Evolution of the accretion disk-corona during bright hard-to-soft state transition: A reflection spectroscopic study with GX 339-4

TL;DR

This study investigates the evolution of the accretion disk and corona during bright state transitions in the black hole binary GX 339-4, revealing that the disk approaches the innermost stable orbit early and remains close during the transition, driven by accretion rate changes.

Contribution

It provides a comprehensive reflection spectroscopic analysis of GX 339-4's state transitions, confirming low disk truncation and highlighting the role of accretion rate variations, with consistent results across different outbursts.

Findings

Disk inner edge approaches ISCO during early bright hard state.

Disk truncation remains low ($ extless$14 Rg) during state transition.

Similar disk evolution observed in different outbursts despite luminosity differences.

Abstract

We present the analysis of several observations of the black hole binary GX 339--4 during its bright intermediate states from two different outbursts (2002 and 2004), as observed by RXTE/PCA. We perform a consistent study of its reflection spectrum by employing the relxill family of relativistic reflection models to probe the evolutionary properties of the accretion disk including the inner disk radius ($R_{\rm in}$), ionization parameter ($\xi$), temperatures of the inner disk ($T_{\rm in}$), corona ($kT_{\rm e}$), and its optical depth ($\tau$). Our analysis indicates that the disk inner edge approaches the inner-most stable circular orbit (ISCO) during the early onset of bright hard state, and that the truncation radius of the disk remains low ($\lesssim 14 R_{\rm g}$) throughout the transition from hard to soft state. This suggests that the changes observed in the accretion disk properties during the state transition are driven by variation in accretion rate, and not necessarily due to changes in the inner disk's radius. We compare the aforementioned disk properties in two different outbursts, with state transitions occurring at dissimilar luminosities, and find identical evolutionary trends in the disk properties, with differences only seen in corona's $kT_{\rm e}$ and $\tau$. We also perform an analysis by employing a self-consistent Comptonized accretion disk model accounting for the scatter of disk photons by the corona, and measure low inner disk truncation radius across the bright intermediate states, using the temperature dependent values of spectral hardening factor, thereby independently confirming our results from the reflection spectrum analysis.