The clumpy absorber in the high-mass X-ray binary Vela X-1

TL;DR

This study uses high-resolution X-ray spectroscopy of Vela X-1 to reveal complex, highly variable absorption and multi-phase gas near the neutron star, challenging simple wind models and supporting structured accretion flow scenarios.

Contribution

It provides new observational evidence of variable, multi-phase accretion structures in Vela X-1, advancing understanding of wind accretion in high-mass X-ray binaries.

Findings

Detection of variable absorption features from multiple ionization states.

Evidence for co-existing cool and hot gas phases near the neutron star.

Observation of spectral variability on kilosecond timescales.

Abstract

Bright and eclipsing, the high-mass X-ray binary Vela X-1 offers a unique opportunity to study accretion onto a neutron star from clumpy winds of O/B stars and to disentangle the complex accretion geometry of these systems. In Chandra-HETGS spectroscopy at orbital phase ~0.25, when our line of sight towards the source does not pass through the large-scale accretion structure such as the accretion wake, we observe changes in overall spectral shape on timescales of a few kiloseconds. This spectral variability is, at least in part, caused by changes in overall absorption and we show that such strongly variable absorption cannot be caused by unperturbed clumpy winds of O/B stars. We detect line features from high and low ionization species of silicon, magnesium and neon whose strengths and presence depend on the overall level of absorption. They imply a co-existence of cool and hot gas phases in the system that we interpret as a highly variable, structured accretion flow close to the compact object such as has been recently seen in simulations of wind accretion in high-mass X-ray binaries.