Models for the lens and source of B0218+357: a LensClean approach to determine H₀

MNRAS 349 (2004) 14-30

O. Wucknitz [1,2,3], A.D. Biggs [1,4], I.W.A. Browne [1]

1. University of Manchester, Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, UK
2. Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
3. Universität Potsdam, Institut für Physik, Am Neuen Palais 10,14469 Potsdam, Germany
4. Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands

Abstract

After presenting detailed results from classical lens modelling, we apply our improved version of the LensClean algorithm which for the first time utilizes the beautiful Einstein ring for lens modelling purposes. The primary result using isothermal lens models is a now very well defined lens position of ( 255 +- 6 , 119 +- 4 ) mas relative to the A image, which allows the first reliable measurement of the Hubble constant from the time-delay of this system. The result of H₀= ( 78 +- 6 ) km/s/Mpc (2 sigma) is very high compared with other lenses. It is, however, compatible with local estimates from the HST key project and with WMAP results, but less prone to systematic errors. We furthermore discuss possible changes of these results for different radial mass profiles and find that the final values cannot be very different from the isothermal expectations. The power-law exponent of the potential is constrained by VLBI data of the compact images and the inner jet to be beta= 1.04 +- 0.02, which confirms that the mass distribution is approximately isothermal (corresponding to beta=1), but slightly shallower. The effect on H₀ is reduced from the expected 4 per cent decrease by an estimate shift of the best galaxy position of ca. 4 mas to at most 2 per cent.

Maps of the unlensed source plane produced from the best LensClean brightness model show a typical jet structure and allow us to identify the parts which are distorted by the lens to produce the radio ring. We also present a composite map which for the first time shows the rich structure of B0218+357 on scales ranging from milli-arcseconds to arcseconds, both in the image plane and in the reconstructed source plane. Finally we use a comparison of observations at different frequencies to investigate the question of possible weakening of one of the images by propagation effects and/or source shifts with frequency. The data clearly favour the model of significant "extinction" without noticeable source position shifts.

The technical details of our variant of the LensClean method are presented in the accompanying Paper I.

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

This is the second of two papers about our new variant of LensClean (Paper I) and its application to the lens B0218+357 (this Paper II). This work was initially submitted in January 2003 as a single paper.

For more details, have a look at my PhD thesis.

Erratum