Episodic Transient Gamma-Ray Emission from the Microquasar Cygnus X-1
S. Sabatini, M. Tavani, E. Striani, A. Bulgarelli, V. Vittorini, G., Piano, E. Del Monte, M. Feroci, F. de Pasquale, M. Trifoglio, F. Gianotti, A., Argan, G. Barbiellini, P. Caraveo, P. W. Cattaneo, A. W. Chen, F. D'Ammando,, E. Costa, G. De Paris, G. Di Cocco, I. Donnarumma

TL;DR
This study reports a significant transient gamma-ray emission from Cygnus X-1 during a hard spectral state, revealing that short-term gamma-ray flares can occur in such states and challenging existing models.
Contribution
First detection of a transient gamma-ray flare from Cygnus X-1 during a hard state, providing new insights into high-energy processes in microquasars.
Findings
Detected a 1-2 day gamma-ray flare in October 2009
Established a spectral cutoff above 1 MeV in the hard state
Set an upper limit for persistent gamma-ray emission
Abstract
Cygnus X-1 is the archetypal black hole (BH) 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 or 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 Cygnus X-1 optical position. This episode, occurred during a hard spectral state of Cygnus 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…
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