The quiescent X-ray spectrum of accreting black holes

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 similar to 570 ks of observations. In contrast to the majority of active galactic nuclei where observations are limited by contamination from diffuse gas, the stellar-mass 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 I = 2.06 +/- 0.03 and kT = 5.03(-0.31)(+0.33) keV, respectively, in the 0.3-10 keV bandpass, at a median luminosity of L-x similar to 5.5 x 10(-7)L(Edd). This result in discussed in the context of our understanding of the accretion flow on to stellar and supermassive black holes at low luminosities.