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We report on the spectroscopic analysis of the black hole binary GX 339-4during its recent 2017-2018 outburst, observed simultaneously by the Swift andNuSTAR observatories. Although during this particular outburst the sourcefailed to make state transitions, and despite Sun constraints during the peakluminosity, we were able to trigger four different observations sampling theevolution of the source in the hard state. We show that even for the lowestluminosity observations the NuSTAR spectra show clear signatures of X-rayreprocessing (reflection) in an accretion disk. Detailed analysis of thehighest signal-to-noise spectra with our family of relativistic reflectionmodels RELXILL indicates the presence of both broad and narrow reflectioncomponents. We find that a dual-lamppost model provides a superior fit whencompared to the standard single lamppost plus distant neutral reflection. Inthe dual lamppost model two sources at different heights are placed on therotational axis of the black hole, suggesting that the narrow component of theFe K emission is likely to originate in regions far away in the disk, but stillsignificantly affected by its rotational motions. Regardless of the geometryassumed, we find that the inner edge of the accretion disk reaches a fewgravitational radii in all our fits, consistent with previous determinations atsimilar luminosity levels. This confirms a very low degree of disk truncationfor this source at luminosities above ~1% Eddington. Our estimates of Rinreinforces the suggested behavior for an inner disk that approaches theinner-most regions as the luminosity increases in the hard state.

The 2017 Failed Outburst of GX 339-4: Relativistic X-ray Reflection near the Black Hole Revealed by NuSTAR and Swift Spectroscopy