Abstract

We report the results of a simultaneous X-ray and optical spectroscopy campaign on the Galactic black hole X-ray binary (BH XRB) V4641 Sgr, carried out with XRISM and the Seimei telescope during a low-luminosity phase toward the end of its 2024 outburst. Despite a very low X-ray luminosity of 10^34 erg s -1 $, the continuum spectrum is well reproduced by a disk blackbody model with a high inner disk temperature ($1.8 keV). XRISM/Resolve provides the highest resolution X-ray spectrum ever obtained from the source. Several strong, narrow emission lines have been detected, resolved, and characterized at a high significance. The continuum shape and narrow emission lines both indicate that the inner disk region is obscured by the surrounding high-density gas, while the intrinsic luminosity is at least one order of magnitude higher. In the simultaneous optical observation from the Seimei telescope, the line features are largely dominated by the optical companion. Although we detected a clear emission component in Hα that could originate from a cold outflow or the disk atmosphere, there are no signs of the strong outflow signatures historically detected in this source. In X-rays, the combination of significantly redshifted (∼ 700 km s -1 $) and weakly blueshifted (∼-250 km s^-1) components, all varying strongly on kilosecond timescales, along with a marginally significant (99.2%) highly blueshifted (∼-1300 km s$^ -1 ) component, indicating a complex, inhomogeneous outflow geometry. This is corroborated by the erratic long-term evolution of the source seen in the complementary X-ray monitoring, as well as radio detections spanning several orders of magnitude.