Abstract: We present a detailed spectral analysis of XMM-Newton and NuSTAR observations of the accreting transient black hole GRS?1739-278 during a very faint low hard state at ~0.02% of the Eddington luminosity (for a distance of 8.5 kpc and a mass of 10 solar mass). The broadband X-ray spectrum between 0.5 and 60 keV can be well-described by a power-law continuum with an exponential cutoff. The continuum is unusually hard for such a low luminosity, with a photon index of G=1.39+/-0.04. We find eviden…