Gamma-Ray Observations of Cygnus X-1 above 100 MeV in the Hard and Soft States
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 which includes a remarkably prolonged "soft state" phase (2010 June-2011 May). 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 × 10-8 photons cm-2 s-1 , 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 2009 October during the hard state, we report a weak but important candidate for enhanced emission which occurred at the end of 2010 June (2010 June 30 10:00-2010 July 2 10:00 UT) exactly coinciding 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- April 2013
- DOI:
- 10.1088/0004-637X/766/2/83
- arXiv:
- arXiv:1303.0161
- Bibcode:
- 2013ApJ...766...83S
- Keywords:
-
- black hole physics;
- gamma rays: general;
- stars: flare;
- stars: individual: Cygnus X-1;
- stars: winds;
- outflows;
- X-rays: binaries;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 16 pages, 12 figures, 1 table, accepted for publication in ApJ