  We present HST/ACS ultraviolet photometry of three quiescent black hole X-ray
transients: X-ray Nova Muscae 1991 (GU Mus), GRO J0422+32 (V518 Per), and X-ray
Nova Vel 1993 (MM Vel), and one neutron star system, Aql X-1. These are the
first quiescent UV detections of these objects. All are detected at a much
higher level than expected from their companion stars alone and are significant
detections of the accretion flow. Three of the four UV excesses can be
characterized by a black body of temperature 5000-13,000K, hotter than expected
for the quiescent outer disk. A good fit could not be found for MM Vel. The
source of the black-body-like emission is most likely a heated region of the
inner disk. Contrary to initial indications from spectroscopy there does not
appear to be a systematic difference in the UV luminosity or spectral shape
between black holes and neutron star systems. However combining our new data
with earlier spectroscopy and published X-ray luminosities there is a
significant difference in the X-ray to UV flux ratios with the neutron stars
exhibiting Lx/Luv about 10x higher than the black hole systems. Since both
bandpasses are expected to be dominated by accretion light this suggests the
difference in X-ray luminosities cannot simply reflect differences in quiescent
accretion rates and so is a more robust discriminator between the black hole
and neutron star populations than the comparison of X-ray luminosities alone.
