An analysis of the broadband (22-3900 MHz) radio spectrum of HB 3 (G132.7+1.3): The detection of thermal radio emission from an evolved supernova remnant?

Abstract

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 cm-3. 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 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 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. © 2007. The American Astronomical Society. All rights reserved.