  We present an analysis of the broadband radio spectrum (from 22 to 3900 MHz)
of the Galactic supernova remnant (SNR) HB3 (G132.7+1.3). Published
observations have revealed that a curvature is present in the radio spectrum of
this SNR, indicating that a single synchrotron component appears is
insufficient to adequately fit the spectrum. We present here a fit to this
spectrum using a combination of a synchrotron component and a thermal
bremsstrahlung component. We discuss properties of this latter component and
estimate the ambient density implied by the presence of this component to be n
\~ 10 cm^-3. We have also analyzed extracted X-ray spectra from archived {\it
ASCA} GIS observations of different regions of HB3 to obtain independent
estimates of the density of the surrounding interstellar medium (ISM). From
this analysis, we have derived electron densities of 0.1-0.4 f^-1/2 cm^-3 for
the ISM for the three different regions of the SNR, where f is the volume
filling factor. By comparing these density estimates with the estimate derived
from the thermal bremsstrahlung component, we argue that the radio thermal
bremsstrahlung emission is emitted from a thin shell enclosing HB3. The
presence of this thermal bremsstrahlung component in the radio spectrum of HB3
suggests that this SNR is in fact interacting with an adjacent molecular cloud
associated with the HII region W3. By extension, we argue that the presence of
thermal emission at radio wavelengths may be a useful tool for identifying
interactions between SNRs and molecular clouds, and for estimating the ambient
density near SNRs using radio continuum data.
