Sabatini, S. ; Tavani, M. ; Striani, E. ; Giommi, P. ; Colafrancesco, S. ; Salotti, L.
Mar - 2010
ISSN : 2041-8205 ;
journal : The Astrophysical Journal

Issue : 1
type: Article Journal

Cygnus X-1 (Cyg X-1) is the archetypal black hole binary system in our Galaxy. We report the main results of an extensive search for transient gamma-ray emission from Cygnus X-1 carried out in the energy range 100 MeV-3 GeV by the AGILE satellite, during the period 2007 July-2009 October. The total exposure time is about 300 days, during which the source was in the hard X-ray spectral state. We divided the observing intervals in 2-4 week periods, and searched for transient and persistent emission. We report an episode of significant transient gamma-ray emission detected on 2009 October 16 in a position compatible with Cyg X-1 optical position. This episode, which occurred during a hard spectral state of Cyg X-1, shows that a 1-2 day time variable emission above 100 MeV can be produced during hard spectral states, having important theoretical implications for current Comptonization models for Cyg X-1 and other microquasars. Except for this one short timescale episode, no significant gamma-ray emission was detected by AGILE. By integrating all available data, we obtain a 2A upper limit for the total integrated flux of F ³,U.L. = 3 × 108 ph cm2 s1 in the energy range 100 MeV-3 GeV. We then clearly establish the existence of a spectral cutoff in the energy range 1-100 MeV that applies to the typical hard state outside the flaring period and that confirms the historically known spectral cutoff above 1 MeV.

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