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

TL;DR

This study investigates how gamma rays are absorbed in Cygnus X-3, revealing that high-energy gamma rays are likely produced at significant distances from the compact object, with absorption patterns influenced by the accretion disk and stellar wind.

Contribution

It provides a detailed numerical analysis of gamma-ray absorption in Cygnus X-3, constraining the gamma-ray source location and ruling out coronal origin for the high-energy emission.

Findings

Gamma rays are significantly absorbed by soft X-rays from the accretion disk.

Absorption patterns are complex and anisotropic, affected by Thomson scattering.

High-energy gamma rays are likely produced at distances of 10^8-10^10 cm from the compact object.

Abstract

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. In this study, we examine quantitatively the effect of pair production in Cygnus X-3 and put constraints on the location of the gamma-ray source. 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, 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. 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 due to 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. The lack of absorption feature in the GeV emission indicates that high-energy gamma rays should be located at a minimum distance ~10^8-10^10 cm from the compact object. The gamma-ray emission is unlikely to have a coronal origin.