Modelling the Energy-dependent broadband variability in the black-hole transient GX 339-4 using Astrosat and NICER

TL;DR

This study conducts a detailed spectro-timing analysis of the black hole transient GX 339-4 during its 2021 outburst, revealing energy-dependent variability and lag features in the low/hard state using Astrosat and NICER data.

Contribution

It introduces a comprehensive model linking accretion rate, disc radius, and coronal heating to observed variability and delays, advancing understanding of black hole accretion physics.

Findings

Detection of two broad PDS features at 0.22 Hz and 2.94 Hz.

Observation of hard lags and decreasing variability with energy.

Correlation of accretion parameters with lag and rms spectra.

Abstract

We present a spectro-timing analysis of the black hole X-ray transient GX 339-4 using simultaneous observations from Astrosat and NICER during the 2021 outburst period. The combined spectrum obtained from NICER, LAXPC, and SXT data is effectively described by a model comprising a thermal disk component, hard Comptonization component, and reflection component with an edge. Our analysis of the Astrosat and NICER spectra indicates the source to be in a low/hard state, with a photon index of ~1.64. The Power Density Spectra (PDS) obtained from both Astrosat and NICER observations exhibit two prominent broad features at 0.22 Hz and 2.94 Hz. We generated energy-dependent time lag and fractional root mean square (frms) at both frequencies in a broad energy range of 0.5-30 keV and found the presence of hard lags along with a decrease in variability at higher energy levels. Additionally, we discovered that the correlated variations in accretion rate, inner disc radius, coronal heating rate, and the scattering fraction, along with a delay between them, can explain the observed frms and lag spectra for both features.