348 research outputs found
Cosmic Ray Spectrum Steepening in Supernova Remnants -- I. Loss-Free Self-Similar Solution
The direct measurements of cosmic rays (CRs), after correction for the
propagation effects in the interstellar medium, indicate that their source
spectra are likely to be significantly steeper than the canonical
spectrum predicted by the standard Diffusive Shock Acceleration (DSA)
mechanism. The DSA has long been held responsible for the production of
galactic CRs in supernova remnant (SNR) shocks. The -ray 'probes' of
the acceleration spectra of CRs on-the-spot, inside of the SNRs, lead to the
same conclusion. We show that the steep acceleration spectrum can be attributed
to the of (i) spherical expansion, (ii) tilting of the magnetic
field along the shock surface and (iii) shock deceleration. Because of (i) and
(ii), the DSA is efficient only on two ``polar caps'' of a spherical shock
where the local magnetic field is within to its normal. The
shock-produced spectrum observed edge-on steepens with the particle energy
because the number of freshly accelerated particles with lower energies
continually adds up to a growing acceleration region. We demonstrate the
steepening effect by obtaining an exact self-similar solution for the particle
acceleration at expanding shock surface with an arbitrary energy dependence of
particle diffusivity . We show that its increase toward higher energy
steepens the spectrum, which deeply contrasts with the standard DSA spectrum
where cancels out.Comment: 18 pages, 9 figures, to Appear in Ap
Detection of diffuse gamma-ray emission near the young massive cluster NGC 3603
We report the Fermi Large Area Telescope's detection of extended gamma-ray
emission towards the direction of the young massive star cluster NGC 3603. The
emission shows a hard spectrum with a photon index of 2.3 from 1 GeV to 250
GeV. The large size and high luminosity of this structure make it unlikely a
pulsar wind nebular. On the other hand the spatial correlation with the ionised
gas indicate a hadronic origin. The total cosmic ray (CR) protons energy are
estimated to be of the order assuming the gamma-ray are
produced in the interaction of CRs with ambient gas . The environment and
spectral features show significant similarity with the Cygnus cocoon. It
reveals that the young star clusters may be a gamma-ray source population and
they can potentially accelerate a significant fraction of the Galactic cosmic
rays.Comment: 6 pages, 5 figures, submitted to A&
Interpretation of the excess of antiparticles within a modified paradigm of galactic cosmic rays
We argue that the anomalously high fluxes of positrons and antiprotons found
in cosmic rays (CR) can be satisfactorily explained by introducing two
additional elements to the current "standard" paradigm of Galactic CRs. First,
we propose that the antiparticles are effectively produced in interactions of
primary CRs with the surrounding gas not only in the interstellar medium (ISM)
but also inside the accelerators. Secondly, we postulate the existence of two
source populations injecting CRs into the ISM with different, (1) soft (close
to ) and (2) hard ( or harder),
energy distributions. Assuming that CRs in the 2nd population of accelerators
accumulate "grammage" of the order of before their leakage
into ISM, we can explain the energy distributions and absolute fluxes of both
positrons and antiprotons, as well as the fluxes of secondary nuclei of the
(Li,Be,B) group. The superposition of contributions of two source populations
also explains the reported hardening of the spectra of CR protons and nuclei
above 200 GV. The 2nd source population accelerating CRs with a rate at the
level below 10 percent of the power of the 1st source population, can be
responsible for the highest energy protons and nuclei of Galactic CRs up to the
"knee" around .Comment: accepted for publication in PR
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
Limitations on the Photo-disintegration Process as a Source of VHE Photons
We consider whether photo-disintegration is ever able to provide an effective
mechanism for the production of VHE -ray emission from astrophysical
sources. We find that the efficiency of this process is always smaller by a
factor () than that of nuclei cooling through Bethe-Heitler
pair-production. Furthermore, for sources optically thin to TeV emission, we
find that the efficiency of this process can be no more than , where is the source
size and is the CR nuclei Larmor radius. We conclude that this
process is unable to provide an effective mechanism for VHE -ray
emission from astrophysical sources.Comment: 10 pages, 5 figure
On the potential of Cherenkov Telescope Arrays and KM3 Neutrino Telescopes for the detection of extended sources
We discuss the discovery potential of extended very-high-energy (VHE)
neutrino sources by the future KM3 Neutrino Telescope (KM3NeT) in the context
of the constraining power of the Cherenkov Telescope Array (CTA), designed for
deep surveys of the sky in VHE gamma rays. The study is based on a comparative
analysis of sensitivities of KM3NeT and CTA. We show that a minimum gamma-ray
energy flux of E^2{\phi}_{\gamma}(10 TeV) > 1x10^{-12} TeV cm^{-2} s^{-1} is
required to identify a possible neutrino counterpart with a 3{\sigma}
significance and 10 years of KM3NeT observations with upgoing muons, if the
source has an angular size of R_{src} = 0.1 deg and emits gamma rays with an
E^{-2} energy spectrum through a full hadronic mechanism. This minimum
gamma-ray flux is increased to the level of E^2{\phi}_{\gamma}(10 TeV) >
2x10^{-11} TeV cm^{-2} s^{-1} in case of sources with radial extension of
R_{src} = 2.0 deg. The analysis methods are applied to the supernova remnant RX
J1713.7-3946 and the Galactic Center Ridge, as well as to the recent HAWC
catalog of multi-TeV gamma-ray sources.Comment: 15 pages, 7 figure
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