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We compile optical and UV spectra of a sample of 'typical' short period blackhole X-ray transients in outburst. We also survey determinations ofinterstellar extinction and distance in order to deredden spectra and compareabsolute fluxes. Hence we perform a comparative study of the broad-bandspectral energy distributions (SED). We find that given such a homogeneoussample of typical sources, the optical SEDs form a relatively uniform set, allexhibiting quasi-power-law spectra with Fnu proportional to nu^alpha, where0.5<alpha<1.5 (steeper than the canonical nu^1/3 disk spectrum). All becomeflatter in the UV, although there is more diversity here. The SEDs studied canbe 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. TheUV-soft spectra, e.g. GRO J0422+32, drop off. XTE J1859+226 evolved fromUV-soft to UV-hard as it decayed indicating that this effect is a realdifference, not a dereddening artifact. All of the spectra can be fitted by ageneralized black body disk model with two forms of heating, resulting in thetwo states. The UV-soft state is consistent with a disk illuminated by acentral point source, with irradiative heating dominating over viscous. TheUV-hard state is well described by a viscously heated disk, although thisrequires very high mass flow rates in the case of Nova Muscae. Alternatively, aUV-hard spectrum can be produced if the disk is illuminated by a verticallyextended X-ray source such as a central scattering corona or jet. [Abridged]Comment: Accepted for publication in the Astrophysical Journal. 20 pages using  emulateapj.cl

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