A&A 447, 515-524 (2006)
DOI: 10.1051/0004-6361:20054012

VLBI phase-reference observations of the gravitational lens JVAS B0218+357

R. Mittal¹, R. Porcas¹, O. Wucknitz^{2, 3}, A. Biggs^{3, 4} and I. Browne<sup>5

1</sup>  MPIfR, Auf dem Hügel 69, 53121 Bonn, Germany
²  Universität Potsdam, Institut für Physik, Am Neuen Palais 10, 14469 Potsdam, Germany
³  JIVE, 7990 AA Dwingeloo, The Netherlands
⁴  UK Astronomy Technology Centre, Royal Observatory, Edinburgh, UK
⁵  JBO, Macclesfield, Cheshire SK11 9DL, UK

(Received 9 August 2005 / Accepted 12 October 2005 )

Abstract
We present the results of phase-referenced VLBA+Effelsberg observations at five frequencies of the double-image gravitational lens JVAS B0218+357, made to establish the precise registration of the A and B lensed image positions. The motivation behind these observations is to investigate the anomalous variation of the image flux-density ratio (A/B) with frequency - this ratio changes by almost a factor of two over a frequency range from 1.65 GHz to 15.35 GHz. We investigate whether frequency dependent image positions, combined with a magnification gradient across the image field, could give rise to the anomaly. Our observations confirm the variation of image flux-density ratio with frequency. The results from our phase-reference astrometry, taken together with the lens mass model of Wucknitz et al. (2004, MNRAS, 349, 14), show that shifts of the image peaks and centroids are too small to account for the observed frequency-dependent ratio.

Key words: gravitational lensing -- techniques: interferometric -- quasars: individual: JVAS B0218+357

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