Annu. Rev. Astron. Astrophys. 1997. 35: 607-36
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2.1. High-Luminosity Core Dominated Objects

The high-luminosity core dominated objects are typically found in systematic flux-density-limited surveys of compact sources (see Wilkinson 1995 for a comprehensive review, Pearson & Readhead 1988, Witzel et al 1988, Wehrle et al 1992, Polatidis et al 1995, Thakkar et al 1995, Xu et al 1995, Taylor et al 1994, Henstock et al 1995). Such surveys are biased towards selecting strongly beamed objects, and indeed a majority (about 80-90%) have "core-jet" type structures, i.e. they contain an unresolved flat-spectrum "core" and a steeper-spectrum one-sided "jet" that in turn may contain distinct structure "components."

The cores are identified based on their compactness, flat or inverted radio spectra, and sometimes their flux density variability. In several cases, phase referencing observations have demonstrated little or no discernible movement for the core component, including 3C 345 (Bartel et al 1986), 1038+528 (Marcaide et al 1994b) and 4C39.25 (Guirado et al 1995). Narrow collimated jets are typical in this class of source.

The degree of alignment between these parsec-scale and the kiloparsec-scale jets shows a bimodal distribution with peaks near 0° and 90°, and BL Lac objects appear to show systematically stronger misalignments than quasars (e.g. Pearson & Readhead 1988, Wehrle et al 1992, Xu et al 1994, Appl et al 1996). This cannot be explained by a single population of slightly distorted jets, but it requires the presence of a second population of sources with small Lorentz factors and intrinsically large bending. Helical motion, perhaps arising in a binary black hole system, provides a likely mechanism to explain some cases that have been studied in detail (e.g. O'Dea et al 1988, Conway & Wrobel 1995). It is not yet clear if this is applicable for the population as a whole (Conway & Murphy 1993, Appl et al 1996), nor is it clear what causes the systematic differences found for BL Lac objects and quasars, of which the former have on average stronger misalignments (Conway & Murphy 1993, Conway 1994).

Structural variability and in particular apparent superluminal motion are frequently observed, and bulk relativistic motion is commonly inferred (Witzel et al 1988, Vermeulen 1995). Many of the observed jets are curved, and in some cases semioscillating trajectories or ridge lines have been observed (see below). This effect is typically most pronounced near the core (Krichbaum & Witzel 1992).

A good example for a parsec-scale jet in a core-dominated source is the quasar 3C 345. Figure 2 shows an image at 5 GHz (Lobanov & Zensus 1997). This source has been regularly monitored with VLBI since 1979 (Biretta et al 1986, Zensus et al 1995a, Brown et al 1994, Wardle et al 1994, Rantakyrö et al 1995, Zensus et al 1995c). At 5 GHz, the jet appears continuous, but from comparison with images made at higher frequencies, it is known that the structure at a given epoch can be attributed primarily to a few distinct features (these are mostly blended in the image shown). These features typically have been tracked for several years. In 3C 345, for example, high-dynamic range work has revealed a complex filamentary underlying jet flow (Unwin & Wehrle 1992).

Figure 2

Figure 2. The parsec-scale jet of 3C 345 at 5 GHz, June 1992 (from JA Zensus, AP Lobanov, KJ Leppänen, SC Unwin, and AE Wehrle, in preparation. Contours are -0.1, -0.06, 0.06, 0.1, 0.2, 0.3, 0.5, 1, 2, 3, 5, 10, 25, 50, 75, and 90% of the peak, 3.17 Jy per beam. The restoring beam is 2.22 × 1.0 milliarcsec, at PA -6°.

An increasing number of core-dominated quasars like 3C 345 have been imaged in fascinating detail; two other examples are 3C 273 (Zensus et al 1990, Davis et al 1991, Unwin et al 1994a, Bahcall et al 1995) and 0836+710 (Hummel et al 1992a). The 18-cm image of 3C 273 has a dynamic range (defined as the ratio of image peak and lowest believable feature) exceeding 5000:1, and the image traces the jet to more than 150 milliarcsec from the core. An extended secondary feature detected as separate from the arcsecond jet has been speculated to be the elusive counterjet feature (Davis et al 1991). In general, however, no convincing counterjet has been found in any core-dominated, i.e. presumably strongly boosted, source. The 2000:1 dynamic range image of 0836+710 was made by combining VLBI data with VLBA and MERLIN observations and traces the complex continuous jet in this source from parsec to kiloparsec scales (Hummel et al 1992a).

These examples demonstrate a significant trend: Images of luminous sources typically show continuous jets with rich substructures, which are markedly different from the simple structures represented in maps from only a few years ago. In some cases, the jets are resolved in transverse directions, and filaments, limb-brightening, and edge-brightening have been reported. The technical advance in imaging capability has brought with it the need for more complicated models to explain the properties of the sources under study. The main features in these images still correspond to the "distinct components" seen in older maps, but state-of-the-art images tend to reveal weaker features and often directly reveal the underlying continuous jet emission. The evidence for the apparent superluminal motions in the best-studied sources has remained strong, but at the same time it is clear that infrequent sampling in time is bound to cause misidentification and confusion. Any motion is typically not uniform, and several values of motion have been observed in a given source. In particular, there coexist in some cases slow or stationary features and fast-moving regions. Little is known about the kinematic properties of the underlying continuous jet component. Although subluminal motions have been observed mostly in radio galaxies (e.g. Cygnus A, M 87, 3C 84, Centaurus A), there are some quasars with subluminal components as well (e.g. 0153+74).

There are a number of highly compact sources that show intraday variability (see Wagner & Witzel 1995). Although refractive interstellar scintillation can explain so far only the lower-frequency behavior in cases like 0917+624, it must have some effect in all very compact objects (Rickett et al 1995); the correlated variability between radio and optical and between optical and X rays, such as those found in 0716+714 (Wagner et al 1996), probably rules out a solely extrinsic origin (Qian et al 1995).

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