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arXiv · 1303.0161

Gamma-ray observations of Cygnus X-1 above 100 MeV in the hard and soft states

S. Sabatini·M. Tavani·P. Coppi·G. Pooley·M. Del Santo·R. Campana·A. Chen·Y. Evangelista·G. Piano·A. Bulgarelli·P. W. Cattaneo·S. Colafrancesco·E. Del Monte·A. Giuliani·M. Giusti·F. Longo·A. Morselli·A. Pellizzoni·M. Pilia·E. Striani·M. Trifoglio·S. Vercellone
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Abstract

We present the results of multi-year gamma-ray observations by the AGILE satellite of the black hole binary system Cygnus X-1. In a previous investigation we focused on gamma-ray observations of Cygnus X-1 in the hard state during the period mid-2007/2009. Here we present the results of the gamma-ray monitoring of Cygnus X-1 during the period 2010/mid-2012 carried out for which includes a remarkably prolonged `soft state' phase (June 2010 -- May 2011). Previous 1--10 MeV observations of Cyg X-1 in this state hinted at a possible existence of a non-thermal particle component with substantial modifications of the Comptonized emission from the inner accretion disk. Our AGILE data, averaged over the mid-2010/mid-2011 soft state of Cygnus X-1, provide a significant upper limit for gamma-ray emission above 100 MeV of F_soft < 20 x 10^{-8} ph/cm^2/s, excluding the existence of prominent non-thermal emission above 100 MeV during the soft state of Cygnus X-1. We discuss theoretical implications of our findings in the context of high-energy emission models of black hole accretion. We also discuss possible gamma-ray flares detected by AGILE. In addition to a previously reported episode observed by AGILE in October 2009 during the hard state, we report a weak but important candidate for enhanced emission which occurred at the end of June 2010 (2010-06-30 10:00 - 2010-07-02 10:00 UT) exactly in coincidence with a hard-to-soft state transition and before an anomalous radio flare. An appendix summarizes all previous high-energy observations and possible detections of Cygnus X-1 above 1 MeV.

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S. Sabatini, M. Tavani, P. Coppi, G. Pooley, M. Del Santo, R. Campana, A. Chen, Y. Evangelista, G. Piano, A. Bulgarelli, P. W. Cattaneo, S. Colafrancesco, E. Del Monte, A. Giuliani, M. Giusti, F. Longo, A. Morselli, A. Pellizzoni, M. Pilia, E. Striani, M. Trifoglio, S. Vercellone. 2013-03-01. Gamma-ray observations of Cygnus X-1 above 100 MeV in the hard and soft states. https://doi.org/10.1088/0004-637x%2F766%2F2%2F83

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