Modelling the disk atmosphere of the low mass X-ray binary EXO 0748-676

TL;DR

This study analyzes high-resolution X-ray spectra of the low mass X-ray binary EXO 0748-676 during eclipses, revealing a complex, multi-component ionized disk atmosphere with high-density plasma and inflow features.

Contribution

It provides the first detailed modeling of the extended disk atmosphere in EXO 0748-676, identifying two distinct photoionization components with different properties.

Findings

Detection of two photoionization components with different ionization parameters.

High-density plasma (>10^13 cm^-3) in the lower ionization component.

Evidence of inflow velocity in the lower ionization gas.

Abstract

Low mass X-ray binaries exhibit ionized emission from an extended disk atmosphere that surrounds the accretion disk. However, its nature and geometry is still unclear. In this work we present a spectral analysis of the extended atmosphere of EXO 0748-676 using high-resolution spectra from archival XMM-Newton observations. We model the RGS spectrum that is obtained during the eclipses. This enables us to model the emission lines that come only from the extended atmosphere of the source, and study its physical structure and properties. The RGS spectrum reveals a series of emission lines consistent with transitions of O VIII, O VII, Ne IX and N VII. We perform both Gaussian line fitting and photoionization modelling. Our results suggest that there are two photoionization gas components, out of pressure equilibrium with respect to each other. One with ionization parameter of 2.5 and a large opening angle, and one with 1.3. The second component is possibly covering a smaller fraction of the source. From the density diagnostics of the O vii triplet using photoionization modelling, we detect a rather high density plasma of > 10^13 cm^-3 for the lower ionization component. This latter component also displays an inflow velocity. We propose a scenario where the high ionization component constitutes an extended upper atmosphere of the accretion disk. The lower ionization component may instead be a clumpy gas created from the impact of the accretion stream with the disk.