An XMM-Newton long look at the accretion disk plasma in the dipping neutron star LMXB 4U 1624–490

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam Science Park 904 NL-1098 XH Amsterdam, The Netherlands
² SRON Space Research Institute Netherlands Niels Bohrweg 4 NL-2333 CA Leiden, The Netherlands
³ ESO Karl-Schwarzschild-Strasse 2 D-85748 Garching bei München, Germany

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

Received: 25 June 2025
Accepted: 2 October 2025

Abstract

Context. Dipping neutron star low-mass X-ray binaries (NS LMXBs) are systems that exhibit periodic drops in their X-ray light curves. These are thought to be caused by material at the impact point of the gas stream onto the accretion disk, the bulge. Dipping systems are observed at high inclination and provide exceptional opportunities to address important open questions about accretion disks, such as the physical properties of the bulge, and the connection between disk atmospheres and disk winds.

Aims. We characterize the accretion disk plasmas in the 21-hour-period NS LMXB 4U 1624–490 and performed a detailed spectral analysis of the material in the impact region.

Methods. We used four XMM-Newton EPIC pn observations that specifically targeted dips and probed the dipping activity on different timescales (i.e., consecutive orbits and ∼6 months). We used a time- and flux-resolved spectroscopic analysis to probe the structural properties of the bulge moving along the line of sight and its homogeneity, respectively.

Results. During dipping, the primary spectrum is modulated by an ionized (log ξ ∼ 3.4) absorber with a varying column density and covering factor, and a colder absorber. This suggests that the bulge is a multiphase and clumpy absorbing medium. From size-scale arguments, we estimate the number of clumps in the bulge to be > 7 × 10³. A highly ionized disk atmosphere only becomes evident when different absorption phases are analyzed individually. We show that a physical picture of the bulge can be constructed, and we highlight that future research might reveal the dependence of its properties on the system parameters and determine whether the bulge might affect the dynamics of the accretion disk.