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 \pm0.09) x10^{21} A_V (mag). We compare our result with the previous studies and assess any systematic uncertainties that may affect these results.