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Abstract:
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The evolution of supernova remnants is linked to the propagation of a collisionless
shock wave, formed during the initial expansion of high-velocity supernova ejecta
through the interstellar environment. Theoretical studies of shock waves are very
important for the analysis of supernova remnants, as well as processes in the interstellar
medium in general. In this doctoral dissertation, some theoretical results
based on the magnetohydrodynamical theory of shock waves are presented, with
special emphasis on ideal radiative magnetohydrodynamics for the optically thick
case. Particularly, solutions for the case when jump in adiabatic index and/or ratio
of gas to total pressure is allowed, are discussed.
The main hypothesis of this dissertation is that thermal bremsstrahlung radiation
at radio continuum frequencies can provide a signi cant contribution in the case
of several Galactic supernova remnants. This hypothesis can give a natural explanation
for nearly concave up radio continuum spectra of several Galactic supernova
remnants that are expanding in the environment with higher than average density.
In this context, it is important to identify the existence of the possible indicators of
ensemble of thermal electrons at su ciently low temperatures and su ciently high
densities so that the thermal bremsstrahlung radiation linked to a particular remnant
could be observed at radio continuum frequencies (vicinity, interaction or expansion
through the molecular cloud, presence of the cooled thermal X-ray electrons during
the post Sedov-Taylor phases, detection of low-frequency turnovers associated with
thermal absorption linked to the remnant, detection in H , identi cation of radio
recombination lines linked to the remnant, etc).
The signi cant presence of thermal component could theoretically explain radiospectral
indices less than 0.5 measured for several evolutionary older supernova remnants,
(mainly of mixed-morphology class) that expand in the high density region.
Actually, these smaller radio-spectral indices, under the assumption of simple power
law, would represent a natural manifestation of a signi cant fraction of thermal
emission at radio continuum frequencies.
However, present knowledge of the radio continuum spectra of Galactic supernova
remnants is still not determined precisely enough for any de nite conclusions
to be made about the inherent thermal radio-emission from supernova remnants. A
thorough analysis is only possible in the case of three Galactic supernova remnants
(3C396, IC443, 3C391) for which the thermal contribution is determined despite
high associated uncertainties. New observations in the near future will lay the
groundwork for making rmer conclusions about the existence of the so-called radio
thermally active supernova remnants.
This dissertation highlights the importance of observations of supernova remnants
in X and
-rays, and multiwavelength analysis is general. Besides, it suggests
a possible detection of
-rays from supernova remnant 3C434.1 based on the observations
made by Fermi. |