On the existence of "radio thermally active" Galactic supernova remnants
  In this paper, we investigate the possibility of significant production of
thermal bremsstrahlung radiation at radio continuum frequencies that could be
linked to some Galactic supernova remnants (SNRs). The main targets for this
investigation are SNRs expanding in high density environments. There are
several indicators of radio thermal bremsstrahlung radiation from SNRs, such as
a flattening at higher frequencies and thermal absorption at lower frequencies
intrinsic to an SNR. In this work we discuss the radio continuum properties of
3 SNRs that are the best candidates for testing our hypothesis of significant
thermal emission. In the case of SNRs IC443 and 3C391, thermal absorption has
been previously detected. For IC443, the contribution of thermal emission at 1
GHz, from our model fit is 3-57%. It is similar to the estimate obtained from
the thermal absorption properties (10-40% at 1 GHz). In the case of the 3C391
the conclusions are not so clear. The results from our model fit (thermal
emission contribution of 10-25% at 1 GHz) and results obtained from the low
frequency absorption (thermal contribution of 0.15-7% at 1 GHz) do not overlap.
For the SNR 3C396 we suggest that if previously detected thermal absorption
could be intrinsic to the SNR then the thermal emission (<47% at 1 GHz from our
model fit) could be significant enough to shape the radio continuum spectrum at
high frequencies. Polarization observations for these SNRs can constrain the
strength of a thermal component. Reliable observations at low frequencies (<100
MHz) are needed as well as more data at high radio frequencies (>1 GHz), in
order to make stronger conclusions about the existence of "radio thermally
active" SNRs.
