Radio observations at 232 MHz and multifrequency spectral studies of the SNR HB21

Radio observational results at 232 MHz and multifrequency studies of the su pernova remnant (SNR) HB21 are presented. Its integrated flux density at 23 2 MHz is about 390 +/- 30 Jy. Both the integrated spectral index and the sp atial variations of spectral index of the remnant were calculated by combin ing the new map at 232 MHz with previously published maps made at 408, 1420 , 2695, and 4750 MHz. The SNR has an integrated spectral index of about alp ha = -0.43 (S-v proportional to v(alpha)) between 232 and 4750 MHz. In gene ral the spectral index varies from -0.5 in southeast and west regions of th e remnant to -0.3 in the central region and near the northwest edge. The ne w data of 232 MHz reveal that there is interaction between the remnant and the surrounding gas along the east edge of the remnant which causes the spe ctrum flattening at low frequency, while the very good agreement between th e structure of X-ray emission and the central flat spectrum area suggests t hat the existence of thermal emission is the reason of spectrum flattening in the area.