Observational Constraints on Cool Disk Material in Quiescent Black Hole Binaries

We consider current observational constraints on the presence of cool,optically thick disk material in quiescent black hole binaries, specificallyfocusing on a case study of the prototypical system A0620-00. Such materialmight be expected to be present theoretically, but is usually claimed to make anegligible contribution at optical and infrared wavelengths. The primaryargument is based on measurements of the veiling of stellar photosphericabsorption lines, in which it is assumed that the disk spectrum is featureless.We use simulated spectra to explore the sensitivity of veiling measurements touncertainties in companion temperature, gravity, and metallicity. We find thatthe derived veiling is extremely sensitive to a mismatch between thetemperature and metallicity of the companion and template, but that the effectof a plausible gravity mismatch is much smaller. In general the resultinguncertainty in the amount of veiling is likely to be much larger than theusually quoted statistical uncertainty. We also simulate spectra in which thedisk has an emergent spectrum similar to the star and find that in this case,optical veiling constraints are moderately robust. This is because therotational broadening of the disk is so large that the two line profileseffectively decouple and the measurement of the depth of stellar lines islargely unbiased by the disk component. We note, however, that this is onlytrue at intermediate resolutions or higher, and that significant bias mightstill affect low resolution IR observations. [Abridged]Comment: 9 pages using emulateapj. To appear in The Astrophysical Journal (in press