Magnetic fields in quasar cores. II.

Multifrequency polarimetry with the Very Long Baseline Array telescope has revealed absolute Faraday rotation measures (RMs) in excess of 1000 rad m(-2) in the central regions of seven out of eight strong quasars studied (e.g., 3C 273, 3C 279, and 3C 395). Beyond a projected distance of similar to 20 pc, however, the jets are found to have \ RM \ < 100 rad m(-2). Such sharp RM gradients cannot be produced by cluster or galactic-scale magnetic fields, but rather must be the result of magnetic fields organized over the central 1-100 pc. The RMs of the sources studied to date and the polarization properties of BL Lacs, quasars, and galaxies are shown to be consistent so far with the predictions of unified schemes. The direct detection of high RMs in these quasar cores can explain the low fractional core polarizations usually observed in quasars at centimeter wavelengths as the result of irregularities in the Faraday screen on scales smaller than the telescope beam. Variability in the RM of the core is reported for 3C 279 between observations taken 1.5 yr apart, indicating that the Faraday screen changes on that timescale or that the projected superluminal motion of the inner jet components samples a new location in the screen with time. Either way, these changes in the Faraday screen may explain the dramatic variability in core polarization properties displayed by quasars.