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Fermi GBM observes another decrease in hard x-rays from Cyg X-1

ATel #3880; G. L. Case (LSU), C. A. Wilson-Hodge (NASA/MSFC), A. Camero-Arranz (IEEC-CSIC), V. Chaplin (UAH), M. L. Cherry (LSU), M. H. Finger (USRA), P. Jenke (NASA/MSFC) for the GBM Earth Occultation Team
on 24 Jan 2012; 17:44 UT
Credential Certification: Gary Case (case@phunds.phys.lsu.edu)

Subjects: X-ray, Gamma Ray, Binary, Black Hole

Referred to by ATel #: 6115

Cygnus X-1 has been quite active over the last ~2 years, undergoing two hard-to-soft state transitions. The GBM Earth Occultation Team has been using GBM to monitor Cyg X-1 in hard x-ray and low energy gamma-ray energies (8-1000 keV) during this time. After the most recent soft state transition (ATel #3534, #3616, #3636), GBM observed the Cyg X-1 hard x-ray fluxes to begin recover to the hard state beginning MJD 55890 (ATel #3802). Cyg X-1 remained in the hard state from about MJD 55909 until MJD 55938. Starting on MJD 55939 (January 13, 2012), GBM has observed the hard x-ray fluxes to steadily decrease from a 30-day average of 748 +/- 8 mCrab, 896 +/- 12 mCrab, 1040 +/- 17 mCrab, and 880 +\- 31 mCrab in the 12-25 keV, 25-50 keV, 50-100 keV, and 100-300 keV energy bands, respectively, to a one day average of 160 +\- 38 mCrab, 74 +\- 61 mCrab, 142 +\- 89 mCrab, and 236 +\- 166 mCrab in the same energy bands on MJD 55947 (January 21, 2012). This is indicative of another transition to the soft state. We will continue to monitor Cyg X-1 through the soft state in anticipation of its return to the hard state. The daily-updated GBM occultation light curve for Cyg X-1 can be found at the link below.

Daily updated GBM light curves for Cyg X-1