Galactic supernova remnants: dependence of radio brightness on galactic height and its implications

Abstract

The variation of radio brightness across individual shell supernova remnants has been investigated; on the average, the brightness decreases with increasing distance from the galactic plane (z), with a scale height of 110 pc. Part of the brightness gradient results from a dependence of diameter (D) on the density of the interstellar medium, but much of the gradient represents a z-dependence intrinsic to the radio emission process. We conclude that the relation Σ ∝ D-3 exp (- |z|/175), describes the typical evolution of the mean brightness (Σ) of supernova remnants at different z. The applicability of this relation extends to the faintest observable remnants and entails considerable revision of the diameters, distances and ages for some remnants; in particular, a long-standing anomaly concerning the faint (but young) remnant of AD 1006 is now explained. Estimates of the galactic distribution of supernovae are also affected by the z-dependence of Σ; we derive corrected values of 200 pc for the scale height of supernovae producing shell remnants, and 80 years for the mean interval between such supernovae. (This rate does not include supernovae which produce Crab-like remnants or no radio remnants.) We suggest that the galactic magnetic field is responsible for the intrinsic Σ-z dependence; the scale height of the field locally is then inferred to be 260 pc - possibly the first experimentally determined estimate.