Broadband X-ray spectra of GX 339-4 and the geometry of accreting black holes in the hard state

A major question in the study of black hole binaries involves our understanding of the accretion geometry when the sources are in the "hard'' state, with an X-ray energy spectrum dominated by a hard power-law component and radio emission coming from a steady "compact'' jet. Although the common hard state picture is that the accretion disk is truncated, perhaps at hundreds of gravitational radii (R(g)) from the black hole, recent results for the recurrent transient GX 339-4 by Miller and coworkers show evidence for disk material very close to the black hole's innermost stable circular orbit. That work studied GX 339-4 at a luminosity of similar to 5% of the Eddington limit (L(Edd)) and used parameters from a relativistic reflection model and the presence of a thermal component as diagnostics. Here we use similar diagnostics but extend the study to lower luminosities (2.3% and 0.8% L(Edd)) using Swift and RXTE observations of GX 339-4. We detect a thermal component with an inner disk temperature of similar to 0.2 keV at 2.3% L(Edd). At both luminosities, we detect broad features due to iron K alpha that are likely related to reflection of hard X-rays off disk material. If these features are broadened by relativistic effects, they indicate that the material resides within 10R(g), and the measurements are consistent with the disk's inner radius remaining at similar to 4R(g) down to 0.8% L(Edd). However, we also discuss an alternative model for the broadening, and we note that the evolution of the thermal component is not entirely consistent with the constant inner radius interpretation.