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(-0.09)(+0.13) DM (pc cm(-3)), which corresponds to an average ionization of 10(-3)(+4)%, 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 HI column density along the same line of sight, while the optical extinction generally gives the best N-H predictions.