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Published online by Cambridge University Press: 25 May 2016
The radio core of BL Lac has been monitored with VLBI techniques since 1980. At least seven superluminal events have been seen, usually associated with flux outbursts. The components have been interpreted as weak relativistic shocks associated with opposed jets. Recent dramatic changes in the nucleus of BL Lac have been seen: optical spectra (Lawrence et al. 1995) show the presence of broad-line Hα and [NII] emission lines. The single-dish radio flux has also increased by 3 times in the past year. Often in BL Lac's past, a flux outburst immediately precedes the ejection of a new superluminal component.