Observational Constraints upon 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, specifically focusing on a case study of the prototypical system A0620– 00. Such material might be expected to be present theoretically, but is usually claimed to make a negligible contribution at optical and infrared wavelengths. The primary argument is based on measurements of the veiling of stellar photospheric absorption lines, in which it is assumed that the disk spectrum is featureless. We use simulated spectra to explore the sensitivity of veiling measurements to uncertainties in companion temperature, gravity, and metallicity. We find that the derived veiling is extremely sensitive to a mismatch between the temperature and metallicity of the companion and template, but that the effect of a plausible gravity mismatch is much smaller. In general the resulting uncertainty in the amount of veiling is likely to be much larger than the usually quoted statistical uncertainty. We also simulate spectra in which the disk has an emergent spectrum similar to the star and find that in this case, optical veiling constraints are moderately robust. This is because the rotational broadening of the disk is so large that the two line profiles effectively decouple and the measurement of the depth of stellar lines is largely unbiased by the disk component. We note, however, that this is only true at intermediate resolutions or higher, and that significant bias might still affect low resolution IR observations. Assuming that the optical veiling is reliable, we then examine the constraints upon the temperature and covering factor of any optically thick disk component. These are stringent if the disk is warm (Teff & 3500K), but very temperature sensitive, and cooler disks are largely unconstrained by optical measurements. Current IR veiling estimates do not help much, representing rather high upper limits. Probably the best constraint comes from the relative amplitudes of ellipsoidal variations in different bands as these are sensitive to differences in veiling which are expected for disks cooler than the companion star. A significant disk contribution in the IR, up to ∼ 25%, is not ruled out in this or any other way considered, however. Subject headings: accretion, accretion disks—binaries: close