Distinct Fe K line complexes in MAXI J1744−294 revealed by XRISM high-resolution spectroscopy

Abstract

The newly discovered Galactic transient MAXI J1744−294 went into its first X-ray outburst in 2025. We study the spectral properties of this source in the 2–10 keV energy band during this outburst using X-ray data from the XRISM satellite for both of its Resolve and Xtend instruments, taken on 2025 March 3. High-resolution spectroscopy has revealed, for the first time, complex iron line features in this source, corresponding to distinct components of Fe XXV emission and Fe XXVI absorption lines. Such a detailed structure has not been reported in other low-mass X-ray binaries to date, prior to the XRISM era. Our analysis shows that the line complexes arise from two highly ionized plasmas with an ionization rate ∼103 erg cm s −1 with distinct turbulent velocities—one broad (vturb ≈ 2513 km s −1) from hot gas at the inner accretion disk and one narrow (vturb ≈ 153 km s−1) scattered by nearby photoionized gas. These results offer new insight into the reprocessing of continuum in stratified media, either in the accretion disk or winds, or both, for X-ray binaries in the soft state. The data are well described by models with spin, mass of the black hole, and accretion disk inclination 0.63─0.70, 7.9 ± 2.2 M⊙, and 19°─24°. The fitted spectral model parameters suggest that the source is in the soft spectral state. The source is situated in a crowded field near the Galactic center, resulting in a large hydrogen column density.