A Swift survey of accretion onto stellar-mass black holes

TL;DR

This paper provides a comprehensive analysis of 476 Swift X-ray spectra from stellar-mass black hole binaries, revealing disk evolution and spectral states across a wide flux range, including the largest sample of cool accretion disks.

Contribution

It offers the first large-scale, high-quality spectral survey of black hole binaries with simultaneous optical/UV data, expanding understanding of accretion disk properties at low luminosities.

Findings

Accretion disks detected in 61% of observations.

Spectral states are consistent with previous RXTE observations.

Sensitive to disks down to 10^{-3} L_Edd.

Abstract

We present a systemic analysis of all of the stellar mass black hole binaries (confirmed & candidate) observed by the Swift observatory up to June 2010. The broad Swift bandpass enables a trace of disk evolution over an unprecedented range in flux and temperature. The final data sample consists of 476 X-ray spectra containing greater than 100 counts, in the 0.6 -- 10 keV band. This is the largest sample of high quality CCD spectra of accreting black holes published to date. In addition, strictly simultaneous data at optical/UV wavelengths are available for 255 (54%) of these observations. The data are modelled with a combination of an accretion disk and a hard spectral component. For the hard component we consider both a simple power-law and a thermal Comptonization model. An accretion disk is detected at greater than the 5sigma confidence level in 61% of the observations. Lightcurves and color-color diagrams are constructed for each system. Hardness luminosity and disk fraction luminosity diagrams are constructed and are observed to be consistent with those typically observed by RXTE, noting the sensitivity below 2 keV provided by Swift. The observed spectra have an average luminosity of ~ 1% Eddington, though we are sensitive to accretion disks down to a luminosity of 10^{-3} L_Edd. Thus this is also the largest sample of such cool accretion disks studied to date. (abridged)