2008ApJ...680..593T

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 ∼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 ∼0.2 keV at 2.3% L_Edd. At both luminosities, we detect broad features due to iron Kα 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 ∼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.