Frequency-dependent flux ratios in B0218+357

Talk given at the last CERES workshop in Linoperamata, Crete, 28. June 2001

O. Wucknitz [1,2]

  1. Hamburger Sternwarte, Gojenbergsweg 112, D-21029 Hamburg, Germany
  2. Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, UK

Abstract

In 0218+357, the flux ratio A:B shows a strong dependence on frequency. At high frequencies (15 GHz and above), it is about 3.8, while it goes down to about 3.0 at 5 and 2.6 at 1.7 GHz. Two scenarios provide possible explanations for this fact which seems to be incompatible with achromatism in lensing at first sight. One is a possible extinction in the A component, due to scattering or other processes. The other is an effective shift of the source with frequency, which, together with the magnification ratio gradient of the lens, would also change flux ratios at lower frequencies. I summarize evidence for the two theories and present some preliminary results from my own LensCLEAN calculations. In these, I compare models and data at 15 and 5 GHz to determine a shift between the two as well as the amount of possible extinction. LensCLEAN seems to favour a situation with about 20 to 25 per cent extinction and with neglibible shift.

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



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