|| © CAMBRIDGE UNIVERSITY PRESS 1991
Unfortunately, even the highest resolution radio maps do not probe deep enough into an AGN to give a crisp picture of what is going on at the 1015 cm scale over which jets are probably being formed, nor is structural information on that scale likely to emerge from higher frequency observations. Neither the possible advent of longer radio baselines provided by an orbiting antenna (e.g., RADIOASTRON) nor the currently contemplated optical interferometers would change this picture in a significant fashion, except perhaps for the very nearest active galaxies (see Section 4.3.2). Several questions concerning the very core AGN relevant to jet formation can, however, be partially addressed with current or planned observational capabilities, and the rest of this chapter will consider some of these.
3.1. Better evidence for black holes
Despite all of the evidence for SMBHs discussed in Section 2.1, the proof that they are responsible for nuclear activity remains merely circumstantial. Several of the arguments may be greatly improved in the near future. For example, the Hubble Space Telescope should be able to clearly examine the stellar spectra of the innermost few parsecs of nearby galaxies thanks to its diffraction-limited resolution. This would give convincing dynamical values for the masses of their cores. Improved spectroscopy of the BLR will put more constraints on the total size of that region. More sophisticated analyses of the differential origins in radii of specific broad lines could also give a handle on the mass distributions within galactic nuclei. Also, many models that rely on dense electron-positron plasmas in the near vicinity of the SMBH would produce -rays of significant intensities. In particular, broadened annihilation lines ought to be produced, and the Gamma Ray Observatory should be capable of detecting them, at least in the brightest sources.
Perhaps the clearest evidence for SMBHs would arise from the detection of binary periods due to black holes in a mutual orbit. Many active galaxies are almost certainly the product of galactic mergers, and if two galaxies housing nuclear SMBHs merge, then the smaller galaxy's hole will sink to the core of the larger galaxy over just a few orbital periods (Begelman, Blandford & Rees 1980). An orbit decaying over time scales 109 yr can then be established until gravitational radiation leads to a rapid coalescence. The lifetime of typical SMBH binaries with orbital periods of Torb years and masses of about 106 M is roughly 4 × 107 Torb8/3 yr, which is a significant fraction of the total expected active lifetime of ~ 3 × 108 yr (Blandford 1986), although it decreases with increasing BH masses. If both SMBHs retain, or reform, accretion discs, then there is a reasonable possibility that eclipsing binaries (of easily detectable periods, Torb 1 yr) could be found. This is because it is likely that between 10-4 and 10-2 of Seyfert and LINER nuclei host such binary accretion discs and a careful monitoring of their optical continua could provide a new, and extremely strong, argument in favour of SMBHs (Blandford 1986). An apparent occultation of the nucleus of the Seyfert galaxy NGC 4151 in February 1983 has been reported (Meaburn et al. 1985) and the observers argue that it is best explained as the blockage of a central source of 3 A.U. by a dark cloud of diameter ~ 6 A.U. It is not clear if this observation can be made consistent with the binary black hole hypothesis, but close monitoring of NGC 4151 is clearly warranted. A binary pair of SMBHs has also been proposed for OJ 287 (Sillanpää et al. 1988).
On the other hand, because of the inexorable pull of BHs, fluid flows around BHs do not appear to be capable of supporting long-term variations with short periods comparable to the horizon crossing time. Therefore, convincing observations of such stable, short-term periods would be powerful evidence in favour of spinar or magnetoid models. The only source where there is any real evidence for minute-scale variability is the BL Lac object OJ 287 (Valtaoja et al. 1985). Although a 15.7 minute period was apparently fairly stable over more than a year, that group also found possible periodicities at 13.0 and 33.0 minutes, but another group found that the strongest periodic variation was at 22.8 minutes with other peaks in the Fourier spectrum at 42.2, 12.0 and 7.7 minutes (Carrasco et al. 1985). These multiple periodicities are probably best explained as a series of "hot spots" on an accretion disc spiralling into a SMBH, which could provide short-lived, quasi-periodic variability (Carrasco et al. 1985).