the GW source region in 4 – 30 minutes after the event and obtained an upper limits for an X-ray emission. Next GW event may occur in the field of view of MAXI. Combination of MAXI with new instruments will make MAXI more valuable. They are CALET (high energy electron and gamma-ray detector) on JEM and NICER (X-ray telescope on ISS coming up in early 2017).
NASA News (2016.7.15)。 (hardcopy of MAXI part)
JAXA ISS news (2016.7.15) (in Japanese)
RIKEN topics (2016.7.22) (in Japanese)
MAXI presentation on ISS R&D conference (pdf 18MB) (2016.7.13)
Suzaku & MAXI special issue of PASJ (2016.6.1)
July 25th, 2016 by Tatehiro Mihara
In the last four years, the Be/X-ray binary EXO 2030+375 has displayed an unusual behaviour: its X-ray activity decreased and type I outbursts were not detected at several periastron passages. At the same time, its pulse-period derivative changed from a constant spin up to almost zero (ATEL #8835). Optical observations revealed a large equivalent width of the H alpha line (ATEL #8927).
We report a shift of the outburst peak phase in the most recent (MJD 57589) type I X-ray outburst of EXO 2030+375. The peak flux of the latest outburst was ~30 mCrab on 2016 July 20 (MJD 57589) for both MAXI/GSC and Swift/BAT. This corresponds to an orbital phase of 0.015 (orbital ephemeris of Wilson et al. 2008) and represents a significant change from the almost constant value of ~0.13 for all the detected type I outbursts since 2007. The peak time changed from ~5 days after the periastron to ~0.7 days after the periastron.
In addition, the latest Fermi/GBM spin frequency measurement (2016 April 25) suggests that the source is now starting to spin down.
All these observations are strikingly similar to the events which occurred from January 1994 to October 1995. The X-ray flux decreased, leading to several undetected outbursts, followed by a slow transition to spin down. In October 1995, a sudden shift of the outburst peak time from ~5 days after the periastron to ~5 days before the periastron was observed (Wilson et al. 2002).
The first giant outburst was detected on MJD 46204 (1985 May). The peak could be 0-30 days earlier than this date. The orbital phase shift occurred at MJD 50040 (1995 October). The time between these events is of 3836 ~ 3866 days. The peak of the second giant outburst was around MJD 53960 (2006 May), which is 3920 days after the phase shift. The time between the second giant outburst and the current phase shift is of 3629 days. All these time spans are similar.
We strongly encourage optical and infra-red observations to better understand the origin of this apparently recurrent events.
The recent outburst compared to the last two brightest outbursts is shown below.
Recent type I outburst of EXO 2030+375 compared to the last two brightest outbursts. Orbit 1 is MJD 50086.967, which is the first periastron passage after the launch of RXTE. The current orbit is 164.
The latest evolution of the outburst can be followed at the BeXRB monitor page: http://integral.esac.esa.int/bexrbmonitor/ which combines data from MAXI/GSC, Swift/BAT and Fermi/GBM. This news has been posted to ATEL #9263.
January 27th, 2016 by Tatehiro Mihara
On 2016 January 24 (MJD 57411), the MAXI/GSC nova-search system detected an X-ray emission from the recurrent Be/X-ray transient pulsar, GX 304-1 (ATel #8592). The Swift/BAT also observed the brightening from January 23. The MAXI trigger time corresponds to the orbital phase of 0.019 (adopting Porb=132.189 d and To=55425.6 (MJD) ; Sugizaki et al. 2015). Thus the present event is a normal outburst.
Comparing it with the recent four normal outbursts, we found that their onsets steadily shifted later and later (from ~0.90 orbital phase on 2014 December to 0.02 on 2016 January). Such a shift has been observed from three other Be X-ray binaries (GS 0834-430 ; Wilson et al. 1997, EXO 2030+375 ; Wilson et al. 2002, A0535+26 ; Nakajima et al. 2014). Although not all of the Be X-ray binaries show the shift, the shift of the outburst phase might be a common phenomenon in the Be X-ray binaries and it might be based on some physical mechanism.
The MAXI 2-20keV light curves of the recent 6 orbits of GX 304-1 from the top to the bottom. The horizontal axis is the orbital phase. A steady shift of the onset phase is seen in the succeeding 4 outbursts.
June 20th, 2014 by Tatehiro Mihara
The neutron-star low-mass X-ray binary GS 1826-238 turned into a soft state on June 8, 2014. This source remained in the hard state since the discovery in 1988 (Cocchi et al, 2010, Ubertini et al. 1999, Barret et al. 1996, Tanaka 1989) including the recent five years of MAXI observation.
However, on 8 June 2014, the MAXI/GSC hardness ratio (7-20keV / 2-7 keV) showed a rapid drop from ~ 0.4 (previous level) into 0.1, which meant a softening. The 2-10 keV X-ray flux accordingly increased from 50 mCrab (before May 2014) to 140 mCrab (15 June 2014) (Fig.1). On the other hand, Swift/BAT light curve shows a decline in the hard X-ray band (15-50 keV) after 2014 June 8. The GSC spectra of the hard state and the soft state are shown in Fig.2. These results indicate that the source entered into a soft state for the first time.
Fig.1. Light curves with MAXI/GSC and Swift/BAT. The MAXI flux started to increase and the BAT flux started to decrease from MJD 56816 (2014/06/08). The two jumping points in MAXI at 56793 and 56815 MJD are X-ray bursts. The BAT light curve is taken from http://swift.gsfc.nasa.gov/results/transients/Ginga1826-238/ .
Fig.2. Spectra of the hard state (black, 56770- 56800 MJD) and the soft state (red, 56820 – 56827 MJD) of GS 1826-238 with MAXI/GSC. The fit model is wabs*(diskbb+compbb). Now the disk-blackbody is dominabt in the soft energy band.
The GSC spectrum can be obtained through MAXI On-demand page http://maxi.riken.jp/mxondem/ .
October 2nd, 2013 by Tatehiro Mihara
The pulse period (about 7.6 s) and change rate of the pulse period (about -2.6 x 10-11 s/s) of the X-ray Binary Pulsar 4U1626-67 are obtained with MAXI/GSC data.
The pulse period (red points in the figure, plotted in frequency) was obtained in each 60-day MAXI/GSC data from MJD 55110 up to 56249. The results were superposed with those with Fermi/GBM (black points). MAXI data are consistent with the Fermi/GBM data. Each red bar with the red point represents an average change-rate of the pulse period with MAXI in the 60 days. The pulse period is getting shorter and shorter (= spinning up) with alomost a constant change rate.
The next figure shows residuals from a linear fit to the Fermi/GBM data. Red points and black points represent MAXI and Fermi/GBM data, respectively. The vertical axis is in frequency. MAXI data points have an error bar, but it is small enough as is sometimes included in the size of the red points. The data is V-shaped, and there is a break in the middle. We found that the flux after this point was brighter than that before. We interprete that amount of gas accreting to the neutron star increased at this point to cause an increase in the spin-up rate.
July 31st, 2013 by Tatehiro Mihara
The gas gain of the GSC unit #0 counter started to increase suddenly on 2013 June 15 (56458 MJD) and reached 1.6 times higher than that before on July 24 (56497 MJD) (see figure). The counter had been operated in a reduced high-voltage mode (1550 V) to avoid breakdown. However, the gain became almost the same as that in 1650 V. We reduced the high voltage to 1484V on July 24 to keep the gas gain at the standard level in 1550V. We exclude data of the GSC unit #0 after 2013 June 15 from the archived products on our web page. They will be included later when the counter response becomes ready.
July 11th, 2013 by Tatehiro Mihara
On 2013 July 09 (MJD 56482), MAXI Nova-search system detected an X-ray flux increase from a Be/X-ray binary pulsar GX 304-1. The daily-averaged X-ray flux in 4-10 keV band reached 33 +- 19 mCrab on MJD 56481, and 57 +- 7 mCrab on MJD 56482. Using the orbital parameters ( Porb = 132.20 d; T0 = MJD 55554.75 ), the epoch when the brightening was detected (MJD 56482) corresponds to an orbital phase of ~0.02.
In prior to the current brightening, the source exhibited anomalous activities; a weak flux increase at ~0.7 orbital phase on 2013 May 29 and a multi-peaked weak outburst at the previous periastron passage (MJD 56349) (please see the figure).
According to the previous researches (Nakajima et al. 2012; Caballero et al. 2013), multi-peaked or double-peaked outbursts are often followed by a giant outburst. Theoretically, the giant outburst is explained by an interaction between the neutron star orbit and the warped Be-disc (Okazaki et al. 2013).
June 13th, 2013 by Tatehiro Mihara
The process version of GSC data was updated to the revision v1.4 in all the archived products of the 336 monitoring X-ray sources on May 13, 2013, one month ago. MAXI/GSC gas counters have been operated with a high voltage of either 1650V (initial nominal) or 1550V (for long life). We have performed the detector response calibration for the 1550V data and revised the analysis software to be able to handle both high-voltage data. The light-curve process version had been updated from v0.3, 0.5, 0.8 to v1.4. The version of the spectrum data was updated from v0.3b to v1.4s. In some emergent targets, light-curve data are updated every 4 hours through a special analysis procedure whose version is v1.4.rkn. We continue the instrument calibration to improve errors on the response function.
We have been correcting errors in the web page for a month. If you find some, please let us know through the contact tab in the left column of the front page.
January 24th, 2013 by Tatehiro Mihara
According to the large flux increase observed with MAXI/GSC during the giant outburst in 2012 November, the MAXI team requested Suzaku ToO observation. It was performed at the outburst peak from 2012-11-20 (MJD 56251) 14:44 to 2012-11-22 5:21 with an exposure of 50 ks. The MAXI/GSC flux was 400 +- 11 mCrab in 4-10 keV. The flux with Suzaku was 6.8 x 10-9 erg cm-2 s-1 in 2-10 keV, and 2.65 x 10-8 erg cm-2 s-1 in 1-100 keV. The luminosity was 8.0 x 1037 erg s-1 in 1-100 keV at the 5 kpc distance (Coe et al. 1994).
Our preliminary analysis of the Suzaku HXD/PIN and GSO data revealed that the spectrum can be reproduced by an NPEX with a cyclabs model with a reduced chi squared (dof) = 0.94 (127). The derived parameters of the cyclotron-line feature are E_cyc = 75.5 (+2.5, -1.5) keV, W = 4.0 (+5.4, -4.0) keV, and D = 1.6 (+9.4, -0.6) with errors of 90% confidence limits. The feature existed throughout the entire phase of the apparent 93.6257 s spin period. The E_cyc were 77.7, 74.7, 75.5, and 76.2 keV for the devided spin phases of rise, peak, fall, and valley of the pulse shape, respectively.
Suzaku spectrum does not show any signature of cyclotron-line feature in 20-50 keV range. Thus, the feature at 76 keV is considered as the fundamental cyclotron line. It corresponds to a magnetic field strength of 6.6 x 1012 G, which is the highest among binary X-ray pulsars ever observed.
The spectra with Suzaku PIN and GSO fitted with a NPEX and cyclabs model. The residuals in the bottom panel are without cyclabs model.
The pulse profile of GRO J1008-57. The pulse phases are divided into four phases : rise, peak, fall, and valley.