Fermi GBM detects a rapid hard X-ray decline in Cyg X-1

ATel #2721; Colleen A. Wilson-Hodge (NASA/MSFC) & Gary Case (LSU) for the GBM Earth Occultation Team
on 3 Jul 2010; 02:30 UT
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Credential Certification: Colleen A. Wilson (Colleen.Wilson@nasa.gov)

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Recent GBM data show gradual spectral softening of the black hole candidate Cyg X-1 since 8 June 2010, consistent with the recent measurements by MAXI/GSC at soft x-ray energies (Atel #2711). Five day average 50-100 keV and 100-300 keV fluxes have decreased from 1040+/-50 and 920+/-100 mCrab, respectively, for 8-12 June 2010 to 760+/-40 and 470+/-80 mCrab, respectively, for 29 June - 1 July 2010. At the same time, the lower energy flux has increased, from 390+/-50 to 620+/-50 mCrab in the 8-12 keV band and 510+/-20 to 642+/-20 mCrab in the 12-25 keV band.

This behavior is consistent with a hard-to-soft state transition as suggested by lower-energy measurements from MAXI-GSC (Atel #2711) and RXTE-ASM (Atel #2714). We note that this relatively smooth decline in hard X-rays resembles the behavior seen in the 1996 soft state transition (Zhang et al, ApJ 477, L95, 1997). If this transition follows a similar trend, we should expect the hard X-ray flux to continue to decline over the next ~50 days. We will continue to monitor the hard x-ray flux with GBM through the state transition.