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 , 000 K , 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 L _ { X } / L _ { UV } about 10 \times higher than the black hole systems .
This is consistent with earlier comparisons based on estimating non-stellar optical light , but since both bandpasses we use are expected to be dominated by accretion light we present a cleaner comparison .
This suggests that the difference in X-ray luminosities can not simply reflect differences in quiescent accretion rates and so the UV/X-ray ratio is a more robust discriminator between the black hole and neutron star populations than the comparison of X-ray luminosities alone .