XRISM reveals a variable, multi-phase outflow-inflow structure during the 2024 X-ray obscured outburst of black hole transient V4641 Sgr

¹ Department of Physics, Ehime University, 2-5, Bunkyocho, Matsuyama, Ehime 790-8577, Japan
² Department of Earth and Space Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan
³ Centre for Extragalactic Astronomy, Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK
⁴ Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain
⁵ Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
⁶ Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), Kanagawa 252-5210, Japan
⁷ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, E-08193 Barcelona, Spain
⁸ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels (Barcelona), Spain
⁹ INAF/IASF Palermo, Via Ugo La Malfa 153, I-90146 Palermo, Italy
¹⁰ Université Paris Cité and Université Paris Saclay, CEA, CNRS, AIM, F-91190 Gif-sur-Yvette, France
¹¹ Departamento de Física, CUCEI, Universidad de Guadalajara, 44430 Guadalajara, Jalisco, Mexico
¹² Doctorado en Ciencias Fisico-Matematicas, CUValles, Universidad de Guadalajara, Carretera Guadalajara–Ameca km. 45.5, 46600 Ameca, Jalisco, Mexico
¹³ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Beijing 100101, China
¹⁴ ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
¹⁵ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹⁶ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
¹⁷ Okayama Observatory, Kyoto University, 3037-5 Honjo, Kamogatacho, Asakuchi, Okayama 719-0232, Japan
¹⁸ Department of Multi-Disciplinary Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
¹⁹ Department of Physics, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
²⁰ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, (LC), Italy
²¹ Department of Physics, Nihon University, 1-8 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8308, Japan
²² University of Manitoba, Winnipeg, MB R3T 2N2, Canada
²³ Institute for Cosmic Ray Research, University of Tokyo, 5-1-5, Kashiwa-no-ha, Kashiwa, Chiba 277-8582, Japan
²⁴ Interdisciplinary Theoretical & Mathematical Science Center (iTHEMS), RIKEN, 2-1, Hirosawa, Wako, Saitama 351-0198, Japan
²⁵ Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto, Kyoto 606-8502, Japan
²⁶ Department of Physics, Missouri University of Science and Technology, Rolla, MO, USA
²⁷ Center for Astrophysics, Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
²⁸ Kapteyn Astronomical Institute, University of Groningen, P.O. BOX 800, 9700 AV, Groningen, The Netherlands
²⁹ Astrophysics, Department of Physics, University of Oxford, Oxford OX1 3RH, UK

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Received: 26 August 2025
Accepted: 22 February 2026

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 10³⁴ 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.