MAXI/GSC detection of a hard-to-soft state transition of XTE J1908+094

ATel #5549; H. Negoro, K. Suzuki (Nihon U.), J. Sugimoto (RIKEN) S. Ueno, H. Tomida, M. Kimura, S. Nakahira, M. Ishikawa, Y. E. Nakagawa (JAXA), T. Mihara, S. Sugizaki, M. Serino, M. Morii, T. Takagi, M. Matsuoka (RIKEN), N. Kawai, R. Usui, K. Ishikawa, T. Yoshii (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Nakano (AGU), H. Tsunemi, M. Sasaki (Osaka U.), M. Nakajima, H. Sakakibara, K. Fukushima, T. Onodera (Nihon U.), Y. Ueda, M. Shidatsu, T. Kawamuro, T. Hori (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 6 Nov 2013; 01:22 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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The black hole candidate XTE J1908+094 currently in outburst (Krimm et al. ATel #5523) had been in the hard state since the beginning of the outburst (Krimm et al. ATel #5529, Miller-Jones et al. #5530, Rushton et al. #5532).

MAXI/GSC data of XTE J1908+094 suffer from the contamination from the nearby, bright persistent emission source GRS 1915+105, especially at the beginning of the outburst. Recent GSC data, however, show clear spectral evolution of the source starting from 2013 November 2 (MJD 56598). Count ratios in the 4-10 keV to 2-4 keV energy bands, avoiding the contamination from GRS 1915+105 as much as possible, started to decrease from 1.0 +/- 0.1 on November 2, implying the state transition set in. We note that the hard X-ray count rate obtained with Swift/BAT also decreased rapidly on November 2 (http://swift.gsfc.nasa.gov/results/transients/weak/XTEJ1908p094/). A spectral softening accompanied by a rapid increase in 2-4 keV X-rays was observed in the MAXI band on November 5 and the count ratio went down to 0.41+/- 0.04, suggesting the source is entering soft state.

The public data archive of this source can be found at
http://maxi.riken.jp/top/index.php?cid=1&jname=J1908+093 .

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