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

Abstract

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.

Gamma-ray binaries beyond one-zone models: an application to LS 5039

“…First, one computes the particle population and the (absorbed) emission from an emitter placed in all the possible locations of the staremitter-observer plane. Once this is done, one can extract any relevant quantity for each location and display it in the form of a A112, page 2 of 13 two-dimensional map (see, e.g., Dubus 2006;Khangulyan et al 2008;Bosch-Ramon et al 2008b;Romero et al 2010, for similar maps of gamma-ray absorption and particle acceleration). We focus here on the total energy flux in the ranges of 0.3-10 keV (F X ), 1-30 MeV (F MeV ), 0.1-10 GeV (F GeV ), and 0.1-10 TeV (F TeV ); the luminosity injected in the relativistic particles (L inj ); and the minimum emitter radius normalized to the stellar distance (R/d).…”

Synergies in extragalactic and Galactic jet research

“…Since the winds are clumpy (e.g., Owocki et al 2009), jetcloud interactions are expected to occur frequently (Araudo et al 2009). Such interactions should result in gamma-ray variability as observed in sources such as Cygnus X-1 and Cygnus X-3 (e.g., Romero et al 2010, Araudo et al 2011, Piano et al 2012. The gammaray radiation can be the result of either hadronic inelastic collisions between relativistic protons in the jet and nuclei in the wind or inverse Compton (IC) up-scattering of UV and X-ray photons locally produced in the shocked cloud.…”

A new multiwavelength lepto-hadronic model of astrophysical jet emission

“…The thermal protons of the wind interact with the relativistic protons in the jets, producing γ-ray photons with energy above the GeV range. If the wind is clumpy, the rate of proton-proton interactions may increase temporarily, resulting in γ-ray flares (see e.g., [31,1,35,32]). However, our work focuses on the steady-state emission of an accreting black hole in a weakly accreting state, we thus choose to model the stellar wind of the companion star by a stationary spherically symmetric wind [19].…”