Gamma-ray absorption and the origin of the gamma-ray flare in Cygnus X-1

• Published in: Astronomy and astrophysics (Berlin) Vol. 518; no. 2; p. A12
• Main Authors: Romero, G. E., del Valle, M. V., Orellana, M.
• Format: Journal Article
• Language: English
• Published: Les Ulis EDP Sciences 01.07.2010
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; gamma-rays: general; outflows; radiation mechanisms: non-thermal; stars: winds; X-rays: binaries; Stellar winds; radiation mechanisms: non-thermal; Galaxies; X-rays: binaries; Black holes; Binary X ray source; Cosmic x-ray sources; Orbits; stars: winds, outflows; Radiation mechanism; gamma-rays: general; Gamma radiation; Inelastic collision; Stellar mass; X-ray binary stars; Pair creation; Cosmology; Jets; Models; Opacity; Energy absorption; Very high energy
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/200913938

Context. The high-mass microquasar Cyg X-1, the best-established candidate for a stellar-mass black hole in the Galaxy, has been detected in a flaring state at very high energies (VHE), E > 200 GeV, by the Atmospheric Cherenkov Telescope MAGIC. The flare occurred at orbital phase ϕ = 0.91, where ϕ = 1 is the configuration with the black hole behind the companion high-mass star, when the absorption of gamma-ray photons by photon-photon annihilation with the stellar field is expected to be highest. Aims. We aim to set up a model for the high-energy emission and absorption in Cyg X-1 that can explain the nature of the observed gamma-ray flare. Methods. We study the gamma-ray opacity due to pair creation along the whole orbit, and for different locations of the emitter. Then we consider a possible mechanism for the production of the VHE emission. Results. We present detailed calculations of the gamma-ray opacity and infer from these calculations the distance from the black hole where the emitting region was located. We suggest that the flare was the result of a jet-clump interaction where the decay products of inelastic p - p collisions dominate the VHE outcome. Conclusions. We are able to reproduce the spectrum of Cyg X-1 during the observed flare under reasonable assumptions. The flare may be the first event of jet-cloud interaction ever detected at such high energies.