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 to deredden spectra and compare absolute fluxes. Hence, we performa comparative study of the broadband spectral energy distributions (SEDs). 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 F-nu proportional to nu(alpha), where 0.5 less than or similar to alpha less than or similar to 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 GS 1124 - 684, 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 the spectra can be fitted by a generalized blackbody 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 GS 1124 - 684. 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. Since scattering is assumed by some numerical simulations, it is worth emphasizing that when illumination comes from (nonlocal) scattering high above the disk, we generically expect a steeper radial dependence of X-ray heating ( F proportional to R-3) than is usually assumed; it is this steep dependence that leads to the UV-hard spectrum.