4U 1624-490 as a possible intermediate-mass X-ray binary: constraints from XRISM and NuSTAR spectroscopy
Abstract
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±14 km s−1 and, if interpreted as orbital motion, implies a mass function f(M)=0.83±0.17 M⊙. Residual structure in the blue wing of Fe XXVI suggests additional kinematic complexity near the dip, but the modulation remains consistent when the two dip-adjacent measurements are excluded. If the absorber traces the neutron-star orbital motion, the inferred donor mass is M2=2.66±0.32 M⊙, compatible with a Roche-lobe-filling B9 V-B9.5 V star. Under this interpretation, 4U 1624−490 may be an intermediate-mass rather than a classical low-mass X-ray binary. (Abridged abstract)
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BibTeX
@misc{iaria20261624490,
title = {{4U 1624-490 as a possible intermediate-mass X-ray binary: constraints from XRISM and NuSTAR spectroscopy}},
author = {R. Iaria and A. Anitra and T. Di Salvo and A. Sanna and F. Barra and L. Burderi and C. Maraventano and S. Caserta and W. Leone and C. Miceli},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.28217},
eprint = {2609.28217},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.28217},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - 4U 1624-490 as a possible intermediate-mass X-ray binary: constraints from XRISM and NuSTAR spectroscopy
AU - R. Iaria
AU - A. Anitra
AU - T. Di Salvo
AU - A. Sanna
AU - F. Barra
AU - L. Burderi
AU - C. Maraventano
AU - S. Caserta
AU - W. Leone
AU - C. Miceli
PY - 2026
DA - 2026/09/23
PB - arXiv
DO - 10.48550/arxiv.2609.28217
UR - https://arxiv.org/abs/2609.28217
AB - 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±14 km s^−1 and, if interpreted as orbital motion, implies a mass function f(M)=0.83±0.17 M_⊙. Residual structure in the blue wing of Fe XXVI suggests additional kinematic complexity near the dip, but the modulation remains consistent when the two dip-adjacent measurements are excluded. If the absorber traces the neutron-star orbital motion, the inferred donor mass is M_2=2.66±0.32 M_⊙, compatible with a Roche-lobe-filling B9 V-B9.5 V star. Under this interpretation, 4U 1624−490 may be an intermediate-mass rather than a classical low-mass X-ray binary. (Abridged abstract)
ER -