Letter to the Editor

ALMA full polarization observations of PKS 1830−211 during its record-breaking flare of 2019

¹ Observatori Astronómic, Universitat de València, Parc Científic, C. Catedràtico José Beltrán 2, 46980 Paterna, València, Spain
e-mail: i.marti-vidal@uv.es
² Departament d’Astronomia i Astrofísica, Universitat de València, C. Dr. Moliner 50, 46100 Burjassot, València, Spain
³ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁴ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
⁵ Instituto de Astrofísica de Andalucía, CSIC, Apartado 3004, 18080 Granada, Spain

Received: 4 April 2020
Accepted: 22 May 2020

Abstract

We report Atacama Large Millimeter Array (ALMA) Band 6 full-polarization observations of the lensed blazar PKS 1830−211 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 in the differential electric-vector position angle at a rate of about two degrees per hour, associated with changes in 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 (Martí-Vidal et al. 2016, A&A, 593, A61) can provide useful information on the polarization state of sources like gravitationally lensed radio-loud quasars.