NuSTAR and XMM-Newton observations of the 2015 outburst decay of GX 339-4

Abstract

The extent of the accretion disk in the low/hard state of stellar mass black hole X-ray binaries remains an open question. There are some evidence suggesting that the inner accretion disk is truncated and replaced by a hot flow, while the detection of relativistic broadened iron emission lines seems to require an accretion disk extending fully to the innermost stable circular orbit. We present comprehensive spectral and timing analyses of six Nuclear Spectroscopic Telescope Array (NuSTAR) and XMM-Newton observations of GX 339-4 taken during outburst decay during the autumn of 2015. Using a spectral model consisting of a thermal accretion disk, Comptonized emission, and a relativistic reflection component we obtain a decreasing photon index, consistent with an X-ray binary during outburst decay. Although, we observe a discrepancy in the inner radius of the accretion disk and that of the reflector, which can be addressed to the different underlying assumptions in each model, both model components indicate a truncated accretion disk that resiles with decreasing luminosity. The evolution of the characteristic frequency in Fourier power spectra and their missing energy dependence support the interpretation of a truncated and evolving disk in the hard state. The XMM-Newton dataset allowed us to study, for the first time, the evolution of the covariance spectra and ratio during outburst decay. The covariance ratio increases and steeps during outburst decay, consistent with increased disk instabilities.

18 pages, 8 figures, accepted by ApJ

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• Colors and patterns of black hole X-ray binary GX 339-4
• 2016 outburst of H~1743--322: \textit{XMM-Newton} and \textit{NuSTAR} view
• Detection of low-frequency breaks in power density spectrum of GX 339-4 in faint low/hard state observations using AstroSat data
• A journey from the hard to the soft state: How do QPOs evolve in the 2021 outburst of GX 339-4?
• Searching for quantum-gravity footprint around stellar-mass black holes
• NuSTAR and Swift observations of Swift J1357.2-0933 during an early phase of its 2017 outburst