Considering the importance of gamma-gamma absorption in gamma-ray binaries

Abstract

Gamma-ray binaries are a rare subclass of high mass binary systems, where the non-thermal emission peaks in the gamma-ray regime. Two scenarios have been proposed to explain the production of the emission; in the pulsar wind scenario the compact object is proposed to be a rapidly rotating pulsar, and the emission originates from particle acceleration that occurs at the shock that forms between the pulsar and stellar wind; in the microquasar scenario emission originates from a relativistic jet. In the pulsar wind scenario, the particle acceleration is normally assumed to occur at the apex of the shock, but hydrodynamic simulations have shown that a second shock could occur due to Coriolis forces. Since gamma-gamma absorption will strongly attenuate TeV emission originating from the apex of the shock, this may imply that it originates from this second shock region. We have undertaken a full calculation of the gamma-gamma optical depth around the gamma-ray binary LS 5039, to investigate whether gamma-gamma absorption may introduce observable features that could be used to constrain the location of the gamma-ray production.