MAXI/GSC detection of a transition back into the hard state in Cygnus X-1

ATel #6115; H. Negoro (Nihon U.), T. Mihara, M. Sugizaki, M. Morii, M. Serino, J. Sugimoto, T. Takagi, A. Yoshikawa, M. Matsuoka (RIKEN), S. Ueno, H. Tomida, S. Nakahira, M. Kimura, M. Ishikawa, Y. E. Nakagawa (JAXA), N. Kawai, T. Yoshii, Y. Tachibana (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Kawakubo, H. Ohtsuki (AGU), H. Tsunemi, D. Uchida (Osaka U.), M. Nakajima, K. Fukushima, T. Onodera, K. Suzuki, M. Fujita, T. Namba, F. Honda (Nihon U.), Y. Ueda, M. Shidatsu, T. Kawamuro, T. Hori (Kyoto U.), Y. Tsuboi, A. Kawagoe (Chuo U.), M. Yamauchi, Y. Morooka (Miyazaki U.), K. Yamaoka (Nagoya U.) report on behalf of the MAXI team
on 4 May 2014; 14:43 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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MAXI/GSC data of Cyg X-1 on 3 May 2014 (MJD 56780) showed that a 2-4 keV count rate was 0.62+/-0.03 c/s/cm^2, which was less than half count rates in the soft state and a typical one in the hard state, and hardness ratios in the 4-10 keV/2-4 keV bands and the 10-20 keV/2-4 keV bands were 0.81+/-0.05 and 0.28+/-0.03, respectively.

Average hardness ratios in the hard state obtained with the GSC, for instance between 15 August 2009 (MJD 55058) and 10 March 2010 (MJD 55320), were 0.93 (4-10 keV/2-4 keV) and 0.41 (10-20 keV/2-4 keV), while average ones in the soft state between 29 September 2012 (MJD 56199) and 19 June 2013 (MJD 56462) were 0.28 (4-10 keV/2-4 keV) and 0.04 (10-20 keV/2-4 keV), respectively. The hardness on 3 May are still slightly soft, but the hardest since 10 January 2012 (MJD 55936) when Cyg X-1 entered this soft state (Case et al. ATel #3880). These data imply that Cyg X-1 is entering the hard state though the energy spectrum still changes on timescales of hours.

Unfortunately, data between 11 April 2014 (MJD 56758) and 2 May 2014 (MJD 56779) have not been obtained with well-calibrated GSCs, which does not allow us to evaluate the transition date more quantitatively. Hardness ratios of Cyg X-1 have gradually increased for about 200 days, and once rapidly increased around 21 March 2014 (Zhang et al. ATel #5995), but decreased again (Grinberg et al. ATel #6021). This soft-to-hard state transition is likely to be the consequence of this long-term trend of the spectral hardening.

We encourage continuous multi-wavelength observations of Cyg X-1 which has exhibited a peculiar soft state for more than 2 years.

The public data of Cyg X-1 can be seen at the following URL:
http://maxi.riken.jp/top/index.php?cid=1&jname=J1958+352

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