Episodic Transient Gamma-Ray Emission From the Microquasar Cygnus X-1

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 2σ upper limit for the total integrated flux of F γ,U.L. = 3 × 10 −8 ph cm −2 s −1 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.