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

During its 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 of 1.38727 s, at a significance of 3.15 sigma based on Monte Carlo simulations. Using this candidate period, phase-resolved spectral analysis was performed with the high spectral resolution of Resolve. The spectra suggest that, if the candidate pulsation is real, the Fe K alpha emission line during the pulse-peak phase has a larger width ( 36-13+60 eV) than that during the remaining phase, with a significance exceeding 3 sigma. This suggests that at least a fraction of the Fe K alpha emission is associated with the ULXP system. The observed width corresponds to a velocity dispersion of (1.7-0.6+2.8)& times;103 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; 104 km, suggesting an origin in the accretion flow. This work demonstrates the capability of XRISM Resolve for pulsation-resolved high-resolution spectroscopy of ULXPs.