Accretion Geometry of GX 339-4 in the Hard State: AstroSat View

TL;DR

This study uses AstroSat data to analyze the accretion geometry of GX 339-4 in the hard state, revealing a truncated disk that evolves with luminosity and confirming the consistency between timing and spectral estimates of the inner disk radius.

Contribution

It provides the first broadband spectral and timing analysis of GX 339-4 in the hard state during multiple outbursts, demonstrating the truncated disk geometry and its evolution with luminosity.

Findings

The disk remains substantially truncated at 2-8% Eddington luminosity.

The inner disk radius decreases with increasing luminosity.

Break frequency in power spectra correlates with luminosity and inner disk radius.

Abstract

We perform broadband ($0.7-100$ keV) spectral analysis of five hard state observations of the low-mass back hole X-ray binary GX~339--4 taken by AstroSat during the rising phase of three outbursts from $2019$ to $2022$. We find that the outburst in 2021 was the only successful/full outburst, while the source was unable to make transition to the soft state during the other two outbursts in 2019 and 2022. Our spectral analysis employs two different model combinations, requiring two separate Comptonizing regions and their associated reflection components, and soft X-ray excess emission. The harder Comptonizing component dominates the overall bolometric luminosity, while the softer one remains relatively weak. Our spectral fits indicate that the disk evolves with the source luminosity, where the inner disk radius decreases with increasing luminosity. However, the disk remains substantially truncated throughout all the observations at the source luminosity of $\sim2-8\%\times$ of the Eddington luminosity. We note that our assumption of the soft X-ray excess emission as disk blackbody may not be realistic, and this kind of soft excess may arise due the non-homogeneity in the disk/corona geometry. Our temporal analysis deriving the power density spectra suggests that the break frequency increases with the source luminosity. Furthermore, our analysis demonstrates a consistency between the inner disk radii estimated from break frequency of the power density spectra and those obtained from the reflection modelling, supporting the truncated disk geometry in the hard state.