A multi-frequency radio study of the supernova remnant HB9

We present a new radio map of HB9 at 232 MHz made at the Beijing Astronomical Observatory. We also present previously unpublished maps at 151 MHz, from the new Cambridge Low-Frequency Synthesis Telescope Survey, and at 4750 MHz, from the Effelsberg Telescope. These radio maps are compared with previously published maps at 408, 1420, and 2695 MHz, thus providing the most complete frequency coverage for a spectral index study of a supernova remnant to date. Spectral index maps are constructed using the running T-T plot method. Several properties of spectral index behavior are determined. The mechanisms which can most naturally account for the observed behavior are: variable thermal electron density near the rim causing variable low frequency absorption and spectral flattening; and variable magnetic field near the rim and a curved electron spectrum causing variable high frequency steepening near the rim. A high spectral index region is likely associated with extended emission from 4C46.09.