512 research outputs found
Constraints on the cosmic ray diffusion coefficient in the W28 region from gamma-ray observations
GeV and TeV gamma rays have been detected from the supernova remnant W28 and
its surroundings. Such emission correlates quite well with the position of
dense and massive molecular clouds and thus it is often interpreted as the
result of hadronic cosmic ray interactions in the dense gas. Constraints on the
cosmic ray diffusion coefficient in the region can be obtained, under the
assumption that the cosmic rays responsible for the gamma ray emission have
been accelerated in the past at the supernova remnant shock, and subsequently
escaped in the surrounding medium. In this scenario, gamma ray observations can
be explained only if the diffusion coefficient in the region surrounding the
supernova remnant is significantly suppressed with respect to the average
galactic one.Comment: To appear in the proceedings of "Journ\'ees de la SF2A 2010"
Marseille 21-24 June 2010, 4 pages, 4 figure
Acceleration of cosmic rays and gamma-ray emission from supernova remnant/molecular cloud associations
The gamma-ray observations of molecular clouds associated with supernova
remnants are considered one of the most promising ways to search for a solution
of the problem of cosmic ray origin. Here we briefly review the status of the
field, with particular emphasis on the theoretical and phenomenological aspects
of the problem.Comment: Invited talk at SUGAR201
Acceleration of cosmic rays and gamma-ray emission from supernova remnants in the Galaxy
Galactic cosmic rays are believed to be accelerated at supernova remnant
shocks. Though very popular and robust, this conjecture still needs a
conclusive proof. The strongest support to this idea is probably the fact that
supernova remnants are observed in gamma-rays, which are indeed expected as the
result of the hadronic interactions between the cosmic rays accelerated at the
shock and the ambient gas. However, also leptonic processes can, in most cases,
explain the observed gamma-ray emission. This implies that the detections in
gamma rays do not necessarily mean that supernova remnants accelerate cosmic
ray protons. To overcome this degeneracy, the multi-wavelength emission (from
radio to gamma rays) from individual supernova remnants has been studied and in
a few cases it has been possible to ascribe the gamma-ray emission to one of
the two processes (hadronic or leptonic). Here we adopt a different approach
and, instead of a case-by-case study we aim for a population study and we
compute the number of supernova remnants which are expected to be seen in TeV
gamma rays above a given flux under the assumption that these objects indeed
are the sources of cosmic rays. The predictions found here match well with
current observational results, thus providing a novel consistency check for the
supernova remnant paradigm for the origin of galactic cosmic rays. Moreover,
hints are presented for the fact that particle spectra significantly steeper
than E^-2 are produced at supernova remnants. Finally, we expect that several
of the supernova remnants detected by H.E.S.S. in the survey of the galactic
plane should exhibit a gamma-ray emission dominated by hadronic processes (i.e.
neutral pion decay). The fraction of the detected remnants for which the
leptonic emission dominates over the hadronic one depends on the assumed values
of the physical parameters and can be as high as roughly a half.Comment: 14 pages, 4 figures, 4 tables, submitted to MNRA
On the plasma temperature in supernova remnants with cosmic-ray modified shocks
Context: Multiwavelength observations of supernova remnants can be explained
within the framework of the diffusive shock acceleration theory, which allows
effective conversion of the explosion energy into cosmic rays. Although the
models of nonlinear shocks describe reasonably well the nonthermal component of
emission, certain issues, including the heating of the thermal plasma and the
related X-ray emission, remain still open.
Aims: To discuss how the evolution and structure of supernova remnants is
affected by strong particle acceleration at the forward shock.
Methods: Analytical estimates combined with detailed discussion of the
physical processes.
Results: The overall dynamics is shown to be relatively insensitive to the
amount of particle acceleration, but the post-shock gas temperature can be
reduced to a relatively small multiple, even as small as six times, the ambient
temperature with a very weak dependence on the shock speed. This is in marked
contrast to pure gas models where the temperature is insensitive to the ambient
temperature and is determined by the square of the shock speed. It thus appears
to be possible to suppress effectively thermal X-ray emission from remnants by
strong particle acceleration. This might provide a clue for understanding the
lack of thermal X-rays from the TeV bright supernova remnant RX J1713.7-3946.Comment: Appendix A added, minor changes and additional references include
Non-linear diffusion of cosmic rays escaping from supernova remnants - I. The effect of neutrals
Supernova remnants are believed to be the main sources of galactic Cosmic
Rays (CR). Within this framework, particles are accelerated at supernova
remnant shocks and then released in the interstellar medium. The mechanism
through which CRs are released and the way in which they propagate still remain
open issues. The main difficulty is the high non-linearity of the problem: CRs
themselves excite the magnetic turbulence that confines them close to their
sources. We solve numerically the coupled differential equations describing the
evolution in space and time of the escaping particles and of the waves
generated through the CR streaming instability. The warm ionized and warm
neutral phases of the interstellar medium are considered. These phases occupy
the largest fraction of the disc volume, where most supernovae explode, and are
characterised by the significant presence of neutral particles. The friction
between those neutrals and ions results in a very effective wave damping
mechanism. It is found that streaming instability affects the propagation of
CRs even in the presence of ion-neutral friction. The diffusion coefficient can
be suppressed by more than a factor of over a region of few tens of pc
around the remnant. The suppression increases for smaller distances. The
propagation of GeV particles is affected for several tens of
kiloyears after escape, while TeV particles are affected for few
kiloyears. This might have a great impact on the interpretation of gamma-ray
observations of molecular clouds located in the vicinity of supernova remnants.Comment: Revised to match the version published in MNRA
The diffuse neutrino flux from the inner Galaxy: constraints from very high energy gamma-ray observations
Recently, the MILAGRO collaboration reported on the detection of a diffuse
multi-TeV emission from a region of the Galactic disk close to the inner
Galaxy. The emission is in excess of what is predicted by conventional models
for cosmic ray propagation, which are tuned to reproduce the spectrum of cosmic
rays observed locally. By assuming that the excess detected by MILAGRO is of
hadronic origin and that it is representative for the whole inner Galactic
region, we estimate the expected diffuse flux of neutrinos from a region of the
Galactic disk with coordinates . Our estimate has
to be considered as the maximal expected neutrino flux compatible with all the
available gamma ray data, since any leptonic contribution to the observed
gamma-ray emission would lower the neutrino flux. The diffuse flux of
neutrinos, if close to the maximum allowed level, may be detected by a
km--scale detector located in the northern hemisphere. A detection would
unambiguously reveal the hadronic origin of the diffuse gamma-ray emission.Comment: submitted to Astroparticle Physic
Gamma ray signatures of ultra high energy cosmic ray accelerators: electromagnetic cascade versus synchrotron radiation of secondary electrons
We discuss the possibility of observing ultra high energy cosmic ray sources inhigh energy gamma rays. Protons propagating away from their accelerators produce secondary electrons during interactions with cosmic microwave background photons. These electrons start an electromagnetic cascade that results in a broad band gamma ray emission. We show that in a magnetized Universe ( G) such emission is likely to be too extended to be detected above the diffusebackground. A more promising possibility comes from the detection of synchrotron photons from the extremely energetic secondary electrons. Although this emission is produced in a rather extended region of size , it is expected to be point-like and detectable at GeV energies if the intergalactic magnetic field is at the nanogauss level
CTA and cosmic-ray diffusion in molecular clouds
Molecular clouds act as primary targets for cosmic-ray interactions and are
expected to shine in gamma-rays as a by-product of these interactions. Indeed
several detected gamma-ray sources both in HE and VHE gamma-rays (HE: 100 MeV <
E 100 GeV) have been directly or indirectly associated with
molecular clouds. Information on the local diffusion coefficient and the local
cosmic-ray population can be deduced from the observed gamma-ray signals. In
this work we concentrate on the capability of the forthcoming Cherenkov
Telescope Array Observatory (CTA) to provide such measurements. We investigate
the expected emission from clouds hosting an accelerator, exploring the
parameter space for different modes of acceleration, age of the source, cloud
density profile, and cosmic ray diffusion coefficient. We present some of the
most interesting cases for CTA regarding this science topic. The simulated
gamma-ray fluxes depend strongly on the input parameters. In some cases, from
CTA data it will be possible to constrain both the properties of the
accelerator and the propagation mode of cosmic rays in the cloud.Comment: In Proceedings of the 2012 Heidelberg Symposium on High Energy
Gamma-Ray Astronomy. All CTA contributions at arXiv:1211.184
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