On the birtrate of galactic supernovae

The birthrate of galactic supernovae is estimated in three different ways:

1. on the basis of the historical record (eight events) the mean time interval between supernovae, τ, is considered to be in the range τ=60±40 yr;
2. on the basis of an approximate total of 120 supernovae events in hundreds of other galaxies, considered similar to our own, the interval obtained is in the range τ=70±50 yr; (iii) on the
3. on the basis of the 130 supernovae remnants in our own Galaxy, the interval is estimated to be in the range τ=80±30 yr. The three ranges overlap, and we suggest that 70±35 yr represents a more realistic estimate of the rate than some that have previously been made.

The galactic radio supernovae remnants, and their observed systematic brightness gradients perpendicular to the galactic plane, imply a scale height of about 200 pc for the remnant progenitors, and indicate that the galactic magnetic field's scale-height is about 300 pc. Long standing anomalies associated with (a) the young remnant AD 1006, (b) the galactic loops, and (c) faint remnants, are accounted for by the brightness gradient effect, providing independent, and firm, corroborating evidence for the fundamental validity of the remnant method of deducing the galactic supernova birthrate.