219 research outputs found

### Cosmic Ray Spectrum in Supernova Remnant Shocks

We perform kinetic simulations of diffusive shock acceleration (DSA) in Type
Ia supernova remnants (SNRs) expanding into a uniform interstellar medium
(ISM). Bohm-like diffusion assumed, and simple models for Alfvenic drift and
dissipation are adopted. Phenomenological models for thermal leakage injection
are considered as well. We find that the preshock gas temperature is the
primary parameter that governs the cosmic ray (CR) acceleration efficiency and
energy spectrum, while the CR injection rate is a secondary parameter. For SNRs
in the warm ISM, if the injection fraction is larger than 10^{-4}, the DSA is
efficient enough to convert more than 20 % of the SN explosion energy into CRs
and the accelerated CR spectrum exhibits a concave curvature flattening to
E^{-1.6}. Such a flat source spectrum near the knee energy, however, may not be
reconciled with the CR spectrum observed at Earth. On the other hand, SNRs in
the hot ISM, with an injection fraction smaller than 10^{-4}, are inefficient
accelerators with less than 10 % of the explosion energy getting converted to
CRs. Also the shock structure is almost test-particle like and the ensuing CR
spectrum can be steeper than E^{-2}. With amplified magnetic field strength of
order of 30 microG, Alfven waves generated by the streaming instability may
drift upstream fast enough to make the modified test-particle power-law as
steep as E^{-2.3}, which is more consistent with the observed CR spectrum.Comment: 15 pages with 8 figures, To be published in the April issue of
Journal of Korean Astronomical Societ

### Nonthermal radiation from relativistic electrons accelerated at spherically expanding shocks

We study the evolution of the energy spectrum of cosmic-ray electrons
accelerated at spherically expanding shocks with low Mach numbers and the
ensuing spectral signatures imprinted in radio synchrotron emission.
Time-dependent simulations of diffusive shock acceleration (DSA) of electrons
in the test-particle limit have been performed for spherical shocks with
parameters relevant for typical shocks in the intracluster medium. The electron
and radiation spectra at the shock location can be described properly by the
test-particle DSA predictions with instantaneous shock parameters. However, the
volume integrated spectra of both electrons and radiation deviate significantly
from the test-particle power-laws, because the shock compression ratio and the
flux of injected electrons at the shock gradually decrease as the shock slows
down in time.So one needs to be cautious about interpreting observed radio
spectra of evolving shocks based on simple DSA models in the test-particle
regime.Comment: corrected typos and figures, 12 pages, 7 figures, Accepted for
publication at Journal of Korean Astronomical Societ

### Diffusive shock acceleration with magnetic field amplification and Alfvenic drift

We explore how wave-particle interactions affect diffusive shock acceleration
(DSA) at astrophysical shocks by performing time-dependent kinetic simulations,
in which phenomenological models for magnetic field amplification (MFA),
Alfvenic drift, thermal leakage injection, Bohm-like diffusion, and a free
escape boundary are implemented. If the injection fraction of cosmic-ray (CR)
particles is greater than 2x10^{-4}, for the shock parameters relevant for
young supernova remnants, DSA is efficient enough to develop a significant
shock precursor due to CR feedback, and magnetic field can be amplified up to a
factor of 20 via CR streaming instability in the upstream region. If scattering
centers drift with Alfven speed in the amplified magnetic field, the CR energy
spectrum can be steepened significantly and the acceleration efficiency is
reduced. Nonlinear DSA with self-consistent MFA and Alfvenic drift predicts
that the postshock CR pressure saturates roughly at 10 % of the shock ram
pressure for strong shocks with a sonic Mach number ranging 20< M_s< 100. Since
the amplified magnetic field follows the flow modification in the precursor,
the low energy end of the particle spectrum is softened much more than the high
energy end. As a result, the concave curvature in the energy spectra does not
disappear entirely even with the help of Alfvenic drift. For shocks with a
moderate Alfven Mach number (M_A<10), the accelerated CR spectrum can become as
steep as E^{-2.1}-E^{-2.3}, which is more consistent with the observed CR
spectrum and gamma-ray photon spectrum of several young supernova remnants.Comment: 12 pages with 6 figures To appear in October 2012 issue of Journal of
Korean Astronomical Societ

### Cosmic Shock Waves on Large Scales of the Universe

In the standard theory of the large scale structure formation, matter
accretes onto high density perturbations via gravitational instability.
Collisionless dark matter forms caustics around such structures, while
collisional baryonic matter forms accretion shocks which then halt and heat the
infalling gas. Here, we discuss the characteristics, roles, and observational
consequences of these accretion shocks.Comment: 3 pages with 1 figure, uses sprocl.sty, to appear in the Proceedings
of the 18th Texas Symposium on Relativistic Astrophysics, ed. A. Olinto, J.
Frieman and D. Schramm, also available upon request to
[email protected]

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