2001MNRAS.327.1071R

External Faraday rotation has been detected in both the core and the parsec-scale jet of BL Lac in a four-frequency very long baseline interferometry (VLBI) experiment. This unexpected result indicates the presence of significant amounts of thermal gas close to the nucleus of this object. The rotation measure (RM) in the jet components is constant, and differs from the currently accepted Galactic RM, indicating that this value (-205rad.m^–2) is not applicable to the components in the parsec-scale jet. The similarity of the RM in these jet components leads us to suspect that the jet RM is caused by a foreground screen in our Galaxy, although we cannot rule out a combination of Galactic RM and RM local to the jet. If the jet RM is due solely to the Galaxy, this would mean that the currently accepted value of the foreground RM (-205rad.m^–2) is not correct, either because the value changed between 1982 and 1997, or because the assumption of no intrinsic source rotation was incorrect, as it was at our later epoch of observation. Our observations suggest a value of -108±19 rad.m^-2.

After correcting for the foreground RM, the core value is -427rad.m^–2, which is unexpected since, owing to the weakness of their line emission, BL Lac objects are often assumed to be depleted in gas. The core RM appears to be variable, probably because of the presence of at least two polarized components close to the core the relative contributions of which vary with time.