Relativistic reflection and reverberation in GX 339-4 with NICER and NuSTAR

TL;DR

This study analyzes NICER and NuSTAR observations of GX 339-4 during its hard state outbursts, revealing how the inner disk radius and reflection features evolve with luminosity, and detecting thermal reverberation lags.

Contribution

It provides detailed spectral-timing analysis of GX 339-4's hard state, highlighting disk truncation behavior and reverberation lags with unprecedented temporal resolution.

Findings

Inner disk radius moves inward with increased flux and ionization.

Hysteresis effect observed in disk truncation during rise and decay.

Detection of thermal reverberation lag at high frequencies.

Abstract

We analyze seven NICER and NuSTAR epochs of the black hole X-ray binary GX 339-4 in the hard state during its two most recent hard-only outbursts in 2017 and 2019. These observations cover the 1-100 keV unabsorbed luminosities between 0.3% and 2.1% of the Eddington limit. With NICER's negligible pile-up, high count rate and unprecedented time resolution, we perform a spectral-timing analysis and spectral modeling using relativistic and distant reflection models. Our spectral fitting shows that as the inner disk radius moves inwards, the thermal disk emission increases in flux and temperature, the disk becomes more highly ionized and the reflection fraction increases. This coincides with the inner disk increasing its radiative efficiency around ~1% Eddington. We see a hint of hysteresis effect at ~0.3% of Eddington: the inner radius is significantly truncated during the rise ($>49R_{g}$), while only a mild truncation ($\sim5R_g$) is found during the decay. At higher frequencies ($2-7$~Hz) in the highest luminosity epoch, a soft lag is present, whose energy dependence reveals a thermal reverberation lag, with an amplitude similar to previous findings for this source. We also discuss the plausibility of the hysteresis effect and the debate of the disk truncation problem in the hard state.