425 research outputs found
Non-thermal emission from standing relativistic shocks: an application to red giant winds interacting with AGN jets
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
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
Clumpy stellar winds and high-energy emission in high-mass binaries hosting a young pulsar
High-mass binaries hosting young pulsars can be powerful gamma-ray emitters.
The stellar wind of the massive star in the system is expected to be clumpy.
Since the high-energy emission comes from the pulsar-star wind interaction, the
presence of clumps can affect the spectrum and variability of this radiation.
We look for the main effects of the clumps on the two-wind interaction region
and on the non-thermal radiation. A simple analytical model for the two-wind
interaction dynamics was developed accounting for the lifetime of clumps under
the pulsar-wind impact. This time plays a very important role with regard to
the evolution of the clump, the magnetic field in the clump-pulsar wind
interaction region, and the non-radiative and radiative cooling of the
non-thermal particles. We also computed the high-energy emission produced at
the interaction of long-living clumps with the pulsar wind. For reasonable
parameters, the clumps will induce small variability on the X-ray and gamma-ray
radiation. Sporadically, large clumps can reach closer to the pulsar increasing
the magnetic field, triggering synchrotron X-ray flares and weakening other
emission components like inverse Compton. The reduction of the emitter size
induced by clumps also makes non-radiative losses faster. Stellar wind clumps
can also enhance instability development and matter entrainment in the shocked
pulsar wind when it leaves the binary. Growth limitations of the clumps from
the wind acceleration region may imply that a different origin for the largest
clumps is required. The large-scale wind structures behind the observed
discrete absorption components in the UV may be the source of these large
clumps. The presence of structure in the stellar wind can produce substantial
energy-dependent variability and should not be neglected when studying the
broadband emission from high-mass binaries hosting young pulsars.Comment: 8 pages, 4 figures, accepted for publication in Astronomy and
Astrophysics (minor corrections after proofs
Secondary emission behind the radio outflows in gamma-ray binaries?
Several binary systems consisting of a massive star and a compact object have
been detected above 100 GeV in the Galaxy. In most of these sources, gamma-rays
show a modulation associated to the orbital motion, which means that the
emitter should not be too far from the bright primary star. This implies that
gamma-ray absorption will be non negligible, and large amounts of secondary
electron-positron pairs will be created in the stellar surroundings. In this
work, we show that the radio emission from these pairs should be accounted for
when interpreting the radio spectrum, variability, and morphology found in
gamma-ray binaries. Relevant features of the secondary radio emission are the
relatively hard spectrum, the orbital motion of the radio peak center, and the
extended radio structure following a spiral-like trajectory. The impact of the
stellar wind free-free absorption should not be neglected.Comment: 6 pages, 4 figures / presented as a contributed talk in HEPRO II,
Buenos Aires, Argentina, October 26-30 2009 / accepted for publication in
Int. Jour. Mod. Phys.
Studying the interaction between microquasar jets and their environments
In high-mass microquasars (HMMQ), strong interactions between jets and
stellar winds at binary system scales could occur. In order to explore this
possibility, we have performed numerical 2-dimensional simulations of jets
crossing the dense stellar material to study how the jet will be affected by
these interactions. We find that the jet head generates strong shocks in the
wind. These shocks reduce the jet advance speed, and compress and heat up jet
and wind material. In addition, strong recollimation shocks can occur where
pressure balance between the jet side and the surrounding medium is reached.
All this, altogether with jet bending, could lead to the destruction of jets
with power . The conditions around the outflow shocks
would be convenient for accelerating particles up to TeV energies. These
accelerated particles could emit via synchrotron and inverse Compton (IC)
scattering if they were leptons, and via hadronic processes in case they were
hadrons.Comment: 4 pages. Contribution to the proceedings of High Energy Phenomena in
Relativistic Outflows, held in Dublin, Ireland, September 24-28, 200
Exploring Particle Acceleration in Gamma-Ray Binaries
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.Comment: 7 pages, 1 figure, proceedings of the 13th ICATPP Conference on
Astroparticle, Particle, Space Physics and Detectors for Physics Applications
(Villa Olmo, Como 3-7 October 2011
Formation of large-scale magnetic structures associated with the Fermi bubbles
The Fermi bubbles are part of a complex region of the Milky Way. This region
presents broadband extended non-thermal radiation, apparently coming from a
physical structure rooted in the Galactic Centre and with a partly-ordered
magnetic field threading it. We explore the possibility of an explosive origin
for the Fermi bubble region to explain its morphology, in particular that of
the large-scale magnetic fields, and provide context for the broadband
non-thermal radiation. We perform 3D magnetohydrodynamical simulations of an
explosion from a few million years ago that pushed and sheared a surrounding
magnetic loop, anchored in the molecular torus around the Galactic Centre. Our
results can explain the formation of the large-scale magnetic structure in the
Fermi bubble region. Consecutive explosive events may match better the
morphology of the region. Faster velocities at the top of the shocks than at
their sides may explain the hardening with distance from the Galactic Plane
found in the GeV emission. In the framework of our scenario, we estimate the
lifetime of the Fermi bubbles as yr, with a total energy injected
in the explosion(s) ergs. The broadband non-thermal radiation from
the region may be explained by leptonic emission, more extended in radio and
X-rays, and confined to the Fermi bubbles in gamma rays.Comment: 5 pages, 4 figures, accepted for A&
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