# Inclination effects on the X-ray emission of Galactic black-hole binaries

## TL;DR

This study investigates how the inclination angle of Galactic black-hole X-ray binaries affects the correlation between X-ray time lags and spectral properties, revealing that the correlation is stronger in systems viewed face-on.

## Contribution

It demonstrates the inclination dependence of X-ray timing and spectral correlations and introduces a jet model explaining these observations.

## Key findings

- Correlation is tight in low-inclination systems.
- Amplitude of lags is larger at low and intermediate inclinations.
- Jet model successfully reproduces observed behaviors.

## Abstract

Galactic black-hole X-ray binaries (BHBs) emit a compact, optically thick, mildly relativistic radio jet when they are in the hard and hard-intermediate states. In these states, BHBs exhibit a correlation between the time lag of hard photons with respect to softer ones and the photon index of the power law component that characterizes the X-ray spectral continuum above $\sim$ 10 keV. The correlation, however, shows large scatter. Our objective is to investigate the role that the inclination of the system plays on the correlation between the time lag and the photon index. We find that the correlation between the time lag and the photon index is tight in low-inclination systems and becomes weaker in high-inclination systems. The amplitude of the lags is also larger at low and intermediate inclination angles than at high inclination. Our jet model that reproduces the process of Comptonization in an extended jet can account for the observations remarkably well.