MAXI/GSC detected a possible hard-to-soft state transition in Cygnus X-1

ATel #3534; H. Negoro, M. Asada, H. Sakakibara, M. Nakajima, F. Suwa (Nihon U.), N. Kawai, M. Morii, K. Sugimori, R. Usui, T. Toizumi (Tokyo Tech), T. Mihara, M. Sugizaki, M. Serino, S. Nakahira, T. Yamamoto, T. Sootome, M. Matsuoka (RIKEN), S. Ueno, H. Tomida, M. Kohama, M. Ishikawa (JAXA), A. Yoshida, K. Yamaoka (AGU), H. Tsunemi, M. Kimura, H. Kitayama (Osaka U.), Y. Ueda, K. Hiroi, M. Shidatsu (Kyoto U.), Y. Tsuboi, T. Matsumura, K. Yamazaki (Chuo U.) report on behalf of the MAXI team
on 8 Aug 2011; 10:02 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

Tweet

MAXI/GSC detected a possible hard-to-soft state transition of Cyg X-1, which had been in the hard state since the end of April, 2011 (Grinberg et al. ATel #3307).
In the last about 10 days, Cyg X-1 showed large X-ray flares with amplitudes of about 1 Crab and with little spectral changes on timescales of days in the MAXI/GSC 2-20 keV light curves. (http://maxi.riken.jp/top/index.php?cid=000000000001&star_dir_id=00000000424) The variation was also seen in the Swift BAT 15-50 keV light curve. (http://heasarc.gsfc.nasa.gov/docs/swift/results/transients/CygX-1/)
Starting on August 7, 2011 (MJD 55780) MAXI/GSC data show a significant spectral softening with a rapid X-ray brightening. For instance, the source flux reached to 1.8+-0.1 Crab in the 2-4 keV band during a scan transit at 00:17 on August 8 (UT), which is almost the same peak level of the previous outburst in 2010 (ATel #2711, #2715, #2721, #2724).
Preliminary spectral analysis of MAXI/GSC data shows that the time averaged spectrum from 05:51 on August 7 to 04:53 on August 8 can be represented with an absorbed power-law with a photon index of about 2.7, implying that the source is currently in the very high (steep power-law) state. Observations at other wavelengths, in particular gamma-ray and radio observations, are encouraged.

MAXI News