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MAXI/GSC detection of a rapid X-ray spectral change in the black hole candidate 4U 1630-472

ATel #4075; S. Nakahira (JAXA), Y. Ueda,(Kyoto U.), K. Yamaoka (Waseda U.), H. Tomida, M. Ishikawa, S. Ueno (JAXA), T. Mihara, M. Sugizaki, M. Serino, T. Yamamoto, M. Matsuoka (RIKEN), H. Negoro, M. Nakajima, F. Suwa, M. Asada, H. Sakakibara, N. Serita (Nihon U.), N. Kawai M. Morii, R. Usui (Tokyo Tech), A. Yoshida (AGU), H. Tsunemi, M. Kimura (Osaka U.), K. Hiroi, M. Shidatsu, R. Sato (Kyoto U.), Y. Tsuboi, M. Higa (Chuo U.) M. Yamauchi, Y. Nishimura, T. Hanayama (Miyazaki U.) report on behalf of the MAXI team
on 29 Apr 2012; 13:03 UT
Credential Certification: Satoshi NAKAHIRA (nakahira@crab.riken.jp)

Subjects: X-ray, Binary, Black Hole, Transient

Referred to by ATel #: 4077, 4395, 4992

We report on a detection of an X-ray spectral change in the black hole candidate 4U 1630-472 with MAXI/GSC. The outburst(ATel#3830,#3945) began on 2011 December 17 (MJD 55912), and the flux peaked at 600 mCrab(2-20 keV) on 2011 December 30 (MJD 55925).
From 2012 January 14 (MJD 55940) until 2012 April 13 (MJD 56030), the source stayed in the soft state holding a similar flux around ~270 mCrab (2-20 keV). In the period, the spectrum was well fitted with a model of absorbed disk blackbody and a hard tail, which yields the hydrogen column density of 8 × 1022 cm-2 and the innermost temperature of 1.4-1.6 keV.
On 2012 April 28 (MJD 56045), MAXI/GSC detected an X-ray brightening in the 10-20 keV band. The Swift/BAT light curve also indicates an increase of the hard X-ray flux above 15 keV. We estimate that the power-law fraction in the 2-20 keV flux increased from ~10% (MJD 56020-56030) to >30% (MJD 56045) by assuming a fixed photon index of 2.5. This may indicate that the source is undergoing a state transition, possibly into the very high state. Follow-up observations are encouraged.
The latest source flux can be checked at the following page:
http://maxi.riken.jp/top/index.php?cid=1&jname=J1634-473