Black hole accretion disks in the canonical low-hard state

TL;DR

This study investigates the inner accretion disks of stellar-mass black holes in the low-hard state, finding evidence that the disks extend close to the black hole without significant truncation down to low luminosities.

Contribution

It provides observational evidence that the inner accretion disk remains close to the black hole in the low-hard state, challenging models that predict disk truncation at low accretion rates.

Findings

Thermal disk emission detected down to 5×10^{-4} L_Edd.

Iron line profiles indicate non-truncated disks in all cases.

No strong evidence of disk truncation at luminosities above 1.5×10^{-3} L_Edd.

Abstract

Stellar-mass black holes in the low-hard state may hold clues to jet formation and basic accretion disk physics, but the nature of the accretion flow remains uncertain. A standard thin disk can extend close to the innermost stable circular orbit, but the inner disk 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 disk. Here, we present an X-ray study of eight black holes in the low-hard state. A thermal disk continuum with a colour temperature consistent with $L \propto T^{4}$ is clearly detected in all eight sources, down to $\approx5\times10^{-4}L_{Edd}$. In six sources, disk models exclude a truncation radius larger than 10rg. Iron-ka fluorescence line emission is observed in half of the sample, down to luminosities of $\approx1.5\times10^{-3}L_{Edd}$. Detailed fits to the line profiles exclude a truncated disk in each case. If strong evidence of truncation is defined as (1) a non-detection of a broad iron line, {\it and} (2) an inner disk temperature much cooler than expected from the ${\rm L} \propto {\rm T}^{4}$ relation, none of the spectra in this sample offer strong evidence of disk truncation. This suggests that the inner disk may evaporate at or below $\approx1.5\times10^{-3}L_{Edd}$.