ROSAT OBSERVATIONS OF THE BLACK-HOLE CANDIDATE V404 CYGNI IN QUIESCENCE

V404 Cyg was detected with the ROSAT PSPC in a 16 ks observation 1265 days after the 1989 May outburst. This is the first unambiguous detect ion of a soft X-ray transient or X-ray nova black hole candidate in qu iescence. The observed X-ray spectrum is extremely soft and can be des cribed equally well by either a blackbody, power-law, or a thermal bre msstrahlung continuum. The best-fit blackbody spectrum is characterize d by a temperature of 0.2 keV. A significant amount of absorption, bot h along the fine of sight and intrinsic to the V404 Cyg system is requ ired to understand the observations. The 0.2-2.4 keV light curve of V4 04 Cyg exhibits substantial variability on timescales of less than a d ay. Assuming a distance of 3.5 kpc, the quiescent X-ray luminosity of V404 Cyg is 8 x 10(33) ergs s-1, nearly two orders of magnitude larger than the upper limit for the X-ray nova and black hole candidate A062 0-00, but comparable to neutron star X-ray novae such as Cen X-4. The lack of a significant hard X-ray luminosity in quiescence, the presenc e of a large and cold neutral region in the accretion disk, and a low- mass accretion rate suggests that an accretion disk instability might account for the outburst of V404 Cyg.