SIMBAD references

New observations are presented of the supernova remnant HB 21 at 408 and 1420 MHz made with the Synthesis Telescope (ST) of the Dominion Radio Astrophysical Observatory. A new method of spectral index analysis is presented that allows for removal of point-source contamination at the spatial resolution of the higher spatial resolution map. This is a great improvement in point-source contamination removal (by a factor ≳10), and allows more detailed spectral index variation studies than previously possible. Significant spatial variations in spectral index are found in HB 21. Different physical mechanisms for spectral index changes are considered, including a detailed consideration of ionization losses in the dense molecular gas interacting with HB 21. The lack of detailed spatial correlation of spectral flattening with positions of shocked clumps indicates that absorption by thermal plasma is the preferred mechanism.