Polarization VLBI observations of the gravitational lens system B0218+357 at 8.4 GHz

We present polarization VLBI observations of the gravitational lens system B0218 + 357 at an observing frequency of 8.4 GHz. The lensed components A a nd B show a core-jet substructure at milliarcsecond (mas) resolution, consi stent with the quantitative lensing properties known for the system. The id entified core is linearly polarized at a level of 8%-12%. The polarization position angle difference between A and B is consistent with the expected b ehavior of polarized emission under gravitational lensing and the known rot ation due to differential Faraday rotation along the two lines of sight thr ough the lensing galaxy. The mas polarization structure confirms the expect ed parity reversal for the two lensed components predicted by generic lens models for this system. Indirect evidence is presented in support of the BL Lacertae optical classification of the lensed object.