Resolving a Clumpy Disk Wind in 4U 1957+115 with XRISM

Abstract

We present a high-resolution XRISM/Resolve observation of the persistent black hole candidate 4U 1957+115, revealing the first detection of an intrinsic disk wind in this source. The absorption is dominated by highly ionized Fe XXV and Fe XXVI and varies substantially throughout the observation, becoming prominent during the final $\sim 40$ ks despite a modest decrease of only $\sim 5$--$10\%$ in source flux. The observed evolution of the absorption cannot be explained solely by ionization changes driven by the modest continuum variability, favoring an inhomogeneous outflow with evolving line-of-sight properties. Time-resolved spectroscopy reveals substantially narrower absorption features in shorter intervals, with the inferred turbulent velocity decreasing from $\sim 350km/s$ in the longer integration to $\lesssim 70km/s$ in the shortest selections. This suggests that the broader absorption profiles measured over longer integrations may result, at least in part, from averaging over an evolving wind rather than from intrinsically larger turbulent velocities. Such time averaging can therefore affect the inferred physical properties of the wind and complicate efforts to identify its launching mechanism. Even though the wind carries only a small fraction of the radiative luminosity in kinetic form, its mass-loss rate may remain dynamically relevant. These results highlight the importance of time-resolved, high-resolution X-ray spectroscopy for characterizing the structure and variability of accretion disk winds.

arXiv comment13 pages plus appendices. 10 figures, 3 tables. Submitted for publication to ApJL