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

Abstract

The origin of accretion disc winds remains disputed to date. High inclination, dipping, neutron star Low Mass X-Ray Binaries (LMXBs) provide an excellent testbed to study the launching mechanism of such winds due to being persistently accreting and showing a nearly ubiquitous presence of highly-ionised plasmas. 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. We leverage the exquisite spectral resolution of XRISM to perform phase-resolved spectroscopy of the full binary orbit to characterise the highly ionised plasma at all phases except during absorption dips. 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 and a column density above 10$^{23}$ cm$^{-2}$. The line profiles are generally narrow, spanning from ~50 to ~100 km/s, depending on the orbital phase, pointing 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. 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.