New Evidence for Black Hole Event Horizons from Chandra
  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. 
Michael R. Garcia (1), Jeffrey E. McClintock (1), Ramesh Narayan (1),   Paul Callanan (2), and Stephen S. Murray (1) ((1) Ha...