Robert Antonucci
Blazars are members of the family of active galactic nuclei and quasars, defined specifically by their strong optical Polarization and variability. These unique defining properties seemed mysterious and even paradoxical in the l960s and 1970s, but now there is a growing consensus that their behavior and their role among quasars is qualitatively understood. Many of the modern ideas started to emerge during an important meeting in 1978 (the Pittsburg Confernce on BL Lac Objects). This is a good place to pick up the historical thread.
BL Lac objects have historically been defined as point-like sources of optical radiation that show little or no line emission, and strong and variable brightness and polarization. Pittsburgh meeting participants made it clear that some nearby objects exhibit all of these properties, along with narrow emission lines of considerable strength. Because they did not seem fundamentally different from the original BL Lacs, they were generally accepted as members of the class.
This was especially reasonable in light of the fact that the narrow emission line equivalent widths (strengths of the lines compared with that of the continuum) vary inversely with continuum flux. Without this unification, an object's class would sometimes be a function of time!
Optically violently variable (OVV) quasars presented a similar situation. They were defined as broad emission line objects which otherwise showed the characteristics of BL Lacs. In fact, in their contributions to the Proceedings, Joseph Miller and collaborators showed that very high signal-to-noise ratio spectroscopy of known BL Lacs sometimes reveals broad emission lines. Furthermore, some OVVs clearly look like BL Lacs when their continua are in high brightness states. These facts are closely related. Since 1978, several of the BL Lacs discussed by the meeting participants have shown broad emission lines when observed carefully in low states. Therefore, it is no longer possible to distinguish BL Lacs and OVVs in a rigorous way and the two classes were merged under the name blazars. (Of course, this does not imply that all blazars are intrinsically exactly the same.)
The old-fashioned view of blazars was that their high polarization and tiny sizes(from variability arguments) meant that they were bare quasars, with the fundamental energy generation process being observed directly, perhaps within a few gravitational radii of supermassive black holes. In this picture, ordinary quasars are surrounded by gas that reprocesses and depolarizes the radiation and damps out the variability.
Roger Blandford and Martin Rees presented a very different idea at Pittsburgh, an idea which has since had many successes and which prevails among most researchers today. The high polarization and "power law" spectra could naturally be produced by synchrotron radiation, as in the Crab nebula. However, Blandford and Rees pointed out the very severe constraints on any such model that result from the rapid optical variability and high observed luminosities. The variability seems to require that even the luminous sources are intrinsically tiny (light-days or less). However, the polarization requires that both the optical depth to electron scattering and the optical depth to the synchrotron self-absorption process must be low; the reason is that both of these processes destroy polarization. A source satisfying all of these constraints basically cannot be as luminous as those observed!
Now all of the constraints would be greatly alleviated if we made one assumption: Suppose the synchrotron sources are not stationary, but are moving in bulk at relativistic speeds toward Earth. (This idea is called the beam model.) Then two things happen. Because the radiation is "beamed forward" by special relativistic aberration, the observed fluxes are greatly boosted. Therefore, the luminosities in the rest frames are much less than was otherwise thought. Also, with the emitting volume moving toward Earth and nearly keeping up with its own past images, the rapid observed variability is partially an illusion. The variations have been compressed in time. In the rest frames they are substantially slower, so the sources can be rather larger than in a stationary model.
After Blandford and Rees' paper was written, the variability constraints became even stronger. Papers by Chris Impey and collaborators and by P. A. Holmes and collaborators reported studies of variability in the infrared. This is where blazars put out most of their energy. Now, independent of the emission mechanism, radiation from black hole accretion is generally not expected to vary on time scales shorter than the travel time of light across the event horizon of a maximally accreting black hole. Yet infrared monitoring showed such enormous apparent luminosities and such rapid variability that even this conservative expectation was violated in at least five cases!
The assumption that all blazars are moving relativistically toward Earth may seem ad hoc or even crazy. In fact, it is very reasonable. Blazars invariably have very bright compact radio cores, and these cores often show very strong evidence for such a scenario. It was well known since the work of Fred Hoyle, Ceoffrey Burbidge, and Wallace Sargent in the 1960s that a stationary synchrotron model for compact radio sources was not tenable. Radio variability seems to require extremely compact sources, and from these sizes and the observed radio fluxes, the surface brightnesses can be calculated. These turned out to be far above the "Compton limit" at 1012 K in brightness temperature. A stationary synchrotron source must emit fantastically large and observationally excluded inverse-Compton x-ray emission in order to have such a high brightness temperature. Therefore, relativistic motion in the line of sight had already been invoked. The idea was confirmed when superluminal(apparent faster-than -light) motion of milliarcsecond-scale radio jets was discovered.
The "time compression" of the observed variability was also verified by James Condon and B. Dennisoh. They showed that if the sources were really as small as naively expected from the radio variability data, they should have such small angular sires that they should show interstellar scintillation (twinkling), and they do not!
Blandford and Rees supplied an astrophysical context for synchrotron sources undergoing relativistic bulk motion in the line of sight. They suggested that the sources were simply the bases of the jets of normal double radio galaxies and quasars that happened to point in our direction. After all, some of these objects must be oriented in that way. Beaming of radiation by the aberration effect referred to earlier boosts the radio core fluxes in such objects, so they should be greatly over-represented in flux-limited surveys.
M. ORR and I. Browne adopted a simplified version of Blandford and Rees' idea. They postulated that all blazars, other core-dominant radio sources, and normal double sources all have similar relativistic bulk speeds, that the motions are along straight lines, and that the jets are linear in shape (rather than, say, conical). They concluded that such a simple model was consistent with a variety of source count data. Finally, Orr and Browne gave the name unified scheme to the hypothesis that flat-spectrum core-dominant sources are just normal doubles seen end-on. (The flat-spectrum core-dominant sources are just a slightly larger superset of blazars.)
The hypothesis that blazars are double radio sources seen along their jet (symmetry)axes obviously predicts that the double lobes should be seen projected as halos on the strong radio cores. It was just becoming possible in the early 1980s to achieve the required dynamic range in interferometer maps that was needed to detect such halos. (Remember that the blazar radio cores are tremendously strong. )Several groups discovered significant diffuse radio emission around many sources; this includes work by R. T. Schilizzi and A. G. de Bruyn with the Westerbork telescope, and Wardle and collaborators and James Ulvestad and collaborators with the NRAO Very Large Array.
The author and Ulvestad carried out an exhaustive blazar mapping program with the VLA and discovered substantial diffuse radio emission in almost all cases. The emission had qualitatively the right power, morphology, and projected linear size for the unified scheme. They critically examined various counter arguments in the literature, and then showed that if the beam model is qualitatively correct, the unified scheme must be, too. Suppose the beam model is correct and the core radio flux is beamed into a small solid angle that includes the direction to Earth. Suppose also that the large diffuse sources discovered in association with blazars emit isotropically. (This is very likely for the large, diffuse, and often two-sided halos.) Some blazars have sufficient flux in the diffuse radio halos alone to qualify for inclusion in the flux-limited radio catalogs.
Therefore, under our two hypotheses, blazars not directed at Earth would still be in the catalogs, but classified as something else. The only candidates are the normal double sources. In fact, statistically, many or most normal doubles would have to be misdirected blazars!
Two exciting recent developments need to be mentioned. First, according to the unified scheme, normal double quasars should show much lower speeds in their cores than blazars do (although they should still be superluminal). Sensitive, very long baseline interferometry experiments are now being carried out, and the speeds are, in fact, coming in at 1-5c rather than the 5-l0c typical of blazers.
The second recent development also seems to be a great success for the beam model and the unified scheme. Luminous double radio sources have two lobes that are generally fairly similar in flux, but jets that are very dissimilar in flux. This is at first sight unexpected because the jets appear to be the source of energy feeding the lobes. In the beam model, the jet radiation asymmetry is nicely explained as the result of beaming of the radiation from the jet closer to the line of sight toward us and beaming of the far jet radiation away from us. This does not require the axis to be very close to the line of sight as the blazar phenomenology does. Now the exciting new development is that Robert Laing and collaborators have discovered that in almost every case, one of the radio lobes is depolarized by passage through a magnetoionic medium (or"Faraday screen"), so that the depolarized lobe would be past the screen and the polarized lobe would be on its near side. (The Faraday screen would then probably be associated with the host galaxy.) The near side determined in this way is essentially always the side with the strong radio jet! This seems to mean that the jet radiation is beamed forward. Other interpretations are still possible but most researchers feel that the discovery of Laing and collaborators is a tremendous boost for the beam model.
Finally, there is evidence that the normal double quasars and broadline radio galaxies that lie very close to the sky plane are observed and classified as narrow-line radio galaxies, at least in some cases. The optical continuum sources and broad emission line regions are apparently obscured by opaque tori composed of dust clouds. The evidence comes from optical spectropolarimetry and from some statistical tests which seem to show that too few objects classified as quasars lie very close to the sky plane. These arguments are summarized and the appropriate references given in a recent paper by the present author, which discusses orientation effects in radio-quiet objects as well.
Additional Reading
Antonucci, R. (1989). Evidence for and Against Relativistic Beaming in Active Galactic Nuclei. Fourteenth Texas Symposium on relativistic Astrophysics. Academy of Sciences Press, New York.
Antonucci, R. and Ulvestad, J. (1985). Extended radio emission and the nature nature of blazars. Astrophys. J. 294 158.
Hoyle, F. ,Burbidge, G. ,and Sargent, W. (1966). On the nature of the quasi-stellar sources. Nature 209 751.
Orr, M. and Browne, I. (1982). Relativistic beaming and quasar statistics. MNRAS 200 1067.
Wolfe, A. M., ed. (1978). Pittsburgh Conference on BL LAC Objects (Physics and Astronomy Department, University of Pittsburgh). Includes papers by J. Miller, H. French, and S. Hawley; J. Miller and H. French; and R. Blandford and M. Rees.
See also Active Galaxies and Quasistellar Objects, Jets; Active Galaxies and Quasistellar Objects, Superluminal Motion; Galaxies nuclei.
Adapted from The Astronomy and Astophysics Encyclopedia, ed. Stephen P. Maran