  Previously we claimed that Black Hole X-ray Novae (BHXN) in quiescence are
much less luminous than equivalent Neutron Star X-ray Novae (NSXN). This claim
was based on the quiescent detection of a single short period BHXN (A0620-00,
P(orb)=7.8 hrs) and two longer period BHXN (GRO J1655-40, P(orb)=62.9 hrs; V404
Cyg, P(orb)=155.3 hrs), along with sensitive upper limits. We announce the
detection of two more short period BHXN (GRO J0422+32, P(orb)=5.1 hrs; GS
2000+25, P(orb)=8.3 hrs), an upper limit for a third which is improved by two
orders of magnitude (4U 1543-47, P(orb)=27.0 hrs) and a new, much lower
quiescent measurement of GRO J1655-40. Taken together, these new Chandra
measurements confirm that the quiescent X-ray luminosities of BHXN are
significantly lower than those of NSXN. We argue that this provides strong
evidence for the existence of event horizons in BHXN.
