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Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE
Abstract:
Binary systems can be powerful sources of non-thermal emission from radio to
gamma rays. When the latter are detected, then these objects are known as
gamma-ray binaries. In this work, we explore, in the context of gamma-ray
binaries, different acceleration processes to estimate their efficiency: Fermi
I, Fermi II, shear acceleration, the converter mechanism, and magnetic
reconnection. We find that Fermi I acceleration in a mildly relativistic shock
can provide, although marginally, the multi-10 TeV particles required to
explain observations. Shear acceleration may be a complementary mechanism,
giving particles the final boost to reach such a high energies. Fermi II
acceleration may be too slow to account for the observed very high energy
photons, but may be suitable to explain extended low-energy emission. The
converter mechanism seems to require rather high Lorentz factors but cannot be
discarded a priori. Standard relativistic shock acceleration requires a highly
turbulent, weakly magnetized downstream medium; magnetic reconnection, by
itself possibly insufficient to reach very high energies, could perhaps
facilitate such a conditions. Further theoretical developments, and a better
source characterization, are needed to pinpoint the dominant acceleration
mechanism, which need not be one and the same in all sources.
