31 research outputs found
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&
The role of supernovae inside AGN jets in UHECR acceleration
Jets of active galactic nuclei are potential accelerators of ultra
high-energy cosmic rays. Supernovae can occur inside these jets and contribute
to cosmic ray acceleration, particularly of heavy nuclei, but that contribution
has been hardly investigated so far. We carried out a first dedicated
exploration of the role of supernovae inside extragalactic jets in the
production of ultra high-energy cosmic rays. We characterized the energy budget
of supernova-jet interactions, and the maximum possible energies of the
particles accelerated in those events, likely dominated by heavy nuclei. This
allowed us to assess whether these interactions can be potential acceleration
sites of ultra high-energy cosmic rays, or at least of their seeds. For that,
we estimated the cosmic ray luminosity for different galaxy types, and compared
the injection rate of cosmic ray seeds into the jet with that due to galactic
cosmic ray entrainment. Since the supernova is fueled for a long time by the
luminosity of the jet, the energy of a supernova-jet interaction can be several
orders of magnitude greater than that of an isolated supernova. Thus, despite
the low rate of supernovae expected to occur in the jet, they could still
provide more seeds for accelerating ultra high-energy particles than cosmic ray
entrainment from the host galaxy. Moreover, these interactions can create
sufficiently efficient accelerators to be a source of cosmic rays with energies
~EeV. Supernova-jet interactions can contribute significantly to
the production of ultra high-energy cosmic rays, either directly by
accelerating these particles themselves or indirectly by providing
pre-accelerated seeds.Comment: 4 pages, Letter accepted for publication in Astronomy and
Astrophysics (in press
On the formation of TeV radiation in LS 5039
The recent detections of TeV gamma-rays from compact binary systems show that
relativistic outflows (jets or winds) are sites of effective acceleration of
particles up to multi-TeV energies. In this paper, we discuss the conditions of
acceleration and radiation of ultra-relativistic electrons in LS 5039, the
gamma-ray emitting binary system for which the highest quality TeV data are
available. Assuming that the gamma-ray emitter is a jet-like structure, we
performed detailed numerical calculations of the energy spectrum and
lightcurves accounting for the acceleration efficiency, the location of the
accelerator, the speed of the emitting flow, the inclination angle of the
system, as well as specific features related to anisotropic inverse Compton
scattering and pair production. We conclude that the accelerator should not be
deep inside the binary system unless we assume a very efficient acceleration
rate. We show that within the IC scenario both the gamma-ray spectrum and flux
are strongly orbital phase dependent. Formally, our model can reproduce, for
specific sets of parameter values, the energy spectrum of gamma-rays reported
by HESS for wide orbital phase intervals. However, the physical properties of
the source can be constrained only by observations capable of providing
detailed energy spectra for narrow orbital phase intervals ().Comment: 14 pages, 26 figures, accepted for publication in MNRAS, submitted on
July 11, 200
Gamma-ray emission from massive stars interacting with AGN jets
Dense populations of stars surround the nuclear regions of galaxies. In
active galactic nuclei, these stars can interact with the relativistic jets
launched by the supermasive black hole. In this work, we study the interaction
of early-type stars with relativistic jets in active galactic nuclei. A
bow-shaped double-shock structure is formed as a consequence of the interaction
of the jet and the stellar wind of each early-type star. Particles can be
accelerated up to relativistic energies in these shocks and emit high-energy
radiation. We compute, considering different stellar densities of the galactic
core, the gamma-ray emission produced by non-thermal radiative processes. This
radiation may be significant in some cases, and its detection might yield
valuable information on the properties of the stellar population in the galaxy
nucleus, as well as on the relativistic jet. This emission is expected to be
particularly relevant for nearby non-blazar sources.Comment: Accepted for publication on MNRAS (15 pages, 9 figures
Transient gamma-ray emission from Cygnus X-3
The high-mass microquasar Cygnus X-3 has been recently detected in a flaring
state by the gamma-ray satellites Fermi and Agile. In the present contribution,
we study the high-energy emission from Cygnus X-3 through a model based on the
interaction of clumps from the Wolf-Rayet wind with the jet. The clumps inside
the jet act as obstacles in which shocks are formed leading to particle
acceleration and non-thermal emission. We model the high energy emission
produced by the interaction of one clump with the jet and briefly discus the
possibility of many clumps interacting with the jet. From the characteristics
of the considered scenario, the produced emission could be flare-like due to
discontinuous clump penetration, with the GeV long-term activity explained by
changes in the wind properties.Comment: Contribution to the proceedings of the 25th Texas Symposium on
Relativistic Astrophysics - TEXAS 2010, December 06-10, Heidelberg, German
Gamma rays from cloud penetration at the base of AGN jets
Dense and cold clouds seem to populate the broad line region surrounding the
central black hole in AGNs. These clouds could interact with the AGN jet base
and this could have observational consequences. We want to study the gamma-ray
emission produced by these jet-cloud interactions, and explore under which
conditions this radiation would be detectable. We investigate the
hydrodynamical properties of jet-cloud interactions and the resulting shocks,
and develop a model to compute the spectral energy distribution of the emission
generated by the particles accelerated in these shocks. We discuss our model in
the context of radio-loud AGNs, with applications to two representative cases,
the low-luminous Centaurus A, and the powerful 3C 273. Some fraction of the jet
power can be channelled to gamma-rays, which would be likely dominated by
synchrotron self-Compton radiation, and show typical variability timescales
similar to the cloud lifetime within the jet, which is longer than several
hours. Many clouds can interact with the jet simultaneously leading to fluxes
significantly higher than in one interaction, but then variability will be
smoothed out. Jet-cloud interactions may produce detectable gamma-rays in
non-blazar AGNs, of transient nature in nearby low-luminous sources like Cen A,
and steady in the case of powerful objects of FR II type.Comment: Accepted for publication in A&A (9 pages, 7 figures
High-energy flares from jet-clump interactions
High-mass microquasars are binary systems composed by a massive star and a
compact object from which relativistic jets are launched. Regarding the
companion star, observational evidence supports the idea that winds of hot
stars are formed by clumps. Then, these inhomogeneities may interact with the
jets producing a flaring activity. In the present contribution we study the
interaction between a jet and a clump of the stellar wind in a high-mass
microquasar. This interaction produces a shock in the jet, where particles may
be accelerated up to relativistic energies. We calculate the spectral energy
distributions of the dominant non-thermal processes: synchrotron radiation,
inverse Compton scattering, and proton-proton collisions. Significant levels of
X- and gamma-ray emission are predicted, with luminosities in the different
domains up to ~ 10^{34} - 10^{35} erg/s on a timescale of about ~ 1 h. Finally,
jet-clump interactions in high-mass microquasars could be detectable at high
energies. These phenomena may be behind the fast TeV variability found in some
high-mass X-ray binary systems, such as Cygnus X-1, LS 5039 and LS I+61 303. In
addition, our model can help to derive information on the properties of jets
and clumpy winds.Comment: Proceeding of the conference "High Energy Phenomena in Massive
Stars". Jaen (Spain), 2-5 February 200