Publications of the Astronomical Society of the Pacific 92:127-133, April 1980

SUPERLUMINAL MOTION IN COMPACT RADIO SOURCES

ALAN P. MARSCHER Center for Astrophysics and Space Sciences University of California, San Diego AND JOHN S. SCOTT Steward Observatory, The University of Arizona Received 1979 December 29

We review recent observations of compact radio sources which are undergoing apparent superluminal expansion. The observations are now sufficient to eliminate certain theoretical models and to render others as highly unlikely. Certain models which invoke relativistic bulk motion of the radio components seem most highly favored, but definitive evidence in support of such models is still lacking. We review the various theoretical models which are still capable of explaining apparent superluminal motion, and extract features which can be used to distinguish among them observationallv. For example, observations of the detailed geometrv of a source can narrow considerably the range of possible explanations for the superluminal phenomenon. Keif words: quasi-stellar objects—radio sources—galaxies (general)

I. Introduction papers. Probably the most exciting results to emerge from very long baseline interferometry (VLBI) thus far are the II. The Observations apparent superluminal (relative velocities exceeding 2 c) Because of the large investment of time required to motions in the brightness distributions of several com- determine the detailed milli-arc-second radio structure pact extragalactic radio sources. The observational evi- of an object, information relating to changes in the dence for this phenomenon has been reviewed by Cohen brightness distribution is only available for a small et al. (1977) and Kellermann and Shaffer (1977). In addi- fraction of all bright compact radio sources. This situa- tion, Blandford, McKee, and Rees (1977) have published tion has not changed much since the Cohen et al. a general review of theoretical models which are ca- (1977) review. These authors found that, out of ten pable of explaining the appearance of superluminal ex- sources whose milli-arc-second structures were mon- pansion. itored over a number of years, at least four (and at During the time since these reviews were published, most six) exhibited apparent superluminal motions. The much has been learned observationally about the nature phenomenon is therefore rather common among bright of superluminal radio sources. Many of the previously vi- compact sources. able models have been found to be inconsistent with the The inferred velocities of expansion depend on the as- newly obtained data. In addition, several new models or sumption of cosmological redshifts and increase as the -1 variations of existing theory have been proposed to ac- values of H0 and q0 are lowered. For Hq = 55 km s -1 commodate the observational details which have recent- Mpc and q0 — 0.05, the angular expansion rates corre- ly been uncovered. spond to velocities of 4.1 ± 0.4 c (1972.3—1974.4) (and Here we attempt to review the current status of this possibly 8.5 ± 0.9 c from 1974.4-1976.5) in 3C 120, 5.2 rapidly changing field. We stress how the information ± 0.5 c in 3C 273, 19 ± 4 c in 3C 279, and 6.7 ± 0.4 c gathered since the 1977 review papers were published in 3C 345 (Seielstad et al. 1979; Cotton et al. 1979). has affected the viability of the various proposed theo- These velocities refer to the relative motions of the two ries. We then compare the observational characteristics dominant components in what may be a multi- of the surviving models in an effort to show how future component source. observations can separate the correct ideas from those Although the data on any given object give a rather which do not apply. limited view of the nature of the superluminal phenome- Because of the existence of the previous reviews of non, the information gathered on all four objects taken this subject, we intend mainly to present an update here. together yields a composite picture which is more il- Further details can be obtained from the individual pa- luminating. It seems that superluminal radio sources pos- pers cited herein and from the aforementioned review sess the following properties (see Cohen et al. 1977; Kel-

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© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 128 MARSCHER AND SCOTT lermann and Shaffer 1977; Seielstad et al. 1979): (1) The Thus, any noncosmological redshifts must apply to the angular separations are, to first order, independent of galaxy as a whole. It is still possible for the velocities to wavelength from about 2 to 6 cm. (2) The rate of expan- be subluminal while the redshifts remain cosmological, if sion during a given outburst, at least in 3C 345, is con- one adopts a non-Friedmann cosmology, such as the stant to within the observational errors. (3) The source is Chronometrie theory (Segal 1979; Köhler 1978). extended along a position angle which remains constant both during a given expansion event and, in 3C 120, Β. Random Fluctuations from one event to another. This position angle may, The idea (Dent 1972) that flares erupting at random in however, bend somewhat as one looks at the larger, various locations of the source could simulate super- lower frequency structure (Readhead et al. 1978). (4) luminal motion was initially an attractive alternative. The two major components are of comparable flux den- However, as is summarized by Cohen et al. (1977), it was sities (within a factor of a few), although the ratio of soon realized that the observed expansions were too sys- their fluxes may change with time. (5) The motions al- tematic for this model to remain valid. In addition, the ways appear as separations; i.e., no systematic con- only contractions (which should occur as often as expan- tractions occur. (6) Finally, the extrapolated epoch of sions, statistically) observed seemed more like "resets" to zero separation generally falls within a few months of new stages of systematic expansion. the onset of an outburst in flux density. C. Light-Echo Model The advent of more detailed, hybrid mapping of com- In a similar vein, Lynden-Bell (1977) has postulated pact sources using phase-closure information has shed a that systematic superluminal expansion can occur if a great deal of light on the milli-arc-second structure of "signar (relativistic blast-wave, for example) traveling at superluminal radio sources. Readhead et al. (1979) find or near the speed of light, propagates throughout the that 3C 120, 3C 273, and 3C 345 all contain unresolved source volume, causing regions of the source at progres- (or barely resolved) components at one end of their sively larger radii to brighten. He points out that the brightness distributions. A more diffuse, elongated fea- same effect occurs in some novae, where, as time prog- ture, which could be described as a thick jet, extends resses, the outburst of optical photons is scattered by to one side of this compact component. Imbedded in clouds at increasingly greater distances from the light this jet are one or more knots of emission. It is appar- source. If the signal propagates in opposite directions ently the motion of the brightest of these knots relative along an axis which makes an angle φ with the line of to the unresolved component which corresponds to the sight, the observed expansion velocity is 2 c/sin

_1 -1 ity of which has remained constant over a period of about 60 km s Mpc . more than six years (Seielstad et al. 1979). The observation of very low circular polarization in Many other possibilities involving phase effects exist superluminal radio sources (Hodge and Aller 1979) has (see Blandford et al. 1977). However, nearly all such been used as an argument against the dipole field model. models lead to nonconstant expansion velocities or fre- However, Bahcall and Milgrom (1980) have found that quency-dependent structure (e.g., Epstein and Geller coherent curvature radiation (as originally suggested by 1977), in conflict with observation (Cohen et al. 1977). Cocke and Pacholczyk (1975)) obviates this difficulty An exception is the dipole field model (see below). since bunches on opposite sides of the magnetic field tend to cancel each other's circular polarizations. Never- E. Gravitational Lens theless, there is no evidence that coherent radiation The possibility that a massive object lying along the mechanisms are operating. The observed brightness tem- line of sight to a radio source could result in a gravita- peratures of superluminal components tend to lie be- tional focusing of the source image has been considered tween 1011 Κ and 1012 Κ (Readhead et al. 1979), similar in detail by Barnothy (1965) and Press and Gunn (1973). to those of other compact radio sources (Kellermann and Blandford et al. (1977) considered the case of a single Pauliny-Toth 1969). This implies that the source bright- component appearing as two images on opposite sides of nesses are regulated by Compton cooling of incoherent the lens mass. They concluded that superluminal motion synchrotron-emitting electrons, which yields a maximum cannot in general be obtained in this way. Recently, predicted brightness temperature of ~ 1012 K. Other at- however, Chitre and Narlikar (1979) found that the mag- tempts to search for evidence of coherent emission (e.g., nification of the image of a subluminally expanding or Condon and Dennison 1978) have turned up no evidence moving source by a distributed mass (such as a galaxy) for coherent mechanisms. could give the appearance of superluminal expansion. The likelihood that a galaxy lies along the line of sight G. Relativistic Bulk Motions to a QSO or active galactic nucleus is quite small. Rees (1966) first showed that relativistic expansion of Nevertheless, Chitre and Narlikar point out that flux a source can yield the illusion of superluminal motion to enhancement by the lens could lead to a selection ef- a distant observer. (In fact, Rees predicted superluminal fect in that the brightest objects of a flux-limited expansion to explain the rapid flux variations observed in sample might be primarily those few sources which many compact sources.) In general, the bulk relativistic happen to lie behind a gravitational lens. motion models can simultaneously explain (with a single A serious problem with the use of gravitational lenses Lorentz factor) both the superluminal motions of source to produce apparent superluminal effects is that no fore- components, and the high inferred brightness temper- ground object which could act as a lens is seen on photo- atures and consequent Compton X-ray problems associ- graphs of 3C 120. All features whose spectra can be ated with the flux density variations (cf., e.g., Burbidge, measured show a redshift ζ = 0.033 (Baldwin et al. Jones, and O'Dell 1974). Since observational evidence 1980), similar to the emission lines in the nucleus which seems to be accumulating against the other reasonable contains the radio source. alternatives (see above), the bulk motion models have be- come quite popular in recent years. The many models F. Dipole Field Model which involve relativistic bulk motion of the emitting Sanders (1974) has devised a model in which an out- plasma that have emerged since the paper by Rees burst of relativistic electrons streams out along the di- (1966) can be separated into four subclasses. pole field lines of a massive central object. The radiation is observed as the particles move onto those parts of the 1. Relativistic Free Expansion field lines whose tangents are parallel to the line of sight. Vitello and Salvati (1976) and Vitello and Pacini The differential delay time for the particles on various (1978) have considered the case of a relativistic plasma which expands freely into a vacuum. These authors find field lines to reach these critical points causes a bright that expansions constrained along one dimension can spot to appear on each side of the central object. The hotspots appear to separate superluminally with a veloc- produce the desired superluminal effects. However, it is difficult to obtain the appearance of two components of ity which depends on the orientation of the dipole axis comparable brightness in this manner. relative to the line of sight. A variation of this model has been studied by Cocke and Pacholczyk (1975). 2. Relativistic Blast-Waves Recently, Milgrom and Bahcall (1978) and Bahcall and Blandford and McKee (1976, 1977) have shown that a Milgrom (1980) have reexamined this model. They find large energy release in an ambient medium could result that it predicts a minimum separation speed of 4.4 c, in the formation of a relativistic blast-wave. Particle ac- which is consistent with the angular velocities quoted by celeration and magnetic-field amplification are expected Seielstad et al. (1979), as long as H0 does not exceed to occur behind the shock, thus producing an incoherent

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 130 MARSCHER AND SCOTT synchrotron source. Marscher (1978α,έ>) and Shapiro solved component (whose radiation is beamed toward (1979) have applied the Blandford and McKee formalism the observer) observed by Readhead et al. (1979). Bland- to an isotropic explosion in a disk-confined ambient me- ford and Königl consider two possible processes which dium. They found that the appearance of two super- could then cause the appearance of a knot of emission luminally-separating radio components could be pro- moving superluminally along the more diffuse portion of duced under certain circumstances. However, these blast the jet. In one case, changes in the plasma flow in the jet waves either decelerate (along the plane) or accelerate are imagined to cause a shock to form and propagate (along the axis), albeit possibly rather slowly, in con- through the jet. However, such a shock is smeared out in tradiction with the 3C 345 data (Seielstad et al. 1979). the observer's frame by front-to-back time delays of light 3. Relativistically Ejected Plasmoids propagation (see Jones and Tobin 1977) and would not The earliest model which was designed to produce the appear as a condensed knot of emission (see Marscher appearance of two superluminally separating com- 1980). ponents was that of Ozernoy and Sazonov (1969), in The second mechanism involves a dense cloud (e.g., an which two discrete plasmoids are relativistically ejected emission-line cloud or nova remnant) which is accelera- in opposite directions along an axis which is nearly par- ted to relativistic speeds by the ram pressure of the jet allel to the line of sight. Seielstad (1974) successfully ap- flow. A relativistic bow shock would then form behind plied the model to early observations of 3C 120. How- the cloud, with incoherent synchrotron emission being ever, as the nature of superluminal sources became more produced behind the shock front. apparent, a conflict with the model arose. The com- Marscher (1980) has considered another knot-forming ponents of 3C 345 were found to have comparable flux mechanism wherein condensations in the jet flow occur densities (see Cohen et al. 1977; Kellermann and Shaffer through radiation-driven thermal instabilities. As long as 1977). Yet, for components traveling with Lorentz fac- the magnetic field is high enough for the radiative life- tors γ 7 c (needed to explain the observed separation times of the relativistic electrons to be shorter than the velocity ν œ 7 c), the receding component should ap- expansion time of a parcel of fluid in the jet (but not so pear roughly γ6 105 (for a flat-spectrum source) times high that the magnetic pressure exceeds that of the elec- fainter than the approaching one. Even the fact that the trons), a perturbed region of slightly enhanced magnetic receding component would be younger (and hence in- field will collapse to form a hotspot of radio emission. trinsically brighter) cannot overcome this discrepancy The higher radiative energy losses cause the spectrum of (Christiansen, Scott, and Vestrand 1978). a knot to steepen as the region flows downstream, in ac- Cohen et al. (1979) have proposed a variation of this cordance with the observations of Readhead et al. model in which an explosion ejects two or more com- (1979). ponents in the same direction along an axis nearly paral- IV. Observational Tests lel to the line of sight. Only those components which are To summarize the discussion of the previous section, approaching the observer are detected. If the com- we can state that the random fluctuation model, general ponents are ejected with different Lorentz factors, then phase effects, and gravitational lens model seem to con- for a fairly wide range of such Lorentz factors the com- flict with presently available observational evidence. ponents exhibit superluminal separation from each other, Furthermore, somewhat more circumstantial evidence yet are observed to have comparable flux densities. seems to argue against overestimate of distance, the Christiansen and Scott (1977) have proposed a dyna- light-echo model, and dipole-field model. This leaves re- mical model in which an isotropic ejection of a large lativistic bulk motion as the most likely explanation of number of plasmoids in a disk-shaped ambient medium the superluminal phenomenon, although certain versions results in collimation sufficient to produce the appear- of these models can also be eliminated (see the previous ance of two superluminally separating components with section). Nevertheless, the cases against some of the oth- similar flux densities. er models are far from airtight. In this section we discuss 4. Processes in Relativistic Plasma Jets some observational tests which can be utilized to dis- Since the observations of Readhead et al. (1979) sug- tinguish among the possible theories. These tests either gest the presence of a jet-like component in three super- fall within the present capabilities of VLBI experiments luminal sources, the study of processes which might oc- or should be feasible within a few years. cur in relativistic jets (Blandford and Rees 1974, 1978) Unfortunately, current VLBI techniques do not allow seems natural in an attempt to explain superluminal phe- a precise determination of the absolute position of any nomena. In the recent model of Blandford and Königl given radio component. This may soon be possible, how- (1979), the narrow end of a diverging jet pointing nearly ever, especially in the case of 3C 345, which lies within directly toward the observer comprises the bright, unre- Io of NRAO 512, which can be used as a phase refer-

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System SUPERLUMINAL MOTION IN RADIO SOURCES 131 ence. (See the recent study of Shapiro et al. (1979), the components from both the old and new event are all who have obtained preliminary relative positions of visible, their relative positions in the source would allow NRAO 512 and 3C 345.) In order to illustrate how im- one to determine approximately where the origin is lo- portant such an observation would be, Figure 1 shows cated. Figure 1 illustrates the relative positions of the the various possible geometries of a two-component su- old (labeled "1") and new (^2") components for the vari- perluminal source. In models where the components ous possible geometries. are positioned symmetrically about the expansion ori- A comparison of observational characteristics of five gin (Fig. lb), the brightness centroid would appear es- models is given in Table 1. An asterisk denotes those sentially stationary (allowing for changes in the flux of properties which can be verified with present observa- one component relative to the other). In the other ex- tional techniques. The overestimate of distance and treme, where two components are moving with differ- gravitational lens hypotheses, which rely on mechanisms ent Lorentz factors in a direction almost parallel to the extrinsic to the source, make no observational predic- line of sight (Fig. Id), the brightness centroid would tions as to the behavior of the radio components. appear to have a superluminal velocity (possibly ex- The observational properties listed in Table I are diffi- ceeding the separation velocity) along the radio-source cult to avoid in the contexts of the models considered. axis. Hence, even an upper limit to the velocity of the Hence, we feel that continued VLBI studies of super- brightness centroid near the speed of light could con- luminal sources will enable one to distinguish among the strain the range of possible models. various alternatives. In objects such as 3C 120, where systematic expansions seem to begin every few years, one may be able to A few of the observations listed in Table I have al- determine the motion of the brightness centroid without ready been made and, if interpreted in the most straight- obtaining the absolute positions of the components. If forward possible manner, could be used as evidence for and against certain models. For example, the data on (α) Dipole Field·, Light-Echo 3C 120, most simply interpreted, indicate the occurrence of several events, each with different separation velocity -· · + · I 2 2 (Seielstad et al. 1979). If this interpretation is correct, these observations eliminate all but the most contrived versions of the light-echo and dipole-field models. (b) Christiansen - Scott Modeli Motion in Edge-on Plane Also, one could argue that the optically thin features of the maps of Readhead et al. (1979) should be matched 2 2 I up in order to compare the structure at 10.7 GHz and 5 GHz. If so, the maps indicate a slightly smaller separa- (c) Motion in Inclined Plane tion at 5 GHz, in accordance with the behavior expected +- in the relativistic jet models. A very interesting recent development in the study of superluminal sources is the direction of very extended 2 structure in 3C 273 and 3C 345 by Reich et al. (1979). The features, which are well-aligned with the inner structure, lie at projected distances 1.7 Mpc (3C 273; χ y ι = 55 and q0 = 0) and 12 Mpc and 14 Mpc (3C 345) from the compact sources. In those models in which the (d) Two Components Ejected in same Direction compact components lie within an angle c/vsep to the line of sight, these distances must be multiplied by at + 2 2 least üsep/c (i.e., by factors of 5 and 7 for 3C 273 and 3C 345, respectively). This implies an enormous separation of almost 100 Mpc for one of the lobes near 3C 345, (e) Relativistic Jet Narrow end Knots of Jet which seems highly unlikely. However, it is impossible a • posteriori to determine whether any particular extend- I 2 1.2 (stationary) ed/compact source association is real or accidental, so this apparent conflict with ejection along an axis cannot Fig. 1—Possible geometries of superluminal radio sources. Cross ( + ) be used as definitive evidence against such a model. indicates position of center of activity and filled circles represent positions of individual radio components. Numbers "1" and "2" re- Baldwin et al. (1980) have found that the motions of fer to two separate expansion events. Arrows indicate direction of gas in 3C 120 indicate a possible kinematical axis of sym- component motion (marked for event "l" only). metry which lies within 7° of the compact radio-source

TABLE I

Summary of Observational Predictions of Various Models for Superluminal Motions Model Light-Echo Dipole Field 2 Ejected Christian s en-Scott Relativistic Jet Plasmoids Collimated (BK = Blandford-Konigl, Plasmoids M=Mar scher)

variable from variable from one may vary from one event Separation ν -2c / sir ν sep >4. 4c Velocity^ sep one event to event to another to another constant constant another

Velocity of less than ν may exceed less than 1 v 1 COS0 sep Τ2 sep -2 ν sep brightnes s sep ν sep centroid Changes in initial acceleration BK-initial acceleration separation or deceleration to to asymptotic velocity velocity5^ asymptotic velocity M-none complicated c/\ any φ allowed c Λ Orientation sin 0 = 2c/ν sep φ sep sep angle of dependence on observer (Φ ) ν sep Source major no firm along dipole rotational rotational axis rotational axis axis5!' prediction axis Flux density does not no prediction strongly mildly decreasing strongly decreasing depend on <) decreasing function of Φ function of Φ function of 4 Separation-^ -independent v-independent ν-independent ;-independent increases with ν below turnover ν of jet

Component ν-independent v-independent ν dependence v-independent; knots: v-independent; sizes^ rapid possible; rapid fluctuations expanding fluctuations systematic plus systematic jet: size increases expansion expansion with ν below turnover ν of jet

Radio no prediction no firm steadily turnover ν fluctuates knot: steadily spectrum^ prediction decreasing about systematic decreasing turnover ν turnover ν decrease; broad jet: broad, constant turnover turnover Epoch of zero coincident coincident coincident somewhat displaced BK-somewhat later than separation with flux with flux with flux from flux outburst flux outburst (t-0/è);:: outburst outburst outburst M-prior to or same time as flux outburst 100 no prediction no prediction no prediction Value of H o H o

Observations possible with present techniques

axis. This is at least partial evidence that, as most models row considerably the range of acceptable models. None- predict, the radio components are ejected along the rota- theless, a variety of models are still possibly relevant. tion axis of the galactic gas distribution. However, it is We have attempted to show how observations possible not at all clear that the chaotic velocity field in 3C 120 in the near future can indicate which, if any, of the pro- is indicative of rotation. Furthermore, the probability of posed theories correctly describes the phenomenon. chance alignment of randomly oriented radio and veloc- From the circumstantial evidence cited in the pre- ity axes is not terribly low, ~ 10%. vious sections, we find that models which involve relativistic bulk motions of the radio components seem to pro- V. Summary vide the most likely explanation of apparent The observations of radio sources whose components superluminal motion. In spite of this, we must admit that appear to move superluminally are now sufficient to nar- none of the observations has revealed any evidence from

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