MAXI/GSC detects a spectral softening of MAXI J1836-194

ATel #3626; S. Nakahira (RIKEN), H. Negoro (Nihon U.), R. Usui (Tokyo Tech), T. Mihara, M. Sugizaki, M. Serino, T. Yamamoto, T. Sootome, M. Matsuoka (RIKEN), S. Ueno, H. Tomida, M. Kohama, M. Ishikawa (JAXA), N. Kawai, M. Morii, Sugimori, T. Toizumi (Tokyo Tech), A. Yoshida, K. Yamaoka (AGU), H. Tsunemi, M. Kimura, H. Kitayama (Osaka U.), M. Nakajima, F. Suwa, M. Asada, H. Sakakibara (Nihon 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 2 Sep 2011; 18:25 UT
Credential Certification: Satoshi NAKAHIRA (nakahira@crab.riken.jp)

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MAXI/GSC detected a spectral softening of the recently discovered
MAXI J1836-194 (ATel #3611). The transition occurred around 12 UT
on August 31, 2011 (MJD=55804.5) after a slight decline in 2-4 keV intensity.

The hardness ratio of 4-10 keV to 2-4 keV bands changed
from 1.08 +/-0.20 before the transition (MJD=55803.25-55804.50)
to the followings:
0.64+/-0.19 (MJD=55805.0-55806.0)
0.40+/-0.17 (MJD=55806.0-55806.6).

Before the transition, the source showed a power-law energy spectrum
suggesting that the source was in the hard or hard intermediate state
(Kennea et al. ATel #3613, Strohmayer and Smith ATel #3618).

Swift/BAT data, however, do not show any decline in hard X-ray intensity.
(http://heasarc.gsfc.nasa.gov/docs/swift/results/transients/weak/MAXIJ1836-194/)
These results imply that the source has undergone a transition from the hard
intermediate state to the soft intermediate state.
These spectral evolution is often observed in black hole transients,
and strengthen the possibility that MAXI J1836-194 is a black hole binary
in addition to similarities to the hard intermediate state behaviors (ATel #3618).

The latest MAXI/GSC light curve and the image of this source are available at the following URL:
http://maxi.riken.jp/top/index.php?cid=1&jname=J1835-193

The latest light curve and the image of MAXI J1836-194