Encounters
X-Ray Burster ○꠹｜encounter｜ENC｜20260912123541-00-•
Stage II—Confirm that the Candidates are True Black Holes: Ideally, in order to show that a dynamical BH candidate (i.e., a massive compact object) is a genuine BH, one would hope to demonstrate that the candidate has an event horizon – the defining characteristic of a BH. Strong evidence has been obtained for the reality of the event horizon from observations that compare BHBs with very similar neutron-star binaries. These latter systems show signatures of the hard surface of a neutron star that are absent for the BH systems. For example, X-ray observations in quiescence show that the BH systems are about 100 times fainter than the nearly identical neutron-star binaries (Narayan et al. 1997; Garcia et al. 2001). The ADAF model (§7) provides a natural explanation for the faintness of the BHs, namely, the low radiative efficiency of the accretion flow allows a BH to “hide” most of its accretion energy behind its event horizon (e.g, Narayan et al. 2002). In quiescence, one also observes that BHs lack a soft thermal component of emission that is very prevalent in the spectra of neutron stars and can be ascribed to surface emission (McClintock, Narayan, & Rybicki 2004). During outburst, the presence of a surface for an accreting neutron star likewise gives rise to distinctive phenomena that are absent in BHBs: (a) type I thermonuclear bursts (Narayan & Heyl 2002; Tournear et al. 2003; Remillard et al. 2006a), (b) high-frequency timing noise (Sunyaev & Revnivtsev 2000), and (c) a distinctive spectral component from a boundary layer at the stellar surface (Done & Gierlinski 2003).
Source: Remillard & McClintock 2006 ┃ X-Ray Properties… ⚬𓂃
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