Black hole accretion discs in the canonical low-hard state

Abstract

Stellar mass black holes in the low-hard state may hold clues to jet formation and basic accretion disc physics, but the nature of the accretion flow remains uncertain. A standard thin disc can extend close to the innermost stable circular orbit, but the inner disc may evaporate when the mass accretion rate is reduced. Blackbody-like continuum emission and dynamically broadened iron emission lines provide independent means of probing the radial extent of the inner disc. Here, we present an X-ray study of eight black holes in the low-hard state. A thermal-disc continuum with a colour temperature consistent with L ~ T⁴ is clearly detected in all eight sources, down to ~5 × 10^-4L_Edd. In six sources, disc models exclude a truncation radius larger than 10r_g. Iron Kα fluorescence line emission is observed in half of the sample, down to luminosities of ~1.5 × 10^-3L_Edd. Detailed fits to the line profiles exclude a truncated disc in each case. If strong evidence of truncation is defined as (1) a non-detection of a broad iron line and (2) an inner disc temperature much cooler than expected from the L ~ T⁴ relation, none of the spectra in this sample offers strong evidence of disc truncation. This suggests that the inner disc may evaporate at or below ~1.5 × 10^-3L_Edd.