FRB 121102: Drastic changes in the burst polarization contrasts with the stability of the persistent emission

We study milliarcsecond-scale properties of the persistent radio counterpart to FRB 121102 and investigate the spectro-polarimetric properties of a bright burst. For the former, we use European VLBI Network (EVN) observations in 2017 at 1.7 and 4.8 GHz. For the latter, we reanalyse the 1.7-GHz data from the 100-m Effelseberg telescope taken in 2016. These observations predate other polarimetric studies of FRB 121102, and yield the highest burst Faraday rotation measure (RM) to date, RM = 1.27 · 10⁵ rad m^-2, consistent with the decreasing RM trend. The fractional polarization of the burst emission is 15 per cent at 1.7 GHz. This can be reconciled with the high-fractional polarization at higher frequencies if the Faraday width of the burst environment is 150 rad m^-2 - a bare 0.1 per cent of the total Faraday rotation. The width may originate from minor non-uniformities in the Faraday screen, or from effects in the emitting region itself. The upper limit on the persistent source size is 1 pc, barely consistent with a young supernova (SN) scenario. The flux variability limit of <10 per cent is not in favour of the young SN scenario, and challenges other interpretations as well. The fractional polarization of the faint persistent source is constrained at <25 per cent at 4.8 GHz ruling out a common origin with the highly polarized individual bursts.