An XMM long look at the accretion disk plasma in the dipping neutron star LMXB 4U1624-490

Abstract

Dipping neutron star low-mass X-ray binaries (NS LMXBs) are systems that exhibit periodic drops in their X-ray light curves. These are believed 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. We aimed to characterize the accretion disk plasmas present in the 21h-period NS LMXB 4U 1624-490, and perform a detailed spectral analysis of the material present at the impact region. We used four XMM EPIC pn observations that were specifically targeting dips, and allow us to probe dipping activity over different timescales (i.e. consecutive orbits and $\sim$6 months). We use both time- and flux-resolved spectroscopic analysis to probe the structural properties of the bulge moving along the line of sight and its homogeneity, respectively. During dipping, the primary spectrum is modulated by an ionized (log$\xi\sim$ 3.4) absorber with varying column density and covering factor, as well as 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 $>$7$\times10^{3}$. A highly ionized disk atmosphere becomes evident only when different phases of absorption are analyzed individually. This work demonstrates the feasibility of constructing a physical picture of the bulge, and highlights how future research could reveal how its properties depend on system parameters, and whether the bulge could influence the dynamics of the accretion disk.