SIMBAD references

We have analyzed 1+2+4+6 cm VLBA polarization data for the TeV BL Lac object Mrk 501 obtained at three epochs separated by 3-4 weeks in 1998 March-April. These observations show no significant total-intensity or polarization variability over the interval covered by the observations, supporting previous claims that variability in the parsec-scale structure of this source (e.g., motions in its VLBI jet) is modest. The B field structure is complex, and shows a sheath-like structure. The rotation-measure distribution shows evidence for variations across the jet, suggesting the presence of a helical B field associated with the VLBI jet.