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

TL;DR

This study establishes an empirical linear relation between dispersion measure and X-ray absorption column density in radio pulsars, confirming the typical ionization ratio and comparing different NH estimation methods.

Contribution

It provides the first comprehensive empirical relation between DM and NH for pulsars, validating the ionization ratio assumption and evaluating various NH estimation techniques.

Findings

Established a linear relation: NH (10^20 cm^-2)= 0.30+0.13-0.09 DM (pc cm^-3)

Confirmed an average ionization of about 10% in the interstellar medium

Found optical extinction provides the most accurate NH estimates

Abstract

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 NH 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 NH (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 ten neutral hydrogen atoms commonly assumed in the literature. We also compare different NH estimates and note that some NH values obtained from X-ray observations are higher than the total Galactic HI column density along the same line of sight, while the optical extinction generally gives the best NH predictions.