170 research outputs found
On the plasma temperature in supernova remnants with cosmic-ray modified shocks
Context: Multiwavelength observations of supernova remnants can be explained
within the framework of the diffusive shock acceleration theory, which allows
effective conversion of the explosion energy into cosmic rays. Although the
models of nonlinear shocks describe reasonably well the nonthermal component of
emission, certain issues, including the heating of the thermal plasma and the
related X-ray emission, remain still open.
Aims: To discuss how the evolution and structure of supernova remnants is
affected by strong particle acceleration at the forward shock.
Methods: Analytical estimates combined with detailed discussion of the
physical processes.
Results: The overall dynamics is shown to be relatively insensitive to the
amount of particle acceleration, but the post-shock gas temperature can be
reduced to a relatively small multiple, even as small as six times, the ambient
temperature with a very weak dependence on the shock speed. This is in marked
contrast to pure gas models where the temperature is insensitive to the ambient
temperature and is determined by the square of the shock speed. It thus appears
to be possible to suppress effectively thermal X-ray emission from remnants by
strong particle acceleration. This might provide a clue for understanding the
lack of thermal X-rays from the TeV bright supernova remnant RX J1713.7-3946.Comment: Appendix A added, minor changes and additional references include
Cosmic ray diffusive acceleration at shock waves with finite upstream and downstream escape boundaries
In the present paper we discuss the modifications introduced into the
first-order Fermi shock acceleration process due to a finite extent of
diffusive regions near the shock or due to boundary conditions leading to an
increased particle escape upstream and/or downstream the shock. In the
considered simple example of the planar shock wave we idealize the escape
phenomenon by imposing a particle escape boundary at some distance from the
shock. Presence of such a boundary (or boundaries) leads to coupled steepening
of the accelerated particle spectrum and decreasing of the acceleration time
scale. It allows for a semi-quantitative evaluation and, in some specific
cases, also for modelling of the observed steep particle spectra as a result of
the first-order Fermi shock acceleration. We also note that the particles close
to the upper energy cut-off are younger than the estimate based on the
respective acceleration time scale. In Appendix A we present a new
time-dependent solution for infinite diffusive regions near the shock allowing
for different constant diffusion coefficients upstream and downstream the
shock.Comment: LaTeX, 14 pages, 4 postscript figures; Solar Physics (accepted
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