191 research outputs found
Non-thermal radiation from molecular clouds illuminated by cosmic rays from nearby supernova remnants
Molecular clouds are expected to emit non-thermal radiation due to cosmic ray
interactions in the dense magnetized gas. Such emission is amplified if a cloud
is located close to an accelerator of cosmic rays and if cosmic rays can leave
the accelerator and diffusively reach the cloud. We consider the situation in
which a molecular cloud is located in the proximity of a supernova remnant
which is accelerating cosmic rays and gradually releasing them into the
interstellar medium. We calculate the multiwavelength spectrum from radio to
gamma rays which emerges from the cloud as the result of cosmic ray
interactions. The total energy output is dominated by the gamma ray emission,
which can exceed the emission from other bands by an order of magnitude or
more. This suggests that some of the unidentified TeV sources detected so far,
with no obvious or very weak counterpart in other wavelengths, might be
associated with clouds illuminated by cosmic rays coming from a nearby source.Comment: 4 pages, 3 figures, proceedings of the "4th Heidelberg International
Symposium on High Energy Gamma-Ray Astronomy" July 7-11, 2008, Heidelberg,
German
Searching for galactic cosmic ray pevatrons with multi-TeV gamma rays and neutrinos
The recent HESS detections of supernova remnant shells in TeV gamma-rays
confirm the theoretical predictions that supernova remnants can operate as
powerful cosmic ray accelerators. If these objects are responsible for the bulk
of galactic cosmic rays, then they should accelerate protons and nuclei to
10^15 eV and beyond, i.e. act as cosmic PeVatrons. The model of diffusive shock
acceleration allows, under certain conditions, acceleration of particles to
such high energies and their gradual injection into the interstellar medium,
mainly during the Sedov phase of the remnant evolution. The most energetic
particles are released first, while particles of lower energies are more
effectively confined in the shell, and are released at later epochs. Thus the
spectrum of nonthermal paticles inside the shell extends to PeV energies only
during a relatively short period of the evolution of the remnant. For this
reason one may expect spectra of secondary gamma-rays and neutrinos extending
to energies beyond 10 TeV only from T \lesssim 1000 yr old supernova remnants.
On the other hand, if by a chance a massive gas cloud appears in the \lesssim
100 pc vicinity of the supernova remnant, ``delayed'' multi-TeV signals of
gamma-rays and neutrinos arise when the most energetic partices emerged from
the supernova shell reach the cloud. The detection of such delayed emission of
multi-TeV gamma-rays and neutrinos allows indirect identification of the
supernova remnant as a particle PeVatron.Comment: ApJ Letters, in press. Reference to recent MILAGRO results adde
Clues to unveil the emitter in LS 5039: powerful jets vs colliding winds
LS 5039 is among the most interesting VHE sources in the Galaxy. Two
scenarios have been put forward to explain the observed TeV radiation: jets vs
pulsar winds. The source has been detected during the superior conjunction of
the compact object, when very large gamma-ray opacities are expected. In
addition, electromagnetic cascades, which may make the system more transparent
to gamma-rays, are hardly efficient for realistic magnetic fields in massive
star surroundings. All this makes unlikely the standard pulsar scenario for LS
5039, in which the emitter is the region located between the star and the
compact object, where the opacities are the largest. Otherwise, a jet-like flow
can transport energy to regions where the photon-photon absorption is much
lower and the TeV radiation is not so severely absorbed.Comment: 3 pages, 3 Figures, contribution to the "Fourth Heidelberg
International Symposium on High-Energy Gamma-Ray Astronomy 2008
Simple analytical approximations for treatment of inverse Compton scattering of relativistic electrons in the black-body radiation field
The inverse Compton (IC) scattering of relativistic electrons is one of the
major gamma-ray production mechanisms in different environments. Often the
target photons for the IC scattering are dominated by black (or grey) body
radiation. In this case, the precise treatment of the characteristics of IC
radiation requires numerical integrations over the Planckian distribution.
Formally, analytical integrations are also possible but they result in series
of several special functions; this limits the efficiency of usage of these
expressions. The aim of this work is the derivation of approximate analytical
presentations which would provide adequate accuracy for the calculations of the
energy spectra of up-scattered radiation, the rate of electron energy losses,
and the mean energy of emitted photons. Such formulae have been obtained by
merging the analytical asymptotic limits. The coefficients in these expressions
are calculated via the least square fitting of the results of numerical
integrations. The simple analytical presentations, obtained for both the
isotropic and anisotropic target radiation fields, provide adequate (as good as
) accuracy for broad astrophysical applications.Comment: 16 pages, 11 figures, accepted for publication in Ap
Multiwavelength Signatures of Cosmic Ray Acceleration by Young Supernova Remnants
An overview is given of multiwavelength observations of young supernova
remnants, with a focus on the observational signatures of efficient cosmic ray
acceleration. Some of the effects that may be attributed to efficient cosmic
ray acceleration are the radial magnetic fields in young supernova remnants,
magnetic field amplification as determined with X-ray imaging spectroscopy,
evidence for large post-shock compression factors, and low plasma temperatures,
as measured with high resolution optical/UV/X-ray spectroscopy. Special
emphasis is given to spectroscopy of post-shock plasma's, which offers an
opportunity to directly measure the post-shock temperature. In the presence of
efficient cosmic ray acceleration the post-shock temperatures are expected to
be lower than according to standard equations for a strong shock. For a number
of supernova remnants this seems indeed to be the case.Comment: Invited review, to appear in the proceedings of "4th Heidelberg
International Symposium on High Energy Gamma-Ray Astronomy 2008
Implications of the VHE Gamma-Ray Detection of the Quasar 3C279
The MAGIC collaboration recently reported the detection of the quasar 3C279
at > 100 GeV gamma-ray energies. Here we present simultaneous optical (BVRI)
and X-ray (RXTE PCA) data from the day of the VHE detection and discuss the
implications of the snap-shot spectral energy distribution for jet models of
blazars. A one-zone synchrotron-self-Compton origin of the entire SED,
including the VHE gamma-ray emission can be ruled out. The VHE emission could,
in principle, be interpreted as Compton upscattering of external radiation
(e.g., from the broad-line regions). However, such an interpretation would
require either an unusually low magnetic field of B ~ 0.03 G or an
unrealistically high Doppler factor of Gamma ~ 140. In addition, such a model
fails to reproduce the observed X-ray flux. This as well as the lack of
correlated variability in the optical with the VHE gamma-ray emission and the
substantial gamma-gamma opacity of the BLR radiation field to VHE gamma-rays
suggests a multi-zone model. In particular, an SSC model with an emission
region far outside the BLR reproduces the simultaneous X-ray -- VHE gamma-ray
spectrum of 3C279. Alternatively, a hadronic model is capable of reproducing
the observed SED of 3C279 reasonably well. However, the hadronic model requires
a rather extreme jet power of L_j ~ 10^{49} erg s^{-1}, compared to a
requirement of L_j ~ 2 X 10^{47} erg s^{-1} for a multi-zone leptonic model.Comment: Accepted for pulication. Several clarifications and additions to the
manuscript to match the accepted versio
The Comparison of the Swift Gamma-Ray Bursts With and Without Measured Redshifts
Gamma-ray bursts, detected by the Swift satellite, are separated into two
samples: the bursts with and without determined redshifts. These two samples
are compared by the standard Student t-test and F-test. We have compared the
dispersions and the mean values of the durations, peak fluxes and fluences in
order to find any differences among these two samples. No essential differences
were found.Comment: Published in the Proceedings of the 4th Heidelberg International
Symposium on High Energy Gamma-Ray Astronomy, 200
Gamma-ray flares from red giant/jet interactions in AGN
Non-blazar AGN have been recently established as a class of gamma-ray
sources. M87, a nearby representative of this class, show fast TeV variability
on timescales of a few days. We suggest a scenario of flare gamma-ray emission
in non-blazar AGN based on a red giant interacting with the jet at the base. We
solve the hydrodynamical equations that describe the evolution of the envelope
of a red giant blown by the impact of the jet. If the red giant is at least
slightly tidally disrupted by the supermassive black hole, enough stellar
material will be blown by the jet, expanding quickly until a significant part
of the jet is shocked. This process can render suitable conditions for energy
dissipation and proton acceleration, which could explain the detected day-scale
TeV flares from M87 via proton-proton collisions. Since the produced radiation
would be unbeamed, such an events should be mostly detected from non-blazar
AGN. They may be frequent phenomena, detectable in the GeV-TeV range even up to
distances of Gpc for the most powerful jets. The counterparts at lower
energies are expected to be not too bright.} {M87, and nearby non-blazar AGN in
general, can be fast variable sources of gamma-rays through red giant/jet
interactions.Comment: 8 pages, 4 figure
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