4U 1624-490 as a possible intermediate-mass X-ray binary: constraints from XRISM and NuSTAR spectroscopy — E8 Intelligence Research

High-inclination dipping neutron-star X-ray binaries provide a direct view of accretion-flow structure because the line of sight intercepts ionised plasma above the disc. We analyse two XRISM/Resolve observations of 4U 1624$-$490 with simultaneous NuSTAR coverage, dividing the data into seven orbital-phase intervals. Phase-resolved fits include neutral partial covering, a photoionised absorber, thermal Comptonisation, and ionised disc reflection. Fe XXV and Fe XXVI absorption are detected over most of the orbit. The absorber column density, ionisation state, covering fraction, and projected velocity vary with orbital phase and the lines are systematically blueshifted, with line-of-sight velocities of about $85$-$440\ {\rm km\,s^{-1}}$. Absorption-only models leave broad Fe-K residuals, while ionised reflection improves the fits. The velocity modulation has a semi-amplitude $K=210\pm14\ {\rm km\,s^{-1}}$ and, if interpreted as orbital motion, implies a mass function $f(M)=0.83\pm0.17\,M Author: Andrew Stewart Caldin, Independent Researcher, UK. Part of the E8 Intelligence Research series. Platform: e8intelligence.com