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Session 29 - Invited Talk.
Invited session, Monday, January 15
1st Floor, La Villita Assembly Building
The quest to identify black holes plays a special role in modern astrophysics. Black holes are not simply another exotic form of star, but a link to the fundamental issues of the nature of space and time. Black hole X-ray transients with measured mass functions have proven to be some of the best established black hole candidates. There may be hundreds of such systems in the Galaxy with low mass companions and only one Cygnus X-1 with a high mass companion. These systems have low duty cycles, with outbursts lasting perhaps a year separated by decades. Many of the systems show a rapid rise and slower exponential decline, but others show more complex light curve morphologies, including repeated secondary outbursts. The instability that triggers the primary outburst is most plausibly related to the accretion disk ionization instability of dwarf novae, but the wide variety of phenomenology challenges all theories. These transient systems show activity throughout the electromagnetic spectrum. They routinely show associated radio outbursts and some have recently revealed ``superluminal" jets that make the connection to QSO's compelling. They also show dramatic outbursts in the optical, UV, X-ray and \gamma-ray bands with soft X-ray spectra generally attributed to an optically thick accretion disk and high energy power-law emission of uncertain origin that is again very reminiscent of QSO's. Some of these systems have lithium-rich companions and some show transient \gamma-ray lines that have been attributed to red-shifted positron annihilation or alternatively to emission from newly-formed lithium. These systems are a rich astrophysical laboratory and hold the promise of revealing clues to distinguish non-magnetic neutron stars from low mass black holes and to probe space-time in regions of large curvature.
