The Quiescent X-ray Spectrum of Accreting Black Holes

TL;DR

This study analyzes the X-ray spectra of eight quiescent stellar-mass black holes using data from Chandra, XMM-Newton, and Suzaku, revealing consistent spectral properties and improving understanding of low-luminosity accretion flows.

Contribution

It provides the first comprehensive analysis of quiescent X-ray spectra from a sample of dynamically-confirmed stellar-mass black holes, using simple models to characterize their emission.

Findings

X-ray spectra are well described by power-law or thermal bremsstrahlung models.

Measured photon index b1 0.03 is 2.06, and plasma temperature is b1 0.33 keV.

Median luminosity is approximately 5.5d7 10^{-7} L_{Edd}.

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 $\rm \Gamma = 2.06 \pm 0.03$ and $\rm kT = 5.03^{+0.33}_{-0.31} keV$ respectively in the 0.3 -- 10 keV bandpass, at a median luminosity of $\rm L_x \sim 5.5\times10^{-7} L_{Edd}$. This result in discussed in the context of our understanding of the accretion flow onto stellar and supermassive black holes at low luminosities.