Fermi GBM Observes Decrease in Hard X-rays from Cyg X-1

ATel #3636; G. L. Case (Louisiana State University); C. A. Wilson-Hodge (NASA/MSFC);A. Camero-Arranz (USRA); V. Chaplin (University of Alabama, Huntsville); M. L. Cherry (Louisiana State University); M. Finger (USRA); P. Jenke (NASA/MSFC) for the GBM Earth Occultation Team
on 8 Sep 2011; 02:28 UT
Credential Certification: Gary Case (case@phunds.phys.lsu.edu)

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Recent x-ray observations of Cyg X-1 by MAXI (ATel #3534) and RXTE (ATel #3616) have indicated that this source has again made the transition to the soft state. The GBM instrument on Fermi is being used to monitor the hard x-ray/low energy gamma-ray sky in the 8-500 keV energy range using the Earth occultation technique. We report that Cyg X-1 has been decreasing in hard x-rays, coincident with the rise in soft x-rays seen by MAXI and RXTE. In particular, the 3-day average 50-100 keV and 100-300 keV fluxes have decreased from 750 +/- 53 mCrab and 551 +/- 99 mCrab, respectively, on Aug 13-15 to essentially undetectable in either band on Sep. 1-3. Cyg X-1 had previously made a hard-to-soft state transition in July 2010 (ATel #2711, #2714, #2721). It remained in the soft state until April 2011 when it made the transition back to the hard state (ATel #3307). The GBM data shows that hard x-ray fluxes in the post-transition hard state had not fully recovered to their pre-transition levels before this latest transition to the soft state began. The average post-transition hard state fluxes in the 12-25, 25-50, 50-100, and 100-300 keV bands were 79%, 73%, 64%, and 52%, of the average pre-transition hard state fluxes, respectively. 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 http://heastro.phys.lsu.edu/gbm/image.php?sourceimage=CYGX-1.