4U 1624-490 as a possible intermediate-mass X-ray binary: Constraints from XRISM and NuSTAR spectroscopy
Year: 2026
Type: article
Abstract: High-inclination dipping neutron-star X-ray binaries provide a unique view of accretion-flow structure because the line of sight intercepts the outer-disc absorber and ionised plasma above the disc. Their orbital variability is generally dominated by absorption, although disc reflection may also contribute. We investigate the orbital evolution of the ionised absorber and reflection component in 4U 1624-490 to constrain the kinematics and nature of its highly ionised plasma. We analysed two XRISM/Resolve observations with simultaneous NuSTAR coverage. The data were divided into seven orbital-phase intervals covering persistent and dipping states. We performed phase-resolved fits including neutral partial covering, a photoionised absorber, thermal Comptonisation, and ionised disc reflection. xxv and Fe, xxvi absorption features are detected over most of the orbit. The absorber column density, ionisation state, covering fraction, and projected velocity vary with orbital phase in agreement with previous results. It is systematically blueshifted, with line-of-sight velocities of approximately 85--440 -1 , consistent with a structured outflow or vertically extended plasma. Absorption-only models leave broad Fe--K residuals, whereas ionised reflection systematically improves the fits and varies in visibility along the orbit. The velocity modulation has a semi-amplitude of K=210 and, under an orbital interpretation, implies a mass function of f(M)=0.83 -1 The orbital spectral variability therefore requires the combined effects of ionised absorption, Comptonised emission, and disc reflection. Residual xxvi may indicate additional kinematic complexity close to the dip; nevertheless, the velocity modulation and its fitted parameters remain consistent when the two dip-adjacent measurements are excluded. If this modulation traces the neutron-star orbital motion, the inferred donor mass, M_2=2.66 is compatible with a Roche-lobe-filling B9 V--B9.5 V star. Under this interpretation, 4U 1624-490 may be an intermediate-mass X-ray binary rather than a classical low-mass one.
Source: Astronomy and Astrophysics
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
OpenAlex ID: W7214162899