Volume 529, May 2011
|Number of page(s)||11|
|Published online||15 April 2011|
Absorption of high-energy gamma rays in Cygnus X-3
UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de
Grenoble (IPAG) UMR 5274, 38041
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
Accepted: 15 March 2011
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.
Key words: radiation mechanisms: non-thermal / stars: individual: Cygnus X-3 / gamma rays: general / X-rays: binaries / accretion, accretion disks
© ESO, 2011
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