1,369 research outputs found
Cosmic Ray Origin, Acceleration and Propagation
This paper summarizes highlights of the OG3.1, 3.2 and 3.3 sessions of the
XXVIth International Cosmic Ray Conference in Salt Lake City, which were
devoted to issues of origin/composition, acceleration and propagation.Comment: To appear in the Summary-Rapporteur Volume of the 26th International
Cosmic Ray Conference, ed. B. L. Dingus (AIP, New York, 2000). Latex, 16
pages, no figures (Minor correction to text
Modelling Hard Gamma-Ray Emission From Supernova Remnants
The observation by the CANGAROO experiment of TeV emission from SN 1006, in
conjunction with several instances of non-thermal X-ray emission from supernova
remnants, has led to inferences of super-TeV electrons in these extended
sources. While this is sufficient to propel the theoretical community in their
modelling of particle acceleration and associated radiation, the anticipated
emergence in the next decade of a number of new experiments probing the TeV and
sub-TeV bands provides further substantial motivation for modellers. In
particular, the quest for obtaining unambiguous gamma-ray signatures of cosmic
ray ion acceleration defines a ``Holy Grail'' for observers and theorists
alike. This review summarizes theoretical developments in the prediction of
MeV-TeV gamma-rays from supernova remnants over the last five years, focusing
on how global properties of models can impact, and be impacted by, hard
gamma-ray observational programs, thereby probing the supernova remnant
environment. Properties of central consideration include the maximum energy of
accelerated particles, the density of the unshocked interstellar medium, the
ambient magnetic field, and the relativistic electron-to-proton ratio. Criteria
for determining good candidate remnants for observability in the TeV band are
identified.Comment: 10 pages, 2 figures, to appear in Proc. of Snowbird TeV Gamma-Ray
Workshop ed. B. L. Dingus (AIP, New York, 2000) (Replacement updates Fig. 2
and references
Using Gamma-Ray Burst Prompt Emission to Probe Relativistic Shock Acceleration
It is widely accepted that the prompt transient signal in the 10 keV - 10 GeV
band from gamma-ray bursts (GRBs) arises from multiple shocks internal to the
ultra-relativistic expansion. The detailed understanding of the dissipation and
accompanying acceleration at these shocks is a currently topical subject. This
paper explores the relationship between GRB prompt emission spectra and the
electron (or ion) acceleration properties at the relativistic shocks that
pertain to GRB models. The focus is on the array of possible high-energy
power-law indices in accelerated populations, highlighting how spectra above 1
MeV can probe the field obliquity in GRB internal shocks, and the character of
hydromagnetic turbulence in their environs. It is emphasized that diffusive
shock acceleration theory generates no canonical spectrum at relativistic MHD
discontinuities. This diversity is commensurate with the significant range of
spectral indices discerned in prompt burst emission. Such system diagnostics
are now being enhanced by the broadband spectral coverage of bursts by the
Fermi Gamma-Ray Space Telescope; while the Gamma-Ray Burst Monitor (GBM)
provides key diagnostics on the lower energy portions of the particle
population, the focus here is on constraints in the non-thermal, power-law
regime of the particle distribution that are provided by the Large Area
Telescope (LAT).Comment: 15 pages, 2 figures. Accepted for publication in Advances of Space
Researc
Diffusive Shock Acceleration of High Energy Cosmic Rays
The process of diffusive acceleration of charged particles in shocked plasmas
is widely invoked in astrophysics to account for the ubiquitous presence of
signatures of non-thermal relativistic electrons and ions in the universe. A
key characteristic of this statistical energization mechanism is the absence of
a momentum scale; astrophysical systems generally only impose scales at the
injection (low energy) and loss (high energy) ends of the particle spectrum.
The existence of structure in the cosmic ray spectrum (the "knee") at around
3000 TeV has promoted contentions that there are at least two origins for
cosmic rays, a galactic one supplying those up to the knee, and even beyond,
and perhaps an extragalactic one that can explain even the ultra-high energy
cosmic rays (UHECRs) seen at 1-300 EeV. Accounting for the UHECRs with familiar
astrophysical sites of acceleration has historically proven difficult due to
the need to assume high magnetic fields in order to reduce the shortest
diffusive acceleration timescale, the ion gyroperiod, to meaningful values. Yet
active galaxies and gamma-ray bursts remain strong and interesting candidate
sources for UHECRs, turning the theoretical focus to relativistic shocks. This
review summarizes properties of diffusive shock acceleration that are salient
to the issue of UHECR generation. These include spectral indices, acceleration
efficencies and timescales, as functions of the shock speed and mean field
orientation, and also the nature of the field turbulence. The interpretation of
these characteristics in the context of gamma-ray burst models for the
production of UHECRs is also examined.Comment: 10 pages, 2 embedded figures, To appear in Nuclear Physics B,
Proceedings Supplements, as part of the volume for the CRIS 2004, Cosmic Ray
International Seminar: "GZK and Surroundings.
Photon Splitting in Magnetar Models of Soft Gamma Repeaters
The recent association of soft gamma repeaters (SGRs) with counterparts in
other wavebands has sparked much interest in these sources. One of the recent
models for these objects is that they originate in the environs of neutron
stars with fields much stronger than the quantum critical field
\teq{B_{cr}=4.413\times 10^{13}} Gauss. Near such neutron stars, dubbed
magnetars, the exotic quantum process of magnetic photon splitting becomes
prolific. Its principal effect is to degrade photon energies and thereby soften
gamma-ray spectra from neutron stars; it has recently been suggested that
splitting may be responsible for limiting the hardness of emission in SGRs, if
these sources originate in neutron stars with supercritical surface fields.
Seed photons in supercritical fields efficiently generate soft gamma-ray
spectra, typical of repeaters. In this paper, the influence of the curved
dipole field geometry of a neutron star magnetosphere on the photon splitting
rate is investigated. The dependence of the attenuation length on the location
and angular direction of the seed photons is explored.Comment: 5 pages including 3 encapsulated figures, as a compressed, uuencoded,
Postscript file. To appear in Proc. of the 1995 La Jolla workshop ``High
Velocity Neutron Stars and Gamma-Ray Bursts'' eds. Rothschild, R. et al.,
AIP, New Yor
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