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MAXI/GSC detection of a hard X-ray re-brightening from the BHC 4U 1630-472

ATel #4992; S. Nakahira, M. Ishikawa (JAXA), H. Negoro (Nihon U.), M. Shidatsu, T. Hori (Kyoto U.), T. Mihara, M. Sugizaki, M. Morii, M. Serino, T. Yamamoto, J. Sugimoto, T. Takagi, M. Matsuoka (RIKEN), S. Ueno, H. Tomida, M. Kimura (JAXA), N. Kawai, R. Usui, K. Ishikawa, T. Yoshii (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Nakano (AGU), H. Tsunemi, M. Sasaki (Osaka U.), K. Suzuki, M. Nakajima, K. Fukushima, T. Onodera, (Nihon U.), Y. Ueda, T. Kawamuro (Kyoto U.), Y. Tsuboi, M. Higa (Chuo U.), M. Yamauchi, K. Yoshidome, Y. Ogawa, H. Yamada (Miyazaki U.), K. Yamaoka (Nagoya U.) report on behalf of the MAXI team
on 19 Apr 2013; 03:44 UT
Credential Certification: Satoshi NAKAHIRA (nakahira@crab.riken.jp)

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

We report on a detection of an X-ray spectral change in the black hole candidate 4U 1630-472. A series of outbursts have been observed by MAXI/GSC since 2011 December (ATel#3830,#3945). During these period, the X-ray lightcurve showed three peaks around MJD 55925, MJD 56050(ATel#4075,4077) and 56190(ATel#4360). The second was accompanied by a prominent hard X-ray flare. Since MJD 56240, the source stayed in a roughly constant flux ~300 mCrab (2-20 keV) with a soft X-ray spectrum from the accretion disk.
We detected an X-ray brightening in the 10-20 keV band since MJD 56395 (2013-04-13 UT). The Swift/BAT light curve also indicates an increase of the hard X-ray flux above 15 keV.

We performed MAXI/GSC spectral analysis employing an absorbed disk-blackbody model with a We performed a spectral analysis of MAXI/GSC data with an absorbed disk-blackbody model modified by Comptonization (simpl; Steiner et al. 2009, PASP, 121, 1279). For the period 2013 March 29-April 6 (MJD 56380-56388) we obtain innermost disk-blackbody temperature Tin = 1.26 (±0.10) keV, scattered fraction Fsc = 0.05 (±0.02) and absorption-corrected flux Fx2-20 keV = 8.8×10-9 ergs cm-2 s-1, for April 13-17 (MJD 56395-56399) Tin = 1.28 (-0.09/+0.10) keV, Fsc = 0.14 (±0.03) and Fx2-20 keV = 1.1×10-8 ergs cm-2 s-1.
This indicates that the source is undergoing a state transition, probably into a very high state again. Follow-up observations are encouraged.
The latest source flux can be checked at the following page