A black hole binary, XTE J1752-223, was discovered by the routine patrol observation of the central galactic area by the U.S.'s X-ray astronomical satellite RXTE in October 2009. The discovery was reported to ATEL (Astronomers' Telegram), a mailing list to report sudden astronomical events quickly. At the same time, MAXI also detected the emergence of this new X-ray nova and was able to observe all the activities until it disappeared in April 2010. Fig. 3 shows the evolution of its X-ray intensity and spectral hardness for about six months. Spectral hardness means the intensity ratio of high- and low-energy X rays, corresponding to "color" in visible light.
In the first half of activity, it was in the "hard" state and its intensity changed in two stages. We presume that this is the state where the gas inflow rate was relatively small and X rays were emitted mainly from thin high-temperature gas that has swelled around the black hole. After that, the spectrum rapidly changed to the "soft state" and X-ray intensity reached its peak. For about two months from the peak, it darkened linearly. We believe that, in this stage, a low-temperature disk was formed close to the black hole. After that, XTE J1752-223 made transition to the "hard state" again and disappeared.
There are three remarkable points concerning the activity of XTE J1752-223.
(1) It is rare for an X-ray nova to stay in the hard state for three months after its emergence. We issued a press release stating the "discovery of a gentle black hole." This provided us with new data for elucidating the mechanism of material inflow to produce the "hard state."
(2) The release of jets was detected by radio observations during the transition from hard to soft states. The data will help solve the mystery of the jet-generation mechanism, which is commonly seen in various objects in the Universe.
(3) Spectrum analysis of the soft state revealed that, even though its intensity dropped continually, the innermost radius of the disk stayed almost constant. If XTE J1752-223 is located near the center of the Galaxy, this radius would be about 100km. Since the mass of the central object corresponding to this radius is about 10 times that of the Sun, this supports our assumption that this object is in fact a black hole.
In contrast to XTE J1752-223, the black hole binary MAXI J1659-152 showed state transition soon after its emergence. MAXI and the Swift satellite discovered the object almost simultaneously on September 25, 2010. MAXI J1659-152 was quickly reported by the Swift team as an extraordinary long-period gamma-ray burst (GRB) and was named GRB 100925A. As a result of analysis of MAXI data, however, it was found that the increase in X-ray brightness started the day before it was detected by Swift.
This source was also detected in visible light and observed by researchers and amateur astronomers around the world. Following MAXI's report, RXTE and Suzaku conducted follow-up observations and revealed that MAXI J1659-152 had the properties of a black hole binary. The data just after the discovery, which was obtained by MAXI and Swift, showed a "hard-state" spectrum. When RXTE and Suzaku observed three days later, however, the source had changed to the state in which the soft spectral component was already dominant, probably originating from the accretion disk. Interestingly enough, RXTE detected that its X-ray brightness is modulated every 2.4 hours and its optical counterpart also showed variation with the same period. We suspect that 2.4 hours is the binary period. A black hole binary with such a short orbital period has never been discovered before. As the X rays of MAXI J1659-152 became weak, its optical light decreased too. Since the Sun is close to it, we have delayed detailed observation of a companion star for several months. We expect that, if we can discover the distance to the binary system and type of companion star by detailed observations in X-ray and optical bands during the quiet phase in the future, the nature of MAXI J1659-152 will be revealed.
Other X-ray sources and future plans
In addition to these black hole binaries, MAXI has achieved many interesting observations including: detection of the largest flare from active galactic nuclei in X-ray observation history; discovery of a new binary X-ray pulsar, MAXI J1409-619; and detection of a number of intense star flares. As long as the ISS is operating, we will use MAXI to monitor the X-ray sky, which changes restlessly and violently.
The MAXI team comprises researchers and graduate students from JAXA, RIKEN, Tokyo Institute of Technology, Aoyama Gakuin University, Osaka University, Nihon University, Kyoto University, Chuo University, and Miyazaki University.