ALMA full polarization observations of PKS1830-211 during its record-breaking flare of 2019

We report Atacama-Large-millimeter-Array (ALMA) Band 6 full-polarization observations of the lensed blazar PKS1830-211 during its record-breaking radio and gamma-ray flare in the spring of 2019. The observations were taken close to the peak of the gamma activity and show a clear difference in polarization state between the two time-delayed images. The leading image has a fractional polarization about three times lower than the trailing image, implying that significant depolarization occurred during the flare. In addition, we observe clear intra-hour variability of the polarization properties between the two lensed images, with a quasi-linear increase of the differential electric-vector position angle at a rate of about two degrees per hour, associated with changes of the relative fractional polarization of ~10%. This variability, combined with the lower polarization close to the peak of gamma activity, is in agreement with models of magnetic turbulence to explain polarization variability in blazar jets. Finally, the comparison of results from the full and differential polarization analysis confirms that the differential polarization technique (Marti-Vidal et al. 2016) can provide useful information on the polarization state of sources like gravitationally lensed radio-loud quasars.