Absorption of high-energy gamma rays in Cygnus X-3

¹ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
e-mail: benoit.cerutti@colorado.edu
² Center for Integrated Plasma Studies, Physics Department, University of Colorado, Boulder, CO 80309, USA
³ Université de Toulouse, UPS-OMP, IRAP, Toulouse, France
⁴ CNRS, IRAP, 9 Av. Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁵ Kavli Institute for Particle Astrophysics and Cosmology, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA
⁶ Astronomical Observatory, Jagiellonian University, Orla 171, 30-244 Kraków, Poland
⁷ Centrum Astronomiczne im. M. Kopernika, Bartycka 18, 00-716 Warszawa, Poland

Received: 25 January 2011
Accepted: 15 March 2011

Abstract

Context. The microquasar Cygnus X-3 was detected at high energies by the gamma-ray space telescopes AGILE and Fermi. The gamma-ray emission is transient, modulated with the orbital period and seems related to major radio flares, i.e. to the relativistic jet. The GeV gamma-ray flux can be substantially attenuated by internal absorption with the ambient X-rays.

Aims. We examine quantitatively the effect of pair production in Cygnus X-3 and put constraints on the location of the gamma-ray source.

Methods. Cygnus X-3 exhibits complex temporal and spectral patterns in X-rays. During gamma-ray flares, the X-ray emission can be approximated by a bright disk black-body component and a non-thermal tail extending in hard X-rays, which is possibly related to a corona above the disk. We calculate numerically the exact optical depth for gamma rays above a standard accretion disk. Emission and absorption in the corona are also investigated.

Results. GeV gamma rays are significantly absorbed by soft X-rays emitted from the inner parts of the accretion disk. The absorption pattern is complex and anisotropic. Isotropization of X-rays caused by Thomson scattering in the companion-star wind tends to increase the gamma-ray opacity. Gamma rays from the corona suffer from strong absorption by photons from the disk and cannot explain the observed high-energy emission, unless the corona is unrealistically extended.

Conclusions. The lack of an absorption feature in the GeV emission indicates that high-energy gamma rays should be located at a minimum distance  ~10⁸−10¹⁰ cm from the compact object. The gamma-ray emission is unlikely to have a coronal origin.