The First Insights into an Ultraluminous X-ray Pulsar with XRISM: Phase-Resolved High-Resolution Spectroscopy of the Fe K-shell Band of M82 X-2

Abstract

During the performance verification phase, XRISM observed the M82 galaxy for a net exposure of 207.7 ks, with the ultraluminous X-ray pulsar (ULXP) X-2 included in the field of view. A pulsation search identified a candidate signal with a period close to the previously known value, 1.38727 s, at a significance of $3.15\ddot{I}ƒ$ based on Monte Carlo simulations. Using this candidate period, phase-resolved spectral analysis with the high spectral resolution of Resolve was performed. The spectra suggest that, if the candidate pulsation is real, the Fe K$\hat{I}\pm$ emission line in the pulse peak phase has a larger width ($36_{-13}^{+60}$ eV) than that in the remaining phase at a significance exceeding $3\ddot{I}ƒ$. This suggests that at least a fraction of the Fe K$\hat{I}\pm$ emission is associated with the ULXP system. The observed width corresponds to a velocity dispersion of $(1.7_{-0.6}^{+2.8})\times 10^3$ km s${}^{-1}$, which is too large to be explained by motions in the companion star atmosphere. The rise time of the pulsation constrains the line-emitting region to be smaller than $6.3\times 10^4$ km, suggesting an origin in the accretion flow. This work demonstrates the capability of XRISM Resolve for pulsation-resolved high-resolution spectroscopy of ULX pulsars.

19 pages, 12 figures, accepted for publication in ApJ