Inclination Dependence of The Time-Lag -- Photon-Index Correlation in BHXRBs and its Explanation with a Simple Jet Model

TL;DR

This paper investigates how the correlation between X-ray time-lag and photon index in black-hole X-ray binaries depends on inclination angle, explaining the findings with a jet model that accounts for spectral and timing differences.

Contribution

It introduces an inclination-dependent analysis of the time-lag and photon index correlation, extending the jet model to explain observed variations and QPO phenomena.

Findings

Correlation varies with inclination angle.

Jet precession explains type-B QPOs.

Quantitative agreement with observations.

Abstract

Recently, we reported an observational correlation between a) the time-lag of the hard (9 - 15 keV) with respect to the soft (2 - 5 keV) X-ray photons in black-hole X-ray binaries (BHXRBs) and b) the power-law photon index $\Gamma$ of the X-ray spectrum. This was physically explained with a simple jet model, i.e., a model where the Comptonization (the Compton upscattering of soft photons) happens in the jet. Here, we report the inclination dependence of this correlation, which we also explain with our jet model. Photons that emerge at different polar angles from the jet axis have different spectra and different time-lags. Because of this, we can explain quantitatively the type-B QPOs of GX 339-4 as resulting from a precessing jet.