  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.
