The Optical and Ultraviolet Spectral Energy Distributions of Short Period Black Hole X-ray Transients in Outburst

We compile optical and UV spectra of a sample of 'typical' short period black hole X-ray transients in outburst. We also survey determinations of interstellar extinction and distance in order to deredden spectra and compare absolute fluxes. Hence we perform a comparative study of the broad-band spectral energy distributions (SED). We find that given such a homogeneous sample of typical sources, the optical SEDs form a relatively uniform set, all exhibiting quasi-power-law spectra with Fnu proportional to nu^alpha, where 0.5<alpha<1.5 (steeper than the canonical nu^1/3 disk spectrum). All become flatter in the UV, although there is more diversity here. The SEDs studied can be broadly divided into two optical-UV spectral states. The UV-hard spectra, e.g. A0620-00 and X-ray Nova Muscae 1991, continue to rise in the far-UV. The UV-soft spectra, e.g. GRO J0422+32, drop off. XTE J1859+226 evolved from UV-soft to UV-hard as it decayed indicating that this effect is a real difference, not a dereddening artifact. All of the spectra can be fitted by a generalized black body disk model with two forms of heating, resulting in the two states. The UV-soft state is consistent with a disk illuminated by a central point source, with irradiative heating dominating over viscous. The UV-hard state is well described by a viscously heated disk, although this requires very high mass flow rates in the case of Nova Muscae. Alternatively, a UV-hard spectrum can be produced if the disk is illuminated by a vertically extended X-ray source such as a central scattering corona or jet. [Abridged]