The Galactic NH - AV Relation and its Application to Historical Galactic SNRs

We refine a classic relation between the hydrogen column density (NH) and optical extinction (Av) by employing 39 Galactic Supernova Remnants (SNRs) with X-rays, optical and/or infra-red data available. We find NH = (1.69+/-0.07)*10^21 Av cm^(-2) mag^(-1) . Applying this relation to three Galactic SNRs with good historical records allows us to further constrain either their progenitor's distances or magnitudes, which is independent access to their distances.