The relation between optical extinction and hydrogen column density in the Galaxy

A linear relation between the hydrogen column density (N-H) and optical extinction (A(V)) in the Galaxy has long been observed. A number of studies found differing results in the slope of this relation. Here, we utilize the data on 22 supernova remnants that have been observed with the latest generation X-ray observatories and for which optical extinction and/or reddening measurements have been performed and find N-H (cm-2) = (2.21 +/- 0.09) x 1021 A(V) (mag). We compare our result with the previous studies and assess any systematic uncertainties that may affect these results.