High ionisation absorption in low mass X-ray binaries

TL;DR

This paper reviews how new X-ray telescopes have advanced understanding of ionised outflows in low mass X-ray binaries, revealing their role in accretion and their state-dependent presence.

Contribution

It provides a comprehensive overview of the current knowledge on ionised absorption features and their connection to accretion states in X-ray binaries.

Findings

Ionised outflows are common in soft states but absent in hard states.

These outflows can carry away more matter than is accreted.

Absorption features are signatures of equatorial outflows impacting accretion processes.

Abstract

The advent of the new generation of X-ray telescopes yielded a significant step forward in our understanding of ionised absorption generated in the accretion discs of X-ray binaries. It has become evident that these relatively weak and narrow absorption features, sporadically present in the X-ray spectra of some systems, are actually the signature of equatorial outflows, which might carry away more matter than that being accreted. Therefore, they play a major role in the accretion phenomenon. These outflows (or ionised atmospheres) are ubiquitous during the softer states but absent during the power-law dominated, hard states, suggesting a strong link with the state of the inner accretion disc, presence of the radio-jet and the properties of the central source. Here, we discuss the current understanding of this field.