Radio spectrum and distance of the SNR HB9

Abstract

New images are presented of the supernova remnant (SNR) HB9 based on 408 MHz and 1420 MHz continuum emission and HI-line emission data from the Canadian Galactic Plane Survey (CGPS). Two different T-T plot methods and new integrated flux densities give spectral index ($S_{\nu}$ $\propto$ ν $^{-\alpha}$) for the whole of HB9 of 0.48 ± 0.03; 0.49; and 0.47 ± 0.06, respectively. These values are lower than the previous spectral index estimated for HB9 ( α = 0.61). The change is mainly due to improved 1420 MHz data. No difference in spectral index is detected between strong and weak filaments. A new result is that the spectral index for interior regions is steeper than for the rim. This can be explained by a standard curved interstellar electron energy spectrum combined with lower interior magnetic field compared to that near the outer shock. This results in a larger proportion of steep spectrum emission for lines-of-sight through the central body of the SNR. HI observations show structures probably associated with the SNR in the radial velocity range -3 to -9 km s -1 , suggesting a kinematic distance of 0.8 ± 0.4 kpc for the SNR. This is consistent with the distance to the radio pulsar PSR B0458+46, offset from the center of HB9 by 23′. However the pulsar spindown and kinematic ages are significantly greater than estimates of the SNR age: the Sedov age for HB9 is 6600 yr and the evaporative cloud model yields ages of 4000–7000 yr.