FERMI GBM Observes Increase in Hard X-rays from Cyg X-1

ATel #3802; 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 9 Dec 2011; 18:42 UT
Credential Certification: Gary Case (case@phunds.phys.lsu.edu)

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Cyg X-1 has been in a soft state since MAXI saw a rapid brightening in the low energy x-rays on August 7 (ATel #3534). The transition to the soft state was verified by RXTE (ATel #3616) and FERMI/GBM (ATel #3636). Recently, FERMI/GBM observations using the Earth occultation technique showed an increase in the hard x-ray flux, beginning about November 25 (MJD 55890). The fluxes in the 12-25, 25-50, 50-100, and 100-300 keV bands, averaged over 25 days from MJD 55860-55885, have increased from 260 +/- 11, 183 +/- 16, 90 +/- 23, and 31 +/- 42 mCrab, respectively, to a 5-day average (MJD 55898-55902) of 671 +/- 20, 605 +/- 33, 492 +/- 47, and 395 +/- 89 mCrab, respectively. This would indicate that Cyg X-1 is making the transition back into the hard state. Unfortunately, Cyg X-1 is currently in a region of the sky that is unobservable for MAXI, and so no low energy x-ray monitoring data are available. Observations at other wavelengths are encouraged to verify that Cyg X-1 has returned to the hard state. The daily-updated FERMI/GBM light curves for Cyg X-1 are available at the link below.

Daily-updated FERMI/GBM Light Curves for Cyg X-1