2007ApJ...655L..41U

We present an analysis of the broadband radio spectrum (from 22 to 3900 MHz) of the Galactic supernova remnant (SNR) HB 3 (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 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 the latter component and estimate the ambient density implied by the presence of this component to be n∼10/cm3. We have also analyzed X-ray spectra extracted from archived ASCA GIS observations of different regions of HB 3 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/cm3 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 HB 3. The presence of this thermal bremsstrahlung component in the radio spectrum of HB 3 suggests that this SNR is in fact interacting with an adjacent molecular cloud associated with the H II 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.