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

A phase-reference study of the quasar pair 1038+528A,B

M.J. Rioja ^1,2 and R.W. Porcas <sup>3

1</sup> Observatorio Astronómico Nacional (OAN), Apartado 1143, 28800 Alcalá de Henares, Spain (rioja@oan.es)
² Joint Institute for VLBI in Europe (JIVE), 7990 AA Dwingeloo, The Netherlands
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany (porcas@mpifr-bonn.mpg.de)

Received 5 October 1999 / Accepted 20 January 2000

Abstract

We present results from 3.6 cm observations of the quasar pair 1038+528 A and B, made in 1995 using the VLBA together with the Effelsberg 100m telescope. We describe the use of a phase-referencing technique to measure the astrometric separation between the quasars. We also introduce a new data analysis method - "hybrid double mapping" - which preserves the relative astrometric information in a single VLBI hybrid map for close source pairs. We combine our measurements with those from three previous epochs, the earliest in 1981. Our new observations confirm the evolution within the structure of quasar B, previously proposed to explain the measured change in the relative separation of the pair. Our upper bound for any systematic proper motion between the mass centres of quasars A and B is 10 µas yr^-1. This is set by the limited precision in defining the reference points in the quasars at different epochs and by possible instabilities of the source "core" locations. A separate analysis enables us to put more stringent upper limits to any core motions along the two source axes.

Key words: instrumentation: interferometers techniques: interferometric astrometry radio continuum: general

Send offprint requests to: María J. Rioja

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Contents

• 1. Introduction
• 2. Observations
• 3. Data reduction
  • 3.1. Hybrid mapping in AIPS
  • 3.2. Phase referencing in AIPS
  • 3.3. New mapping method for astrometry of close source pairs
• 4. Analysis of the maps
  • 4.1. Source structures
  • 4.2. Estimating positions of reference features
  • 4.3. Position error analysis
  • 4.4. Astrometry results
• 5. Comparison of astrometry at all 4 epochs
  • 5.1. Comparison between the techniques used at different epochs
  • 5.2. Astrometric separations at the 4 epochs
  • 5.3. Vector decomposition
  • 5.4. Structural evolution within 1038+528 B
  • 5.5. Relative proper motion
  • 5.6. Possible "core" motions?
• 6. Conclusions
• Appendices
  • A: Hybrid Double Mappping (HDM)
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: March 9, 2000
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