The Correlation between Dispersion Measure and X-Ray Column Density from Radio Pulsars

Pulsars are remarkable objects that emit across the entire electromagnetic spectrum, providing a powerful probe of the interstellar medium. In this study, we investigate the relation between dispersion measure (DM) and X-ray absorption column density N _H using 68 radio pulsars detected at X-ray energies with the Chandra X-Ray Observatory or XMM-Newton. We find a best-fit empirical linear relation of N_{H}\;(10^{20}\,cm^{-2})= 0.30^{+0.13}_{-0.09}\;DM\; (pc\,cm^{-3}), which corresponds to an average ionization of 10^{+4}_{-3}%, confirming the ratio of one free electron per 10 neutral hydrogen atoms commonly assumed in the literature. We also compare different N _H estimates and note that some N _H values obtained from X-ray observations are higher than the total Galactic H I column density along the same line of sight, while the optical extinction generally gives the best N _H predictions.