  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 M_sun ). The broad-band X-ray spectrum between 0.5-60 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
Gamma = 1.39 +/- 0.04. We find evidence for an additional reflection component
from an optically thick accretion disk at the 98% likelihood level. The
reflection fraction is low with R_refl = 0.043(+0.033,-0.023). In combination
with measurements of the spin and inclination parameters made with NuSTAR
during a brighter hard state by Miller and co-workers, we seek to constrain the
accretion disk geometry. Depending on the assumed emissivity profile of the
accretion disk, we find a truncation radius of 15-35 Rg (5-12 R_ISCO ) at the
90% confidence limit. These values depend strongly on the assumptions and we
discuss possible systematic uncertainties.
