Inclination-dependent spectral and timing properties in transient black hole X-ray binaries

TL;DR

This study reveals that in transient black hole X-ray binaries, spectral hardness varies with inclination, indicating a flattened corona origin for the power-law emission, with potential state-dependent geometric changes.

Contribution

It introduces a simple parameterisation linking inclination to spectral and timing properties, highlighting the inclination dependence of spectral hardness and the origin of QPOs and broadband noise.

Findings

Harder spectra at higher inclination in certain states.

Broadband noise shape is inclination-independent.

Type C QPOs are inclination-dependent, likely geometric.

Abstract

We use a simple one-dimensional parameterisation of timing properties to show that hard and hard-intermediate state transient black hole X-ray binaries with the same power-spectral shape have systematically harder X-ray power-law emission in higher-inclination systems. We also show that the power-spectral shape and amplitude of the broadband noise (with low-frequency quasi-periodic oscillations, QPOs, removed) is independent of inclination, confirming that it is well-correlated with the intrinsic structure of the emitting regions and that the "type C" QPO, which is inclination-dependent, has a different origin to the noise, probably geometric. Our findings suggest that the power-law emission originates in a corona which is flattened in the plane of the disc, and not in a jet-like structure which would lead to softer spectra at higher inclinations. However, there is tentative evidence that the inclination-dependence of spectral shape breaks down deeper into the hard state. This suggests either a change in the coronal geometry and possible evidence for contribution from jet emission, or alternatively an even more optically thin flow in these states.