Galactic and extragalactic relativistic jets have rich environments that are
full of moving objects, such as stars and dense clumps. These objects can enter
into the jets and generate shocks and non-thermal emission. We characterize the
emitting properties of the downstream region of a standing shock formed due to
the interaction of a relativistic jet with an obstacle. We focus on the case of
red giants interacting with an extragalactic jet. We perform relativistic
axisymmetric hydrodynamical simulations of a relativistic jet meeting an
obstacle of very large inertia. The results are interpreted in the framework of
a red giant whose dense and slow wind interacts with the jet of an active
galactic nucleus. Assuming that particles are accelerated in the standing shock
generated in the jet as it impacts the red giant wind, we compute the
non-thermal particle distribution, the Doppler boosting enhancement, and the
non-thermal luminosity in gamma rays. The available non-thermal energy from
jet-obstacle interactions is potentially enhanced by a factor of ∼100
when accounting for the whole surface of the shock induced by the obstacle,
instead of just the obstacle section. The observer gamma-ray luminosity,
including the flow velocity and Doppler boosting effects, can be ~300(g/10)^2
times higher than when the emitting flow is assumed at rest and only the
obstacle section is considered, where g is the jet Lorentz factor. For a whole
population of red giants inside the jet of an AGN, the predicted persistent
gamma-ray luminosities may be potentially detectable for a jet pointing to the
observer. Obstacles interacting with relativistic outflows, for instance clouds
and populations of stars for extragalactic jets, or stellar wind
inhomogeneities in microquasar jets and in winds of pulsars in binaries, should
be taken into account when investigating the non-thermal emission from these
sources.Comment: 7 pages, 6 figures, version after proofs to appear in Astronomy &
Astrophysic
