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Bright soft state and a soft-to-intermediate state transition of the new X-ray transient Swift J1910.2-0546/MAXI J1910-057 observed by MAXI

ATel #4198; M. Kimura (Osaka U.), H. Tomida, S. Nakahira (JAXA), H. Negoro (Nihon U.), S. Ueno, M. Ishikawa (JAXA), T. Mihara, M. Serino, M. Sugizaki, T. Yamamoto, J. Sugimoto, M. Matsuoka (RIKEN), N. Kawai, M. Morii, R. Usui, K. Ishikawa (Tokyo Tech), M. Nakajima, M. Asada, H. Sakakibara, N. Serita (Nihon U.), A. Yoshida (AGU), H. Tsunemi (Osaka U.), Y. Ueda, K. Hiroi, M. Shidatsu, R. Sato (Kyoto U.), M. Yamauchi, Y. Nishimura, T. Hanayama, K. Yoshidome (Miyazaki U.), Y. Tsuboi, M. Higa (Chuo U.), K. Yamaoka (Waseda U.) report on behalf of the MAXI team
on 21 Jun 2012; 15:00 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

Subjects: X-ray, Black Hole, Transient

Referred to by ATel #: 4210

The new X-ray transient Swift J1910.2-0546/MAXI J1910-057 (Krimm et al. 2012, ATel #4139; Usui et al. 2012, ATel #4140) has exhibited very bright soft-state behavior. MAXI/GSC light curves (http://maxi.riken.jp/top/index.php?cid=000000000001&jname=J1910-057) show that the 2-4 keV X-ray flux has reached a peak of about 2 Crab on 10-12 June 2012, and gradually decreased. An energy spectrum obtained with MAXI/SSC on 12-16 June 2012 is well described by a disk-blackbody model with an inner disk temperature of 0.65 +/- 0.01 keV and a hydrogen column density of 0.37 +/- 0.02 x 1021. A MAXI/GSC spectrum obtained on 12-14 June 2012 is also well fitted by a disk-blackbody model, and adding a power-law model does not improve the fit significantly. The spectra dominated by the thermal disk component are typical of the black hole candidate in the soft state. We also note that the column density is lower than that obtained with Swift/XRT on 4 June (Kennea et al. 2012, ATel #4149). While the 2-4 keV flux decreases since 16 June 2012, the 4-10 keV flux remains roughly constant and the 10-20 keV flux tends to increase, indicating that a soft-to-intermediate transition has taken place. No radio flux was detected on 12 June 2012 (Fogasy et al. 2012, ATel #4171) which is consistent with that the source was in the soft state. Further radio observation is encouraged during the transition.