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

TL;DR

This work uses four XMM-Newton EPIC-pn observations of the dipping NS LMXB 4U 1624-490 to map the accretion-disk bulge via time- and flux-resolved spectroscopy. The authors model the spectrum with a direct continuum plus a two-phase local absorber (cold hot$_{loc}$ and ionized xabs) and a dust-scattering halo, revealing a multiphase, clumpy bulge with high column densities and a persistent, highly ionized disk atmosphere that emerges when different absorption phases are analyzed separately. They estimate bulge substructure consisting of >7×10^3 clumps and infer clump sizes around ⪅10^3 km, set against a disk-scale height of ~6×10^6 km and disk radius ~1.1×10^6 km, highlighting complex disk-disk-wind interactions. The results underscore the bulge's potential influence on disk dynamics and suggest that future XRISM observations could resolve the ionization structure and its dependence on system parameters.

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$6 months). We use both time- and flux-resolved spectroscopic analysis to probe the structural properties of the bulge moving along the line of sight and its homogeneity, respectively. During dipping, the primary spectrum is modulated by an ionized (log$ξ\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.

Figures (12)

• Figure 1: Composite 0.3 - 10 keV light curves of the four EPIC pn observations, with binning in time of 5s. The light curves show both persistent and dipping intervals.
• Figure 2: Histogram of the total number of photons counts vs RAWX for the original event file of the full observation (solid line) and for the one from the deep-dipping time interval (dotted line). The defined source region RAWX = [28 -- 48] and background region RAWX = [3 -- 5] are indicated with vertical lines. The plot shows the background contamination by the source emission displayed by the difference in counts around the wings of the distributions. Data shown for obs 301.
• Figure 3: Phase folded light curves of all EPIC pn observations based on the ephemeris from Smale2001 - with phase 0 defined as the centre of the dip. The uncertainty on phase due to error propagation of the ephemeris is displayed as an error bar in the bottom right corner of each plot. All light curves are 0.3 -- 10 keV and have a 5s time binning. In the top panel of each plot the hardness ratios (5 -- 10 keV/1.7 -- 5 keV) for each observation are shown. The average persistent count rate is indicated as a dashed line in each light curve. The time intervals selected for the time-resolved analysis are shown for all observations: clumps$\#$1, clumps$\#$2, shallow-dipping, and persistent (light blue, orange, dark green, and pink colors, respectively - also see Section $\S$\ref{['subsec: time intervals']}). The duration of the deep-dipping interval is shown in all observations (light green). The intensity ranges for the flux-resolved analysis of the deep-dipping in obs 401 are also indicated (a, b, c) - see Section $\S$\ref{['sub: deep dip']}.
• Figure 4: Schematic of our employed spectral model. The sketch illustrates the direct source emission coming from a point in time t, and the halo - delayed source emission from the moment t - $\Delta$t. A legend for the model components (SPEX syntax) used is also shown.
• Figure 5: Spectra from the selected time intervals of obs 601 with the same color-coding as Fig \ref{['fig: lc_fold_int']}. Black solid lines represent the model for each spectrum, and residuals are shown in the bottom panel.