2005A&A...436..187T

We present images of the SNR HB3 at both 408MHz and 1420MHz from the Canadian Galactic Plane Survey mainly based on data from the Synthesis Telescope of the Dominion Radio Astrophysical Observatory. We reproduce the 100m-Effelsberg HB3 image at 2695MHz at large scale, and find that there exists a background emission gradient across the HB3 area. Based on our analysis of background emission and the boundary between W3 and HB3, we give HB3's flux density as 68.6±11.5Jy at 408MHz and 44.8±12.0Jy at 1420MHz, after subtracting flux from compact sources within HB3. The integrated flux-density-based spectral index between 408MHz and 1420MHz is 0.34±0.15. The averaged T-T plot spectral index using all subareas is 0.36. Our measurement values are less than a previously published value of 0.6. The 408-1420MHz spectral index varies spatially in HB3 in the range 0.1 to 0.7. We investigate the data used by previous authors, and consider more data at 232MHz, 3650MHz and 3900MHz which are not included in previous calculations. There is evidence for two spectral indices for HB3 in the radio band, i.e., 0.63 (38-610MHz) and 0.32 (408-3900MHz). This is consistent with the spatial variations: the low frequency data mainly reflects the steeper indices and the high frequency data mainly reflects the flatter indices.