X-ray Reflection Spectroscopy of the Black Hole GX 339-4: Exploring the Hard State with Unprecedented Sensitivity

TL;DR

This study uses high-precision calibration and advanced reflection modeling to analyze multiple spectra of GX 339-4, revealing disk truncation and high-energy cutoff variations across different luminosities in the hard state.

Contribution

It introduces a new calibration tool { t pcacorr} achieving 0.1\% precision and applies it with { t relxill} to obtain detailed measurements of disk properties and high-energy features in GX 339-4.

Findings

Disk becomes slightly truncated at a few percent of Eddington luminosity.

High-energy cutoff varies significantly with luminosity, exceeding 890 keV.

Black hole spin estimated at approximately 0.95 with high confidence.

Abstract

We analyze {\it simultaneously} six composite {\it RXTE} spectra of GX 339--4 in the hard state comprising 77 million counts collected over 196 ks. The source spectra are ordered by luminosity and spanthe range 1.6\% to 17\% of the Eddington luminosity. Crucially, using our new tool {\tt pcacorr}, we re-calibrate the data to a precision of 0.1\%, an order of magnitude improvement over all earlier work. Using our advanced reflection model {\tt relxill}, we target the strong features in the component of emission reflected from the disk, namely, the relativistically-broadened Fe K emission line, the Fe K edge and the Compton hump. We report results for two joint fits to the six spectra: For the first fit, we fix the spin parameter to its maximal value ($a_*=0.998$) and allow the inner disk radius $R_{\rm in}$ to vary. Results include (i) precise measurements of $R_{\rm in}$, with evidence that the disk becomes slightly truncated at a few percent of Eddington; and (ii) an order-of-magnitude swing with luminosity in the high energy cutoff, which reaches $>890$ keV at our lowest luminosity. For the second fit, we make the standard assumption in estimating spin that the inner edge of the accretion disk is located at the innermost stable circular orbit ($R_\mathrm{in} = R_\mathrm{ISCO}$) and find $a_* = 0.95^{+0.03}_{-0.05}$ (90\% confidence, statistical). For both fits, and at the same level of statistical confidence, we estimate that the disk inclination is $i = 48\pm 1$ deg and that the Fe abundance is super-solar, $A_\mathrm{Fe} = 5\pm1$.