| © CAMBRIDGE UNIVERSITY PRESS 1991
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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).