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Cygnus X-1 is entering its X-ray hard state

ATel #5995; Hui Zhang, Wenda Zhang and Wenfei Yu (Shanghai Astronomical Observatory
on 21 Mar 2014; 15:26 UT
Credential Certification: Wenfei Yu (wenfei@shao.ac.cn)

Subjects: X-ray, Binary, Black Hole

Referred to by ATel #: 6021, 6115, 6244, 6344, 7316

In the last few days, MAXI/GSC's monitoring data shows a decrease of the X-ray intensity of Cygnus X-1 from 0.678+/-0.011 Crab on MJD 56732 to 0.512+/-0.009 Crab on MJD 56736 in 2-10 keV. On the other hand, Swift/BAT data indicates an increase from 0.385+/-0.009 Crab on MJD56732 to 0.584+/-0.012 Crab on MJD 56736 in 15-50 keV. The hardness ratio between the Swift/BAT intensity (15-50 keV) and the MAXI/GSC intensity (2-10 keV) has increased from 0.57+/-0.02 on MJD 56732 to 1.14+/-0.03 on MJD 56736. According to the hardness ratio distribution of the Galactic X-ray binaries (see Yu & Yan 2009, ApJ, 701, 1940), the daily averaged hardness ratio of Cyg X-1 is now above 1.0, which indicates that it is entering the hard state. The X-ray light curves of Cygnus X-1 and the evolution of the hardness ratio are shown at

We encourage additional X-ray and multi-wavelength observations of Cygnus X-1 to cover the end of the transition back to the hard state and to monitor the emergence of the hard X-ray component and the possible disappearance of the soft disk component in the X-ray band. We thank Swift and MAXI team for making public of X-ray monitoring data.