Abstract
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, Porb = 7.8 hr) and two longer period BHXNs (GRO J1655-40, Porb = 62.9 hr; V404 Cygni, Porb = 155.3 hr), along with sensitive upper limits. Here we announce the detection of two more short-period BHXNs (GRO J0422+32, Porb = 5.1 hr; GS 2000+25, Porb = 8.3 hr), an upper limit for a third that is improved by 2 orders of magnitude (4U 1543-47, Porb = 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.
Highlights
Black Hole X-ray Novae (BHXN) as part of a complementary AO1 GO program.The very low, but non-zero, quiescent X-ray luminosity of the BHXN A0620–00 is difficult to understand in the context of standard viscous accretion disk theory (McClintock, Horne & Remillard 1995), given the continued mass transfer from the companion evidenced by an optically bright disk
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, Porb=7.8 hrs) and two longer period BHXN (GRO J1655–40, Porb=62.9 hrs; V404 Cyg, Porb=155.3 hrs), along with sensitive upper limits
Summary
BHXN as part of a complementary AO1 GO program. The very low, but non-zero, quiescent X-ray luminosity of the BHXN A0620–00 is difficult to understand in the context of standard viscous accretion disk theory (McClintock, Horne & Remillard 1995), given the continued mass transfer from the companion evidenced by an optically bright disk. For the same mass accretion rate, a black hole would be significantly less luminous than a compact star with a surface (Narayan & Yi 1995) Such a comparison of otherwise similar systems is a promising method for proving the reality of event horizons (Narayan, Garcia & McClintock 1997a, hereafter NGM). NGM, Garcia et al 1998 (hereafter G98) and M99 showed that quiescent BHXN are much less luminous in X-rays than quiescent NSXN of similar orbital periods, and argued that this provides direct evidence that BHXN are able to “hide” their accretion energy behind an event horizon.
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