New evidence for black hole event horizons from Chandra

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 (A 0620-00, 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+3 2, 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 togethe r, these new Chandra Advanced CCD Imaging Spectrometer measurements confirm that the quiescent X-ray luminosities of BHXNs are significantly lower tha n those of NSXNs. We argue that this provides strong evidence for the exist ence of event-horizons in BHXNs.