Properties of the ionised plasma in the vicinity of the neutron-star X-ray binary EXO 0748-676

TL;DR

This study analyzes high-resolution X-ray spectra of the neutron-star binary EXO 0748-676, revealing complex ionised gas properties, including the first detection of collisionally ionised absorption in such a system, and explores the geometry and physical conditions of the absorbing material.

Contribution

It provides the first evidence of collisionally ionised absorption in a low-mass X-ray binary and models the physical properties and geometry of the ionised gas near the neutron star.

Findings

Detection of both collisionally and photoionised absorbers.

Collisionally ionised absorber may be dense filaments at >10^11 cm.

During dips, the photoionised absorber's column density increases.

Abstract

We present the spectral analysis of a large set of XMM-Newton observations of EXO 0748-676, a bright dipping LMXB. In particular, we focus on the dipping phenomenon as a result of changes in the properties of the ionised gas close to the source. Using the high-resolution spectra collected with the RGS, we explored two simple geometrical scenarios for which we derived physical quantities of the absorbing material like the density, size, and mass. We find that the continuum is absorbed by a neutral gas, and by both a collisionally (temperature T~70 eV) and photoionised (ionisation parameter log\xi~2.5) absorbers. Emission lines from OVII and OVIII are also detected. This is the first time that evidence of a collisionally ionised absorber has been found in a low-mass X-ray binary. The collisionally ionised absorber may be in the form of dense (n>10^14 cm^-3) filaments, located at a distance r>10^11 cm. During dips, the photoionised absorber significantly increases its column density (factor 2--4) while becoming less ionised. This strengthens the idea that the colder material of the accretion stream impinging the disc is passing on our line of sight during dips. We find that the distance from the neutron star to the impact region (~ 5x10^10 cm) is similar to the size of the neutron star's Roche lobe. The gas observed during the persistent state may have a flattened geometry. Finally, we explore the possibility of the existence of material forming an initial, hotter portion of a circumbinary disc.