@article{WucknitzBiggsBrowne2004,
  author    = {Wucknitz, Olaf and Biggs, Andy D. and Browne, Ian W. A.},
  title     = {Models for the lens and source of B0218+357 : a LensClean approach to determine H-0},
  issn      = {0035-8711},
  year      = {2004},
  abstract  = {B0218 + 357 is one of the most promising systems to determine the Hubble constant from time-delays in gravitational lenses. Consisting of two bright images, which are well resolved in very long baseline interferometry (VLBI) observations, plus one of the most richly structured Einstein rings, it potentially provides better constraints for the mass model than most other systems. The main problem left until now was the very poorly determined position of the lensing galaxy. 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 H-0 = (78 +/- 6) km s(-1) Mpc(-1) (2sigma) is very high compared with other lenses. It is, however, compatible with local estimates from the Hubble Space Telescope (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 H-0 is reduced from the expected 4 per cent decrease by an estimated shift of the best galaxy position of circa 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 mas to arcsec, 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.},
  language  = {en}
}
@article{YorkJacksonBrowneetal.2005,
  author    = {York, T. and Jackson, N. and Browne, Ian W. A. and Wucknitz, Olaf and Skelton, J. E.},
  title     = {The Hubble constant from the gravitational lens CLASS B0218+357 using the Advanced Camera for Surveys},
  issn      = {0035-8711},
  year      = {2005},
  abstract  = {We present deep optical observations of the gravitational lens system CLASS B0218 + 357, from which we derive an estimate for the Hubble constant (H-0). Extensive radio observations using the VLA, MERLIN, the VLBA and VLBI have reduced the degeneracies between H-0 and the mass model parameters in this lens to one involving only the position of the radio-quiet lensing galaxy with respect to the lensed images. B0218 + 357 has an image separation of only 334 mas, so optical observations have, up until now, been unable to resolve the lens galaxy from the bright lensed images. Using the new Advanced Camera for Surveys (ACS), installed on the Hubble Space Telescope in 2002, we have obtained deep optical images of the lens system and surrounding field. These observations have allowed us to determine the separation between the lens galaxy centre and the brightest image, and so estimate H-0. We find an optical galaxy position, and hence an H0 value, that varies depending on our approach to the spiral arms in B0218 + 357. If the most prominent spiral arms are left unmasked, we find H-0 = 70 +/- 5 km s(-1) Mpc(-1) (95 per cent confidence). If the spiral arms are masked out, we find H-0 = 61 +/- 7 km s(-1) Mpc(-1) (95 per cent confidence)},
  language  = {en}
}
@article{MittalPorcasWucknitzetal.2006,
  author    = {Mittal, Rupal and Porcas, Richard and Wucknitz, Olaf and Biggs, Andy D. and Browne, Ian W. A.},
  title     = {VLBI phase-reference observations of the gravitational lens JVAS B0218+357},
  doi       = {10.1051/0004-6361:20054012},
  year      = {2006},
  abstract  = {We present the results of phase-referenced VLBA+Effelsberg observations at five frequencies of the double-image gravitational lens WAS 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},
  language  = {en}
}