New Constraints on the Geometry and Kinematics of Matter Surrounding the Accretion Flow in X-ray Binaries from $Chandra$ HETG X-ray Spectroscopy

TL;DR

This study reanalyzes Chandra HETG spectra of X-ray binaries to better understand the geometry, kinematics, and Fe abundance of matter around accretion flows, challenging previous assumptions of uniform spherical distributions.

Contribution

It introduces a self-consistent model for the X-ray reprocessor and tests it against a large sample, revealing variable Fe abundances and complex geometries not previously identified.

Findings

Most sources require non-solar Fe abundance.

Spherical distribution models are generally inconsistent with data.

Fe Kα line widths up to 5000 km/s were measured.

Abstract

The narrow, neutral Fe K{\alpha} fluorescence emission line in X-ray binaries (XRBs) is a powerful probe of the geometry, kinematics and Fe abundance of matter around the accretion flow. In a recent study it has been claimed, using $Chandra$ High-Energy Transmission Grating (HETG) spectra for a sample of XRBs, that the circumnuclear material is consistent with a solar-abundance, uniform, spherical distribution. It was also claimed that the Fe K{\alpha} line was unresolved in all cases by the HETG. However, these conclusions were based on ad hoc models that did not attempt to relate the global column density to the Fe K{\alpha} line emission. We revisit the sample and test a self-consistent model of a uniform, spherical X-ray reprocessor against HETG spectra from 56 observations of 14 Galactic XRBs. We find that the model is ruled out in 13/14 sources because a variable Fe abundance is required. In 2 sources a spherical distribution is viable but with non-solar Fe abundance. We also applied a solar-abundance Compton-thick reflection model, which can account for the spectra that are inconsistent with a spherical model, but spectra with a broader bandpass are required to better constrain model parameters. We also robustly measured the velocity width of the Fe K{\alpha} line and found full width half maximum values up to ~5000 km s$^{-1}$. Only in some spectra was the Fe K{\alpha} line unresolved by the HETG.