2001ApJ...553L..47G

Previously we claimed that black hole X-ray novae (BHXNs) in quiescence are much less luminous than equivalent neutron star X-ray novae (NSXNs). This claim was based on the quiescent detection of a single short-period BHXN (A0620-00, P_orb=7.8 hr) and two longer period BHXNs (GRO J1655-40, P_orb=62.9 hr; V404 Cygni, P_orb=155.3 hr), along with sensitive upper limits. Here we announce the detection of two more short-period BHXNs (GRO J0422+32, P_orb=5.1 hr; GS 2000+25, P_orb=8.3 hr), an upper limit for a third that is improved by 2 orders of magnitude (4U 1543-47, P_orb=27.0 hr), and a new, much lower quiescent measurement of GRO J1655-40. Taken together, these new Chandra Advanced CCD Imaging Spectrometer measurements confirm that the quiescent X-ray luminosities of BHXNs are significantly lower than those of NSXNs. We argue that this provides strong evidence for the existence of event horizons in BHXNs.