A highly ionised outflow in the X-ray binary 4U 1624–49 detected with XRISM

¹ ESO, Karl-Schwarzschild-Strasse 2, 85748, Garching bei München, Germany
² SRON Space Research Organisation Netherlands, Niels Bohrweg 4, NL-2333, CA Leiden, The Netherlands
³ Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), Kanagawa, 252-5210, Japan
⁴ Faculty of Science and Technology, Tokyo University of Science, Chiba, 278-8510, Japan
⁵ Department of Physics, Ehime University, Ehime, 790-8577, Japan
⁶ Villanova University, Department of Physics, Villanova, PA, 19085, USA
⁷ NASA/Goddard Space Flight Center, Greenbelt, MD, 20771, USA
⁸ Center for Research and Exploration in Space Science and Technology, NASA/GSFC (CRESST II), Greenbelt, MD, 20771, USA
⁹ Center for Space Science and Technology, University of Maryland, Baltimore County (UMBC), 1000 Hilltop Circle, Baltimore, MD, 21250, USA
¹⁰ Department of Astronomy, University of Michigan, MI, 48109, USA
¹¹ Anton Pannekoek Institute/GRAPPA, University of Amsterdam, Science Park 904, 1098, XH Amsterdam, The Netherlands

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 2 December 2025
Accepted: 22 January 2026

Abstract

Context. The origin of accretion-disc winds remains disputed. High-inclination, dipping, neutron-star (NS) low-mass X-ray binaries (LMXBs) provide an excellent testbed for studying the launching mechanism of such winds due to them persistently accreting and showing a nearly ubiquitous presence of highly ionised plasmas.

Aims. We aim to establish or rule out the presence of a wind in the high-inclination LMXB 4U 1624−49, for which a highly ionised plasma has been repeatedly observed in X-ray spectra by Chandra and XMM-Newton, and a thermal–radiative pressure wind is expected.

Methods. We leveraged the exquisite spectral resolution of the X-ray Imaging and Spectroscopy Mission (XRISM) to perform phase-resolved spectroscopy of the full binary orbit to characterise the highly ionised plasma at all phases except during absorption dips.

Results. An outflow is clearly detected via phase-resolved spectroscopy of the source with XRISM Resolve. Based on analysis of the radial-velocity curve, we determine an average velocity of ∼200−320 km s⁻¹ and a column density above 10²³ cm⁻². The line profiles are generally narrow, spanning ∼50−100 km s⁻¹, depending on the orbital phase; this points to a low-velocity sheer or turbulence of the highly ionised outflow and a potential increase of turbulence as the absorption dip is approached, likely due to turbulent mixing.

Conclusions. The line profiles, together with the derived launching radius and wind velocity, are consistent with a wind being launched from the outskirts of the disc and without stratification, pointing to a thermal-radiative pressure origin.