The current state of disk wind observations in BHLMXBs through X-ray absorption lines in the iron band

TL;DR

This study reviews X-ray absorption line observations of winds in Black Hole Low-Mass X-ray Binaries, confirming their occurrence mainly in high-inclination, bright soft states, and provides a comprehensive dataset and analysis tools for future research.

Contribution

It offers an updated, systematic analysis of iron band absorption lines across all available data, highlighting consistent wind properties and the need for better outburst sampling.

Findings

Winds are detected mainly in high-inclination, bright soft states.

Blueshift velocities are generally a few hundred km/s.

Few sources have sufficient data to study wind evolution during outbursts.

Abstract

The presence of blueshifted absorption lines in the X-ray spectra of Black Hole Low Mass X-ray Binaries is the telltale of massive outflows called winds. These signatures are found almost exclusively in soft states of high-inclined systems, hinting at equatorial ejections originating from the accretion disk and deeply intertwined with the evolution of the outburst patterns displayed by these systems. In the wake of the launch of the new generation of X-ray spectrometers, studies of wind signatures remain mostly restricted to single sources and outbursts, with some of the recent detections departing from the commonly expected behaviors. We thus give an update to the current state of iron band absorption lines detections, through the analysis of all publicly available XMM-$Newton$-PN and $Chandra$-HETG exposures of known Black Hole Low-Mass X-ray Binary candidates. Our results agree with previous studies, with wind detections exclusively found in dipping, high-inclined sources, and almost exclusively in bright ($L_{X}>0.01L_{Edd}$) soft ($HR<0.8$) states, with blueshift values generally restricted to few 100 km s$^{-1}$. The line parameters indicate similar properties between objects and outbursts of single sources, and despite more than 20 years of data, very few sources have the HID sampling necessary to properly study the evolution of the wind during single outbursts. We provide an online tool with details of the wind signatures and outburst evolution data for all sources in the sample.