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 5-10c 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.
ACTIVE GALAXIES AND QUASISTELLAR OBJECTS, BLAZERS
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.