12,589 research outputs found
The Cosmic Ray - X-ray Connection: Effects of Nonlinear Shock Acceleration on Photon Production in SNRs
Cosmic-ray production in young supernova remnant (SNR) shocks is expected to
be efficient and strongly nonlinear. In nonlinear, diffusive shock
acceleration, compression ratios will be higher and the shocked temperature
lower than test-particle, Rankine-Hugoniot relations predict. Furthermore, the
heating of the gas to X-ray emitting temperatures is strongly coupled to the
acceleration of cosmic-ray electrons and ions, thus nonlinear processes which
modify the shock, influence the emission over the entire band from radio to
gamma-rays and may have a strong impact on X-ray line models. Here we apply an
algebraic model of nonlinear acceleration, combined with SNR evolution, to
model the radio and X-ray continuum of Kepler's SNR.Comment: 7 pages including 4 figures; to appear in ``The Acceleration and
Transport of Energetic Particles Observed in the Heliosphere,'' Proceedings
of the ACE-2000 Symposium held on January 5 - 8, 2000, Indian Springs, C
Experimental stagnation point velocity gradients and heat transfer coefficients for a family of blunt bodies at Mach 8 and angles of attack
Stagnation pressure and heat transfer measurements of blunt axisymmetric bodie
Acceleration Rates and Injection Efficiencies in Oblique Shocks
The rate at which particles are accelerated by the first-order Fermi
mechanism in shocks depends on the angle, \teq{\Tbone}, that the upstream
magnetic field makes with the shock normal. The greater the obliquity the
greater the rate, and in quasi-perpendicular shocks rates can be hundreds of
times higher than those seen in parallel shocks. In many circumstances
pertaining to evolving shocks (\eg, supernova blast waves and interplanetary
traveling shocks), high acceleration rates imply high maximum particle energies
and obliquity effects may have important astrophysical consequences. However,
as is demonstrated here, the efficiency for injecting thermal particles into
the acceleration mechanism also depends strongly on obliquity and, in general,
varies inversely with \teq{\Tbone}. The degree of turbulence and the resulting
cross-field diffusion strongly influences both injection efficiency and
acceleration rates. The test particle \mc simulation of shock acceleration used
here assumes large-angle scattering, computes particle orbits exactly in
shocked, laminar, non-relativistic flows, and calculates the injection
efficiency as a function of obliquity, Mach number, and degree of turbulence.
We find that turbulence must be quite strong for high Mach number, highly
oblique shocks to inject significant numbers of thermal particles and that only
modest gains in acceleration rates can be expected for strong oblique shocks
over parallel ones if the only source of seed particles is the thermal
background.Comment: 24 pages including 6 encapsulated figures, as a compressed,
uuencoded, Postscript file. Accepted for publication in the Astrophysical
Journa
Inverse Bremsstrahlung in Shocked Astrophysical Plasmas
There has recently been interest in the role of inverse bremsstrahlung, the
emission of photons by fast suprathermal ions in collisions with ambient
electrons possessing relatively low velocities, in tenuous plasmas in various
astrophysical contexts. This follows a long hiatus in the application of
suprathermal ion bremsstrahlung to astrophysical models since the early 1970s.
The potential importance of inverse bremsstrahlung relative to normal
bremsstrahlung, i.e. where ions are at rest, hinges upon the underlying
velocity distributions of the interacting species. In this paper, we identify
the conditions under which the inverse bremsstrahlung emissivity is significant
relative to that for normal bremsstrahlung in shocked astrophysical plasmas. We
determine that, since both observational and theoretical evidence favors
electron temperatures almost comparable to, and certainly not very deficient
relative to proton temperatures in shocked plasmas, these environments
generally render inverse bremsstrahlung at best a minor contributor to the
overall emission. Hence inverse bremsstrahlung can be safely neglected in most
models invoking shock acceleration in discrete sources such as supernova
remnants. However, on scales > 100pc distant from these sources, Coulomb
collisional losses can deplete the cosmic ray electrons, rendering inverse
bremsstrahlung, and perhaps bremsstrahlung from knock-on electrons, possibly
detectable.Comment: 13 pages, including 2 figures, using apjgalley format; to appear in
the January 10, 2000 issue, of the Astrophysical Journa
- …