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) 10 21 A V (mag). We compare our result with the previous studies and assess any systematic uncertainties that may affect these results.

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