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Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE
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.
