Absorption of high-energy gamma rays in Cygnus X-3
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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- May 2011
- DOI:
- 10.1051/0004-6361/201116581
- arXiv:
- arXiv:1103.3875
- Bibcode:
- 2011A&A...529A.120C
- Keywords:
-
- radiation mechanisms: non-thermal;
- stars: individual: Cygnus X-3;
- gamma rays: general;
- X-rays: binaries;
- accretion;
- accretion disks;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 11 pages, 9 figures, accepted for publication in Astronomy and Astrophysics