1,873 research outputs found
The Important Role of Cosmic-Ray Re-Acceleration
In the last decades, the improvement of high energy instruments has enabled a
deeper understanding of the Cosmic Ray origin issue. In particular, the
gamma-ray satellites AGILE (Astrorivelatore Gamma ad Immagini LEggero) and
Fermi-LAT (Fermi-Large Area Telescope) have strongly contributed to the
confirmation of direct involvement of Supernova Remnants in Cosmic Ray
energization. Despite several attempts to fit experimental data assuming the
presence of freshly accelerated particles, the scientific community is now
aware that the role of pre-existing Cosmic Ray re-acceleration cannot be
neglected. In this work, we highlight the importance of pre-existing Cosmic Ray
re-acceleration in the Galaxy showing its fundamental contribution in middle
aged Supernova Remnant shocks and in the forward shock of stellar winds.Comment: 16 pages, 4 figure
Fermi Large Area Telescope observations of the supernova remnant HESS J1731-347
Context: HESS J1731-347 has been identified as one of the few TeV-bright
shell-type supernova remnants (SNRs). These remnants are dominated by
nonthermal emission, and the nature of TeV emission has been continuously
debated for nearly a decade.
Aims: We carry out the detailed modeling of the radio to gamma-ray spectrum
of HESS J1731-347 to constrain the magnetic field and energetic particles
sources, which we compare with those of the other TeV-bright shell-type SNRs
explored before.
Methods: Four years of data from Fermi Large Area Telescope (LAT)
observations for regions around this remnant are analyzed, leading to no
detection correlated with the source discovered in the TeV band. The Markov
Chain Monte Carlo method is used to constrain parameters of one-zone models for
the overall emission spectrum.
Results: Based on the 99.9% upper limits of fluxes in the GeV range, one-zone
hadronic models with an energetic proton spectral slope greater than 1.8 can be
ruled out, which favors a leptonic origin for the gamma-ray emission, making
this remnant a sibling of the brightest TeV SNR RX J1713.7-3946, the Vela
Junior SNR RX J0852.0-4622, and RCW 86. The best-fit leptonic model has an
electron spectral slope of 1.8 and a magnetic field of about 30 muG, which is
at least a factor of 2 higher than those of RX J1713.7-3946 and RX
J0852.0-4622, posing a challenge to the distance estimate and/or the energy
equipartition between energetic electrons and the magnetic field of this
source. A measurement of the shock speed will address this challenge and has
implications on the magnetic field evolution and electron acceleration driven
by shocks of SNRs.Comment: 7 pages, 3 fogures, A&A in pres
Evolution of High-Energy Particle Distribution in Mature Shell-Type Supernova Remnants
Multi-wavelength observations of mature supernova remnants (SNRs), especially
with recent advances in gamma-ray astronomy, make it possible to constrain
energy distribution of energetic particles within these remnants. In
consideration of the SNR origin of Galactic cosmic rays and physics related to
particle acceleration and radiative processes, we use a simple one-zone model
to fit the nonthermal emission spectra of three shell-type SNRs located within
2 degrees on the sky: RX J1713.7-3946, CTB 37B, and CTB 37A. Although radio
images of these three sources all show a shell (or half-shell) structure, their
radio, X-ray, and gamma-ray spectra are quite different, offering an ideal case
to explore evolution of energetic particle distribution in SNRs. Our spectral
fitting shows that 1) the particle distribution becomes harder with aging of
these SNRs, implying a continuous acceleration process, and the particle
distributions of CTB 37A and CTB 37B in the GeV range are harder than the
hardest distribution that can be produced at a shock via the linear diffusive
shock particle acceleration process, so spatial transport may play a role; 2)
the energy loss timescale of electrons at the high-energy cutoff due to
synchrotron radiation appears to be always a bit (within a factor of a few)
shorter than the age of the corresponding remnant, which also requires
continuous particle acceleration; 3) double power-law distributions are needed
to fit the spectra of CTB 37B and CTB 37A, which may be attributed to shock
interaction with molecular clouds.Comment: Accepted for publication in The Astrophysical Journal, 11 pages, 3
figures, 1 tabl
Detection of Gamma-Ray Emission from the Starburst Galaxies M82 and NGC 253 with the Large Area Telescope on Fermi
We report the detection of high-energy gamma-ray emission from two starburst
galaxies using data obtained with the Large Area Telescope on board the Fermi
Gamma-ray Space Telescope. Steady point-like emission above 200 MeV has been
detected at significance levels of 6.8 sigma and 4.8 sigma respectively, from
sources positionally coincident with locations of the starburst galaxies M82
and NGC 253. The total fluxes of the sources are consistent with gamma-ray
emission originating from the interaction of cosmic rays with local
interstellar gas and radiation fields and constitute evidence for a link
between massive star formation and gamma-ray emission in star-forming galaxies.Comment: Submitted to ApJ Letter
The origin of galactic cosmic rays
The origin of galactic cosmic rays is one of the most interesting unsolved
problems in astroparticle physics. Experimentally, the problem is attacked by a
multi-disciplinary effort, namely by direct measurements of cosmic rays above
the atmosphere, by air shower observations, and by the detection of TeV
rays. Recent experimental results are presented and their implications
on the contemporary understanding of the origin of galactic cosmic rays are
discussed.Comment: Invited talk given at the Roma International Conference on
Astro-Particle physics (RICAP07) June 20th - 22nd, 2007. To be published in
Nuclear Instruments and Methods
- …