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Astron. Astrophys. 330, 79-89 (1998)

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1. Introduction

Extragalactic jets are believed to be formed in the vicinity of an accretion disk around a supermassive black hole in the center of active galactic nuclei (AGN). Pressure and density gradients, as well as a toroidally shaped magnetic field produced by the rotation of the disk, may collimate the outflowing material into two bipolar streams, and accelerate it up to relativistic speeds (see reviews by Begelman, Blandford, & Rees, 1984; Begelman 1995). Physical conditions in the immediate proximity of an accretion disk determine many aspects of jet evolution at larger spatial scales. The extent of this ultracompact fraction of the jet is estimated to be [FORMULA] pc. Roland et al. (1994) model it as a turbulent region of [FORMULA] pc in size. Marscher & Gear (1985) estimate the projected size of ultracompact jet to be [FORMULA] pc, based on VLBI observations of a radio flare in 3C 273. Königl (1981) and Zensus, Cohen, & Unwin (1995) regard an unresolved core as an ultracompact jet.

In images obtained with Very Long Baseline Interferometry (VLBI), the core is usually identified with the most compact (often unresolved) feature exhibiting a substantial flux and flat spectrum across the radio band. At any given frequency, the core is believed to be located in the region of the jet where the optical depth is [FORMULA]. The core absolute position, [FORMULA], should therefore depend on the observing frequency, [FORMULA]. Königl (1981) gives [FORMULA]. The power index [FORMULA] depends on the shape of electron energy spectrum and on the magnetic field and particle density distributions in the ultracompact jet. Observed offsets of the core position at different frequencies have been reported for several sources including 1038+528 A (Marcaide & Shapiro 1984), 4C 39.25 (Guirado et al. 1995), 3C 395 (Lara et al. 1996), and 3C 309.1 (Aaron 1996). Several studies of the core position offset have been undertaken for 3C 345 (Biretta, Moore, & Cohen. 1986; Unwin et al. 1994; Zensus et al. 1995).

In this paper, we discuss synchrotron self-absorption and free-free absorption in the nuclear regions of AGN. We use the frequency dependence of the VLBI core position as a tool for determining the physical conditions of ultracompact jets. In section 2, we describe a model adopted for ultracompact jets, and outline the relations between core shift and physical properties of the jets. Measurements of the shift of VLBI core in radio sources are discussed in section 3. In section 4, the measured core offsets are applied to deriving the magnetic field distribution and physical conditions in the central regions of Cygnus A, 3C 309.1, 3C 345, 3C 395, 4C 39.25, and 1038+528 A.

Throughout the paper, we use a Hubble constant [FORMULA] km s-1 Mpc-1 and deceleration parameter [FORMULA]. Unless defined otherwise, all quantities are in the cgs units, except for distances which are given in parsecs.

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© European Southern Observatory (ESO) 1998

Online publication: January 8, 1998