Optical depths for gamma-rays in the radiation field of a star heated by external X-ray source in LMXBs: Application to Her X-1 and Sco X-1

TL;DR

This paper investigates gamma-ray optical depths in low mass X-ray binaries with heated stars, showing these systems can produce high-energy gamma-rays similar to high mass binaries, with implications for gamma-ray observations.

Contribution

It introduces calculations of gamma-ray optical depths in irradiated low mass X-ray binaries and compares their gamma-ray production potential to high mass systems.

Findings

Low mass X-ray binaries can be gamma-ray sources.

Gamma-ray optical depths depend on binary phase.

High-energy electrons can efficiently produce gamma-rays in these systems.

Abstract

The surface of a low mass star inside a compact low mass X-ray binary system (LMXB) can be heated by the external X-ray source which may appear due to the accretion process 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 wonder 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 for the gamma-ray photons in the radiation field of such 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 interaction of electrons and gamma-rays are quite similar to these ones observed within the high mass TeV gamma-ray binaries such as LS 5039 and LSI 303 +61. However, due to differences in the soft radiation field, the expected gamma-ray light curves can significantly differ between low mass and high mass X-ray binaries. As an example, we apply such 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 find enough soft photon target from the companion star for efficient gamma-ray production.