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Astron. Astrophys. 355, 552-563 (2000)

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1. Introduction

The quasar pair 1038+528 A,B (Owen et al. 1978) consists of two flat-spectrum radio sources, with redshifts 0.678 and 2.296 (Owen et al. 1980), separated on the sky by only 33". This system provides a unique opportunity to carry out high precision, relative astrometric studies using the full precision of VLBI relative phase measurements, since most sources of phase errors are common for the 2 sources (Marcaide & Shapiro 1983).

VLBI studies of the mas-scale structure of flat-spectrum quasars show that they typically have "core-jet" morphologies, consisting of a highly compact feature (the "core") located at the base of an extended linear feature or line of lower brightness components (the "jet"). Both 1038+528 A and B exhibit such structures. In standard models of extragalactic radio sources, these radio-emitting features arise from a collimated beam of plasma which is ejected with a highly relativistic bulk velocity from a region close to a central massive object such as a black hole (see eg. Blandford & Königl 1986). Whilst jet features may correspond to shocks in the moving plasma, and can give rise to the observed "superluminal" component motions in some sources (Porcas 1987), the "core" emission is thought to arise from a more-or-less permanent location close to the origin of the beam, where the ambient conditions correspond to a transition from optically thick to optically thin emission at the observed frequency. Although the "core" position may thus be frequency-dependent, for a fixed observing frequency the core should provide a stable marker, anchored to the central mass of the quasar, whose location can be used to define a precise position for the object as a whole. Although short time-scale variations in physical conditions may cause small changes in the "core" location, over long time-scales it may be used to track any systematic proper motion of the quasar.

The results from a near decade-long VLBI monitoring program on 1038+52A,B at [FORMULA] 3.6 and 13 cm (from 1981.2 to 1990.5) are reported by Rioja et al. (1997a), whose main conclusions can be summarized as follows:

  1. There is no evidence of any relative proper motion between the quasars A and B. The uncertainties in the astrometric parameters result in an upper bound to any systematic relative motion between the cores of 10 µas yr-1, consistent with zero.

  2. A compact feature within the jet of quasar B, chosen as the reference point for the structure, expands away from the core at a steady, slow rate of [FORMULA]as yr-1, corresponding to v=[FORMULA] c for a Hubble constant, H0 = 100 h km s-1 Mpc-1; q0=0.5. These values are used here throughout.

  3. The accuracy of the relative separation measurement is limited by noise and source structure, with estimated precisions of about [FORMULA]as at [FORMULA] 3.6 cm at any epoch.

  4. Confirmation of the consistently large offset (about 0.7 mas) between the positions of the peak of brightness ("core") at [FORMULA] 3.6 and 13 cm in quasar A.

New VLBI observations of this pair were made in November 1995 (1995.9) at [FORMULA] 2, 3.6 and 13 cm. In this paper we report on results from our analysis of the 3.6 cm observations and investigate the temporal evolution of the source structures and relative separation from all four epochs spanning [FORMULA] years. Investigations of frequency-dependent source structure have also been made from a comparison of the astrometric measurements of the separations between A and B at all 3 wavelengths observed in 1995; these will be presented elsewhere (Rioja & Porcas in preparation).

Our new observations are described in Sect. 2. In Sect. 3 we describe the data reduction and mapping techniques used, and in Sect. 4 an analysis of the measurements in the maps. In Sect. 5 we compare the astrometric results from these observations with those from previous epochs and analyse the changes in separation. Conclusions are presented in Sect. 6.

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Online publication: March 9, 2000