So far, only emission up to a few GeV has been considered. However, TeV astronomy is now in full swing, as we have heard at this meeting, and it is therefore interesting to consider the situation at these energies.
Three extragalactic sources have been detected at E > 0.3 TeV (Lorenz 1998) and these are all BL Lacs. That is, even at energies above those of EGRET (and exactly for the same reasons) the only -ray emitting AGN are still blazars! The difference here is that, unlike the situation in the EGRET band where the majority of detected blazars are flat-spectrum radio quasars, only BL Lacs have been detected so far. Furthermore, these BL Lacs are the three nearest confirmed BL Lacs in the recent catalogue by Padovani and Giommi (1995) namely MKN 421 (redshift z = 0.031), MKN 501 (z = 0.055) and 1ES 2344+514 (z = 0.044). The fact that only relatively nearby BL Lacs have been detected is probably related to absorption of TeV photons by the infrared background (see, e.g., Biller et al. 1995 and references therein). Additionally, the position of the synchrotron peak might anticorrelate with bolometric luminosity in blazars (Fossati et al. 1997). One would then expect the less luminous, and therefore nearer, objects to have the synchrotron peak at UV/X-ray energies and the peak of the inverse Compton emission (within the SSC model) in the TeV band. These objects, therefore, would be, on average, stronger TeV sources.
Why have no flat-spectrum radio quasars been detected at TeV energies? These sources are typically at higher redshifts and so the effect on them of the cosmological absorption by infrared photons is more severe. However, there are at least four strong radio sources classified as flat-spectrum quasars at z < 0.1, including 3C 120 and 3C 111, the latter having been looked at by the Whipple experiment, with negative results (Kerrick et al. 1995; 3C 111, however, although superluminal [Vermeulen and Cohen 1994] is lobe-dominated [fcore / fextended 0.2; Hes et al. 1995], which suggests it is an unlikely blazar. Also, it has not been detected by EGRET.)
This is certainly small number statistics and definite conclusions should only be drawn after a larger number of relatively local flat-spectrum radio quasars have been observed at TeV energies. However, the non-detection of flat-spectrum radio quasars could simply mean that internal absorption is significant in these sources. In fact, the cross-section for photon-photon interaction for ~ 1 TeV photons is maximum at ~ 1014 Hz or ~ 2.5 µ and quasars have a larger photon density than BL Lacs at these frequencies, be it emission from the obscuring torus (3) or even the accretion disk (see e.g., Protheroe and Biermann 1997).
The new, more sensitive projects which are underway in the field of TeV
will certainly shed light on these issues and
on the processes which are responsible for the GeV/TeV emission in blazars.
The origin of such emission is in fact still debated as being due to inverse
Compton radiation, either SSC or Comptonization of external radiation (e.g.,
Sikora et al. 1994),
or to hadronic processes, that is pion production from
accelerated protons, with subsequent pion decay and -ray production
(e.g., Mannheim 1993).
In the latter case, these so-called ``proton blazars''
would also be sources of cosmic rays and of neutrinos and could be detectable
by planned km3 neutrino detectors
(Halzen and Zas 1997).
3 It is not clear if the presence of an obscuring torus is required in BL Lacs as well as in radio quasars: see discussion in Urry and Padovani (1995) and Padovani (1997) and references therein.