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Published online by Cambridge University Press: 14 August 2015
The variability of the highly redshifted 21 cm absorption spectrum in AO 0235+164 presents us with two essential facts. First, rapid changes occur in the line depths of foreground clouds with redshift z ≃ 0.524. Second, the background radio source is among the most violently variable objects known. Thus it is reasonable to suppose that changes in the lines stem from activity in the background BL Lac object. The question is how?