4.6. The Black Hole Paradigm
The intriguing paper by Lynden-Bell (1969) still did not launch a widespread effort to understand AGN in terms of accretion disks around black holes. Further impetus came from the discovery of black holes of stellar mass in our Galaxy. Among the objects discovered by Uhuru and other early X-ray experiments were sources involving binary star systems with a neutron star or black hole. "X-ray pulsars" emitted regular pulses of X-rays every few seconds as the neutron star turned on its axis. The X-ray power was essentially thermal emission from gas transferred from the companion star, impacting on the neutron star with sufficient velocity to produce high temperatures. Another class of source, exemplified by Cyg X-1, showed no periodic variations but a rapid flickering (Oda et al. 1971) indicating a very small size. Analysis of the orbit gave a mass too large to be a neutron star or white dwarf, and the implication was that the system contained a black hole (Webster and Murdin 1972; Tananbaum et al. 1972). The X-ray emission was attributed to gas from the companion O-star heated to very high temperatures as it spiraled into the black hole by way of a disk (Thorne and Price 1975).
Galactic X-ray sources, along with cataclysmic variable stars, protostars, and AGN, stimulated efforts to develop the theory of accretion disks. In many cases, the disk was expected to be geometrically thin, and the structure in the vertical and radial directions could be analyzed separately. A key uncertainty was the mechanism by which angular momentum is transported outward as matter spirals inward. In a highly influential paper, Shakura and Sunyaev (1973) analyzed disks in terms of a dimensionless parameter that characterized the stresses that led to angular momentum transport and local energy release. General relativistic corrections were added by Novikov and Thorne (1973). This "-model" remains the standard approach to disk theory, and only recently have detailed mechanisms for dissipation begun to gain favor (Balbus and Hawley 1991). The -model gave three radial zones characterized by the relative importance of radiation pressure, gas pressure, electron scattering, and absorption opacity. The power producing regions of AGN disks would fall in the "inner" zone dominated by radiation pressure and electron scattering. Electron scattering would dominate in the atmosphere as well as the interior, and modify the local surface emission from an approximate black body spectrum. The "inner" disk zone suffers both thermal and viscous instabilities (Pringle 1976; Lightman and Eardley 1974), but the ultimate consequence of these was unclear. A model in which the ions and electrons had different, very high temperatures was proposed for Cyg X-1 by Eardley, Lightman, and Shapiro (1975). This led to models of "ion supported tori" for AGN (Rees et al. 1982). The related idea of "advection dominated accretion disks" or "ADAFs" (Narayan and Yi 1994) recently has attracted attention.
A key question was, do expected physical processes in disks explain the phenomena observed in AGN? In broad terms, this involved producing the observed continuum and, at least in some objects, generating relativistic jets, presumably along the rotation axis. Shields (1978b) proposed that the flat blue continuum of 3C 273 was thermal emission from the surface of an accretion disk around a black hole. For a mass ~ 109 M and accretion rate 3 M yr-1, the size and temperature of the inner disk was consistent with the observed blue continuum. This component dominated an assumed nonthermal power law, which would explain the infrared upturn and the X-rays. Combining optical, infrared, and ultraviolet observations, Malkan (1983) successfully fitted the continua of a number of QSOs with accretion disk models. Czerny and Elvis (1987) suggested that the soft X-ray excess of some AGN could be the high frequency tail of the thermal disk component or "Big Blue Bump", which appeared to dominate the luminosity of some objects.
Problems confronted the simple picture of thermal emission from a disk radiating its locally produced energy. Correlated continuum variations at different wavelengths in the optical and ultraviolet were observed in the optical and ultraviolet on timescales shorter than the expected timescale for viscous or thermal processes to modify the surface temperature distribution in an AGN disk (e.g., Clavel, Wamsteker, and Glass 1989; Courvoisier and Clavel 1991). This suggested that reprocessing of X-rays incident on the disk made a substantial contribution to the optical and ultraviolet continuum (Collin-Souffrin 1991). Also troublesome was the low optical polarization observed in normal QSOs, typically one percent or less. The polarization generally is oriented parallel to the disk axis, when this can be inferred from jet structures (Stockman, Angel, and Miley 1979). Except for face on disks, electron scattering in disk atmospheres should produce strong polarization oriented perpendicular to the axis. Yet another problem was the prediction of strong Lyman edge absorption features, given effective temperatures similar to those of O stars (Kolykhalov and Sunyaev 1985). These issues remain under investigation today.
The question of fueling a black hole in a galactic nucleus has been difficult. Accretion rates of only a few solar masses a year suffice to power a luminous quasar, and even a billion solar masses is a small fraction of the mass of a QSO host galaxy. However, the specific angular momentum of gas orbiting a black hole at tens or hundreds of gravitational radii is tiny compared to that of gas moving with normal speeds even in the central regions of a galaxy. The angular momentum must be removed if the gas is to feed the black hole. Moreover, some galaxies with massive central black holes are not currently shining. Indeed, the rapid increase in the number of quasars with increasing look back time (Schmidt 1972), implies that there are many dormant black holes in galactic nuclei. What caused some to blaze forth as QSOs while others are inert? A fascinating possibility was the tidal disruption of stars orbiting close to the black hole (Hills 1975). However, the rate at which new stars would have their orbits evolve into disruptive ones appeared to be too slow to maintain a QSO luminosity (Frank and Rees 1976). The probability of an AGN in a galaxy appeared to be enhanced if it was interacting with a nearby galaxy (Adams 1977; Dahari 1984), which suggested that tidal forces could induce gas to sink into the galactic nucleus. There, unknown processes might relieve it of its angular momentum and allow it to sink closer and closer to the black hole.
The growing acceptance of the black hole model resulted, not from any one compelling piece of evidence, but rather from the accumulation of observational and theoretical arguments suggestive of black holes and from the lack of viable alternatives (Rees 1984).