Astrosat view of GX 339-4 during the peak of the recent outburst

TL;DR

This study analyzes AstroSat observations of GX 339-4 during its 2024 outburst peak, revealing spectral and timing features characteristic of the steep power law state, including a high black hole spin and a compact corona.

Contribution

First detailed AstroSat spectral and timing analysis of GX 339-4 during the 2024 outburst peak, estimating black hole spin and coronal geometry.

Findings

Black hole spin estimated at a = 0.99 ± 0.003

Detected strong QPOs at ~4.6 Hz with harmonic features

Coronal height estimated at about 2.5 Rg

Abstract

We present the spectral and timing analyses of \textit{AstroSat} observations of the Black Hole X-ray Binary GX 339-4 when the source was close the peak of the outburst in 2024. We find that both the spectral and timing variability of the source is indicative of it in its steep power law (SPL) state during the observations. We used phenomenological and physical models to understand the physics and geometry of accretion during this spectral state of the source. Spectral fits indicate the presence of an accretion disc with a temperature of $kT\sim$0.82 keV and a hot corona with a spectral index of $\sim$2.2 along with a significant contribution from iron line emission from the accretion disc. Strong QPOs were detected at $\sim$4.6 Hz in the Power Density Spectra of the source along with a harmonics feature. Time and phase lag at the QPO frequency are studied and we find a hard lag at the QPO frequency and at the same time a soft lag at the harmonic frequency. We estimate the spin of the black hole and it was found that $a = 0.99 \pm 0.003$. The height of the coronal region is estimated to be about 2.5 $R_{g}$, which is found to be similar to that observed during the previous outbursts of the source. We attempt to discuss the possible physical scenario for the observed spectral and timing features exhibited by the source.