Large-scale flow dynamics and radiation in pulsar gamma-ray binaries

Abstract

Several gamma-ray binaries show extended X-ray emission that may be associated to interactions of an outflow with the medium. Some of these systems are, or may be, high-mass binaries harboring young nonaccreting pulsars, in which the stellar and the pulsar winds collide, generating a powerful outflow that should terminate at some point in the ambient medium. This work studies the evolution and termination, as well as the related radiation, of the shocked-wind flow generated in high-mass binaries hosting powerful pulsars. A characterization, based on previous numerical work, is given for the stellar/pulsar wind interaction. Then, an analytical study of the further evolution of the shocked flow and its dynamical impact on the surrounding medium is carried out. Finally, the expected nonthermal emission from the flow termination shock, likely the dominant emitting region, is calculated. The shocked wind structure, initially strongly asymmetric, becomes a quasi-spherical, supersonically expanding bubble, with its energy coming from the pulsar and mass from the stellar wind. This bubble eventually interacts with the environment on ~pc scales, producing a reverse and, sometimes, a forward shock. Nonthermal leptonic radiation can be efficient in the reverse shock. Radio emission is expected to be faint, whereas X-rays can easily reach detectable fluxes. Under very low magnetic fields and large nonthermal luminosities, gamma rays may also be significant. We conclude that the complexity of the stellar/pulsar wind interaction is likely to be smoothed out outside the binary system, where the wind-mixed flow accelerates and eventually terminates in a strong reverse shock. This shock may be behind the extended X-rays observed in some binary systems. For very powerful pulsars, part of the unshocked pulsar wind may directly interact with the large-scale environment.