Optical depths for gamma-rays in the radiation field of a star heated by an external X-ray source in LMXBs

Application to Hercules X-1 and Scorpius X-1

Department of Astrophysics, University of Łódź, 90-236 Łódź, ul. Pomorska 149/153, Poland e-mail: bednar@astro.phys.uni.lodz.pl

Received: 1 October 2009
Accepted: 24 January 2010

Abstract

The surface of a low mass star inside a compact low mass X-ray binary system (LMXB) can be heated by an external X-ray source apparently caused by the accretion of mass onto a companion compact object (a neutron star or a black hole). As a result, the surface temperature of the star can become significantly higher than it is in the normal state resulting from thermonuclear burning. We investigate whether high energy electrons and gamma-rays, injected within the binary system, can efficiently interact with this enhanced radiation field. To decide this, we calculate the optical depths of the gamma-ray photons in the radiation field of such an irradiated star as a function of the phase of the binary system. Based on these calculations, we conclude that compact low mass X-ray binary systems may also become sources of high energy gamma-rays since conditions for the interaction of electrons and γ-rays are quite similar to those found within high mass TeV γ-ray binaries such as LS 5039 and LSI 303 +61. However, because of differences in the soft radiation field, the expected γ-ray light curves can differ significantly between low mass and high mass X-ray binaries. As an example, we apply these calculations to two well known LMXBs Her X-1 and Sco X-1. It is concluded that electrons accelerated to high energies inside these binaries should collide with a sufficient number of soft photon targets from the companion star to achieve efficient γ-ray production.