The quiescent X-ray spectrum of accreting black holes

Abstract

The quiescent state is the dominant accretion mode for black holes on all mass scales. Our knowledge of the X-ray spectrum is limited due to the characteristic low luminosity in this state. Herein, we present an analysis of the sample of dynamically confirmed stellar-mass black holes observed in quiescence in the Chandra/XMM-Newton/Suzaku era resulting in a sample of eight black holes with ~570 ks of observations. In contrast to the majority of active galactic nuclei where observations are limited by contamination from diffuse gas, the stellarmass systems allow for a clean study of the X-ray spectrum resulting from the accretion flow alone. The data are characterized using simple models.We find a model consisting of a power law or thermal bremsstrahlung to both provide excellent descriptions of the data, where we measure {box drawings light down and right} = 2.06 ± 0.03 and kT = 5.03+0.33-0.31 keV, respectively, in the 0.3-10 keV bandpass, at a median luminosity of Lx ~ 5.5 × 10-7LEdd. This result in discussed in the context of our understanding of the accretion flow on to stellar and supermassive black holes at low luminosities. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.