BL Lac Objects: Proceedings of a Workshop Held in Como, Italy, September 20–23, 1988

485 Pages·1989·25.173 MB·English

by  J. Anton Zensus (auth.), L. Maraschi, T. Maccacaro, M. -H. Ulrich (eds.)

.I OIDAR JETS

SUPERLUMINAL MOTION IN QUASARS AND BL LAC OBJECTS
J. Anton Zensus
National Radio Astronomy Observatoryt
P. O. Box O; Socorro, New Mexico 87801 - USA
ABSTRACT
Apparent superluminal expansion in compact radio sources has provided strong evidence
for relativistic motion in these objects. Originally seen only in bright, core-dominated
sources, the effect has been found in sources with different morphologies, core strengths,
and identifications. The list of superluminals is still dominated by quasars but it now
includes several BL Lac objects.
INTRODUCTION
The discovery of superluminal sources in the early seventies has become one of the
master keys to our understanding of compact radio sources, by presenting direct evidence
for bulk motion of matter at relativistic velocities. In particular, it has confirmed
Martin Rees's prediction that "an object moving relativistically in suitable directions
may appear to a distant observer to have a transverse velocity much greater than c"
(Rees 1966). At the time of the Pittsburgh meeting, there was little indication for
the existence of superluminal motion in any BL Lac object (Shaffer 1978). However,
the beaming model of Blandford and Rees (1978), discussed at the same conference,
did suggest that the effect should be found in at least some BL Lacs. Today, several
BL Lacs are confirmed superluminals, and furthermore it has become clear that there is
a close relationship between the presence of blazar properties and superluminal motion
(Impey 1987 and at this Symposium).
In this review, I will describe the observational approach in the study of superlu-
minals, present as an example the quasar 3C 345, a prototypical core-dominated source,
and comment on the list of superluminals found so far. Marshall Cohen (at this Sym-
posium) discusses the statistics of superluminal sources. An extensive overview of this
field can be found in the text SuperIuminal Radio Sources (Zensus and Pearson, ed.,
1987; and references therein). For a theoretical perspective see also, e.g., the articles by
Begelmann, Blandford, and aees (1984), Phinney (1985), and Blandford (1987).
t The National Radio Astronomy Observatory is operated by Associated Univer-
sities, Inc., under cooperative agreement with the National Science Foundation of the
United States.
OBSERVATIONS
We measure in superluminal sources the rate of change of the apparent separation of two
features in the brightness distribution, observed with Very Long Baseline Interferometry
(VLBI). Such observations are made routinely at cm-wavelengths A( = 50, 18, 6, 2.8,
and 3.1 cm), but experiments at shorter wavelengths of 7 and 3 mm are possible. Most
of the available images have been made at 6 cm with resolution ~,, 1 mas; in some cases,
a resolution of 0.1 mas and less has been achieved. The dynamic range (the ratio of the
strongest feature in the image to the noise level) varies from about 02 : 1 to 1000 : .1
In terms of linear resolution, 1 mas ranges from about 0.25 pc in nearby active galactic
nuclei to about 4 pc in most quasars. For comparison, the Schwarzschild radius of a
109M® black hole is ~, 01 -4 pc, the size of the "broad emission line region" in active
nuclei is typically ~ 1 pc, and the size of the "narrow emission line region" is typically
001 pc. Thus VLBI can resolve the broad-line region, but it is still some way from
resolving the "central engine".
The angular velocity # is converted into an apparent linear velocity ppaV ppa~/C
~-
(projected on the sky), usually by inferring the distance to the source from the red-
shift, z (see, e.g., Pearson and Zensus 1987). Speeds given here are derived, assuming
a Friedmann-Robertson-Walker model, parameterized by the Hubble constant H0 and
the deceleration parameter .0q The values of H0 and 0q are unfortunately still subject
to debate. Merely for consistency, we use H0 = 100h km s -1 Mpc~-land 0q = 0.5 (the
dimensionless parameter h is the Hubble constant in units of 001 km s -1 Mpc -1).
Two primary approaches are followed in the observational study of superluminal
sources: (a) Monitoring of selected objects, which serves to determine the properties of
individual components, their evolution, and especially, the parameters required to de-
scribe the physical conditions in the underlying jet. )b( Statistical studies of well-defined
samples. These concentrate on the morphology of compact sources, on measuring the
frequency of occurrence of superluminal motion, and on the distribution of velocities.
3C 345: A PROTOTYPE
Only few sources have been studied in sufficient detail to describe the evolution of
individual components. A good example is the quasar 3C 345, which is one of the
"classic" superluminals (Cohen et al. 1977). This object is typical for the class that
contains strong, core-dominated sources as found in radio samples selected at high
frequencies.
The compact structure of 3C 345 has the common asymmetric "core-jet" character,
with several "jet" components separating superluminally from the "core" D (Figure 1).
The identification of core and jet components is usually based on the finding that one
component, the core, is compact, located at one end of the structure, and has a flat radio
spectrum, whereas the jet c6mponents tend to be resolved and have steeper spectra. In
3C 345, however, Barrel et al. (1986) have shown from phase-referencing observations
that the core indeed is stationary on the sky. Several superluminal components (C2, C3,
C4, C5) in 3C 345 have been monitored for a number of years at different wavelenghts.
Here we list some of their properties (Biretta, Moore, and Cohen 1986; Zensus et al., in
preparation).
, , , , ,
3C345 22'.: ~ GHz Oct 84
D C5 C4
i i t i i i
3C'345 22'.2 GHz - Ha~ B6
r i (~ i r i L
D C5 C4
~ i i i i i i i
3C345 ' 22'.2 GHz - $vn 87
t i ~ t Q i i i i
Figure 1 A sequence of hybrid maps of 3G345 at 22.2 GHz (Oct. 84, May 86, Nov. 86, and
June 87), showing the stationary core C, and the inner superluminal components C4 and C5 (Zensus $e
al., in preparation). At this wavelength, the outer superluminal components (C3 and C2) are resolved
(cf. Biretta, Moore, and Cohen 6891 for maps at longer wavelengths). North is at the top; tick marks
are 0.3 mas apart. The CLEAN maps have been restored with a gaussian beam of 0.3 mas (FWHM).
.1 Component sizes and separation from the core can be frequency-dependent, as seen
for several components in 3C 345.
.2 Superluminal components in a given source do not always all move with the same
velocity. This is seen not only in 3C345, but also in 3C 021 and 3C 279, and several
other sources.
.3 Acceleration is seen for component C4 in 3C 543 with changes in both magnitude
and direction of the superluminal velocity.
.4 The size of the superluminal components in 3C 543 increases roughly linearly with
separation from the core, corresponding to a projected jet opening angle of 62 °.
.5 An important question is whether individual components follow the same path
or are moving in different directions. There is evidence that at least the outer
components in 3C 345 (and also in 3C 273) follow a common curved path, but this
is still tentative. Closer to the core, however, components seem to take different
paths (Zensus ¢e al., in preparation).
.6 The structure in 3C 345 is curved northward in the direction of the extended emis-
sion regions located 1-3 arcsec from the core. Such curvature is quite common. The
early finding that curvature and also misalignments with the large-scale symmetry
axis axe more pronounced in core-dominated sources has been confirmed.
.7 Polarization measurement at 5 GHz by Wardle et al. (1988) show weak polarization
of the core (less than 1%), and moderate polarization of the superluminal knots,
with changes of the electric vectors as they move out.
Note that some of these properties have been seen in 3C 345 only; it appears likely,
however, that similar conditions exist in other core-dominated sources.
Biretta and Cohen (1987, 1988) have applied a simple physical model to the obser-
vations of 3C 345. Assuming that the emission is dominated by incoherent synchrotron
radiation, they model the jet components as homogeneous optically thin spheres, and
the core as an inhomogeneous conical jet (KSnigl 1981). These assumptions lead to a
consistent description of the source properties; in particular, it suggests the presence of
bulk relativistic motion in all components.
STATISTICAL STUDIES
Two well-defined samples of core-dominated sources are being studied: Witzel and his
colleagues (Witzel et al. 1989) have selected a complete sample of 31 sources from the
NRAO-MPIfR 5-GHz $5 survey, with flux densities > 1 Jy and flat spectra for a multi-
wavelength observing program. They find four quasars to be superluminal and two
more as superluminal candidates. Four sources without redshifts (listed by them as
BL Lacs) show apparent expansion which would be superluminal if, as seems probable,
the redshifts exceed 0.3. The comparison of measured and predicted inverse-Compton
X-ray fluxes suggests bulk relativistic motion in 21 of the 31 objects, which suggests
that this is a common phenomenon in core-dominated sources.
Pearson, Readhead, and Barthel (1987) are studying a larger sample of 56 sources,
also selected at 5 GHz; the selection criterion was flux density > 1.3 Jy, with no spectral
selection. This work has resulted in a detailed classification into different structure types
(Pearson and Readhead 1988), ranging from unresolved to complex sources with multiple
components. At least nine sources in this sample are superluminal. Superluminal
sources are found in all types of source except the "compact double" sources (Hodges
and Mutel 1987; Mutel and Phillips 1988).
If the beaming theories are correct, sources in such flux-density limited samples are
not randomly oriented but are preferentially beamed towards the observer. To avoid this
bias, several groups have attempted to define orientation-unbiased samples for study,
selecting sources by the flux density of the extended lobes, which are assumed to be
unbeamed, rather than the compact core (Hough and Readhead 1987; Zensus and Porcas
1987; Barthel et al. 1989). Several superluminals have emerged from these studies, and
the results so far indicate a trend that velocities in lobe-dominated superluminals are
smaller than in core-dominated objects, in agreement with the expectations of beaming
models.
LIST OF KNOWN SUPERLUMINALS
The total number of superluminals known at the time of this Symposium is at least 24
(Zensus and Pearson 1988; Cohen et aL 1988). These are listed in Table ,1 which gives
names, redshift, identification, apparent angular velocity, and inferred apparent velocity.
Identifications are Q: quasar, BL: BL Lac, and G: galaxy. The bottom part of Table 1
contains stationary and subluminal sources. Note that the velocity measurements do
vary in accuracy. See the references for details on individual measurements. Some
sources have entries for several superluminal components. A number of objects are not
listed, where motion has not yet been established clearly (cf. Zensus and Pearson 1988).
The galaxy 3C 021 is the closest superluminal source and the detailed observations
by Walker, Benson, and Unwin (1987) have revealed a jet with continuous properties on
scales from 0.5h -1 pc to more than 100h -1 kpc, indicating that the jet characteristics
are defined in the regions of the superluminal components. Recently, Benson et aL
(1988) reported superluminal motion in this source beyond 05 mas from the core, in
support of the idea that large scale jets are also relativistic.
The quasar 3C 971 stands out from the small group of well-established superluminal
sources. This source is the first classical double radio source with a relatively weak core
in which superluminal motion was found. Although much weaker, the components in
this source exhibit properties similar to those in 3C 345 (Porcas 1987). It seems likely
that 3C 971 is typical of the weak cores that are now being studied in well-defined
samples.
In the quasar 3C 273, superluminal motion has been monitored for more than
20 years (Zensus ~e aL 1988). The source contains a narrow jet which extends to
at least 521 h -1 pc from the core (at 5 GHz) and multiple superluminal components
have been observed with roughly similar speed of about 1 milliarcsecond per year. As
in 3C 120, this suggests that relativistic motion is present in the jet to much larger core
distances than previously expected.
The quasar 4C 39.25 was earlier considered the standard example of a source that
does not show superluminal components, but rather has a stationary double structure.
Subsequent observations indicated an apparent superluminal contraction, but it has now
become clear that we see a superluminal component moving between two stationary
components (Shaffer et al. 1987). eSroN of these components appears to have the typical
properties of a core. A similar situation is present in the quasar 3C 395, but Simon
Table 1 Radio Sources with Measured Velocities
Source z Identification # mas yr -1 flapph Ref.
537+2120 763.2 lB 90.0 9.3 1
123+3330 NRAO 041 852.1 Q 51.0 8.4 2
250+0340 C3 021 330.0 G 53.1 1.2 3
2.53 3.9 3
2.47 3.8 3
2.66 4.1 3
2.54 3.9 3
0723+679 3C 179 0.846 Q 0.19 4.8 4
0735+178 0.424 1B 0.18 2.8 5
0836+710 2.16 Q 0.13-0.25 5.2-10.4 1
0850+581 1.322 Q 0.12 3.9 6
0851+202 OJ 287 0.306 1B 0.28 3.3 7
0906+430 3C216 0.669 Q 0.11 2.4 8
0923+392 4C 39.25 0.699 Q <0.006 <0.1 9
0.16 3.5 9
1040+123 3C 245 1.029 Q 0.11 3.1 10
1137+660 3C 263 0.652 Q 0.06 1.3 11
1150+812 1.25 Q 0.13 4.1 1
320+6221 C3 372 851.0 Q 97.0 3.5 21
99.0 6.6 21
02.1 1.8 31
67.0 1.5 31
550-3521 C3 972 835.0 Q 5.0 2.9 41
11.0 0.2 51
1641+399 3C 345 0.595 Q 0.48 9.5 16
0.30 5.9 61
0.07,0.3 1.4,5.9 16
1642+690 0.751 Q 0.34 7.9 17
1721+343 4c 34.47 0.206 Q 0.36 3.1 18
1901+319 3C 395 0.635 Q <0.06 <1.2 91
0.64 13.2 19
1928+738 0.302 Q 0.6 7.0 20
1951+498 0.466 Q ,,,0.07 ~1.2 21
2200+420 BL Lac 0.0695 1B ,,-0.76 ~2.4 22
2230+114 CTA102 1.037 Q ,,,0.65 ~18.5 23
2251+158 3C454.3 0.859 Q <0.05 <1.3 24
0.35 8.9 24
447+3510 33.2 Q 30.0< 3.1< 1
314+6130 3C 84 2710.0 G 42.0 2.0 52
934+0170 715.0 G 40.0< 7.0< 8
70.0< 3.1< 8
653+1170 1.62 Q -0.05 -1.8 26
721+8221 M 87 0.004 G <0.3 <0.1 27
628+7361 NGC 6251 320.0 G 3.0< 3.0< 82
487+3081 86.0 LB 70.0< 47.0< 1
836-4391 381.0 Q 4.0< 1.3< 92
416+1202 6622.0 Q 40.0< 4.0< 62
400+4312 1.936 Q <0.01 <0.4 30
Note: Top part lists superluminal, bottom part stationary and subluminal sources. References:
1. Witzel et al. 1989. 2. Marscher and Broderick 1985. 3. Walker, Benson, and Unwin 1987. 4. Porcas
1987. 5. Bhhth 1984. 6. Barthel et al. 1986. 7. Gabuzda, Wardle, and Roberts 1989. 8. Pearson,
Re~lhead, and Barthel 1987. 9. Shaffer et al. 1987. 10. Hough and Readhem:l 1987. 11. Zensus, Hough,
and Porcas 1987. 12. Unwin el al. 1985. 13. Cohen et al. 1987. 14. Cotton et al. 1979. 15. Unwin
et al. 1989. 16. Biretta, Moore;and Cohen 1986. 17. Pearson el al. 1986. 18. Barthel et al. 1989.
19. Simon et al. 1988. 20. Eckart et al. 1985. 21. Zensus and Porcas 1987. 22. Mutel and Phillips 1987.
23. Bh£th 1987. 24. Pauliny-Toth et al. 1987. 25. Romney et al. 1984. 26. Readhead, Pearson, and
Unwin 1984. 27. Schmitt and Reid 1985. 28. Jones 1986. 29. A. K. Tzioumis, private communication.
30. Pauliny-Toth et al. 1984.
et al. (1988) reported that the superluminal motion has stopped. There is still no
clear evidence in any source for contractions, although the identification of changes as
expansion is not in all cases unique.
The quasar 3C 454.3 appears to be a good example of an object that exhibits rapid
brightness changes in a stationary structure, in this case together with superluminally
moving components (Pauliny-Toth et al. 1987). Similar sources are perhaps 3C 111,
3C 147, and 3C 390.3 which have shown some superluminal behaviour yet to be con-
firmed (Preuss, Alef, and Kellermann 1988; Alef et al. 1988).
SUPERLUMINAL BL LAC OBJECTS
Most of the superluminal objects listed in Table 1 have been identified as quasars;
exceptions are the galaxy 3C 120, and four BL Lacs (three of which are "officially"
recognized as such in the list of Burbidge and Hewitt 1987). Not included are the four
sources listed as BL Lacs by Witzel et al. (1989), which become superluminals if their
redshifts exceed certain critical values (0454+84, 0716+71, 1749+70, 2007+77).
BL Lac itself has been studied by Phillips and Mutel (1988). They find a correlation
between events of flux density/polarization changes and the occurrence of superlumi-
hal components. They also find deceleration to rest for these components (Mutel and
Phillips 1987).
OJ 287 is a special case, since the superluminal motion in this source was derived
from polarization VLBI measurements (Roberts and Wardle 1987; Gabuzda, Wardle,
and Roberts 1989). These show a moderately polarized, variable core, and two super-
luminal jet components, one of which is extremely polarized (64%). As the components
move away from the core, they maintain the orientation of the electric vector, but the
fractional polarization drops significantly.
Gabuzda (this Symposium) is studying a number of BL Lac objects, some of which
may be expected to show superluminal motion in the future. Impey (1987) list a number
of objects, which based on their blazar characteristics might be possible superluminal
candidates. Some of these have already been shown to show structural variations.
Generally, the compact radio cores seen in BL Lacs do not show distinctly different
properties from those in quasars. However, a trend exists, which suggests, that (a) the
speeds in those BL Lacs where motion has been established is comparatively low (2-
4c); (b) BL Lacs seem to be very compact, i.e. a higher fraction of the cm-wavelength
radio flux density originates from the compact VLBI region than in quasars. It may
be important in this context that the effect of variability at cm-wavelengths recently
reported by Quirrenbach et al. (1989) occurs prominantly in the extremely compact BL
Lac object 0716+61, which also is a superluminal candidate.
BEAMING MODELS
The superluminal effect is commonly explained by models which invoke bulk relativistic
motion of the emitting plasma along a jet which is oriented close to the line-of-sight.
These models are often favored since they can explain a range of source properties.
01
Among these are: the one-sidedness of compact structures; the excessive brightness tem-
peratures (Rees 1966); the apparent lack of synchrotron self-Compton x-rays in some
core-dominated sources (Marscher 1987); the trend that in lobe-dominated sources su-
perluminal motion occurs with small speeds; the connection between blazars and super-
luminals (Impey 1987); and the depolarization asymmetry observed in lobe-dominated
quasars (Laing 1988; Garrington et al. 1988). Two of the problems for the beaming
hypothesis are: the deprojected linear sizes in some superluminal quasars are too large
(Schilizzi and deBruyn 1983; Browne 1987); and the observed jet/counterjet ratios in
some quasars are too high to be solely due to beaming effects (Barthel 1989).
Several unified schemes have been proposed, which identify the underlying pop-
ulation, i.e. the sources that are not beamed in the direction to the observer (e.g.,
Blandford and Rees 1978; Scheuer and Readhead 1979; Orr and Browne 1982; see also
the discussion at the end of Scheuer 1987). Barthel (1989) has suggested that all quasars
are beamed, and that the underlying population is formed by powerful radio galaxies
(Fanaroff-Riley class II). Blazars might represent the objects most closely aligned to the
line-of-sight. This scheme alleviates the problems of the beaming hypothesis mentioned
above and offers a number of observational tests. A comprehensive discussion of unifi-
cations is given by Browne (at this Symposium), who favors a scheme of different parent
populations (i.e. FR I and FR II galaxies) for BL Lacs and HPQ/OVV's, respectively.
CONCLUSIONS
Superluminal motion is established as a common phenomenon in active galactic nuclei.
The available observational results are beginning to constrain the parameters of physical
models for individual sources, and to test more general source models. The beaming
hypothesis and related unified schemes have proven to be more successful than expected
in explaining these sources (e.g., Blandford 1987), even though modifications of the most
naive models seem to be necessary.
I thank Tim Pearson and Marshall Cohen for their comments; my participation in this Symposium
saw possible through financial support by the organizers, the California Institute of Technology, and
the Max-Planck Institut fur Radioastronomie.
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B££th, .L B. 1987, in Zensus and Pearson 1987, p. 206.
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