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 \textit{Chandra/XMM-Newton/Suzaku} era resulting in a sample of 8 black holes with $\sim$ 570 ks of observations. In contrast to the majority of AGN 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 $\mathrm{\Gamma }=2.06\pm 0.03$ and $\mathrm{kT}=5.03_{-0.31}^{+0.33}\mathrm{keV}$ respectively in the 0.3 -- 10 keV bandpass, at a median luminosity of ${\mathrm{L}}_{\mathrm{x}}\sim 5.5\times 10^{-7}{\mathrm{L}}_{\mathrm{Edd}}$. This result in discussed in the context of our understanding of the accretion flow onto stellar and supermassive black holes at low luminosities.

12 pages, 5 figures, 2 tables, MNRAS accepted

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