Downstream Depolarization in the Sausage Relic: A 1-4 GHz Very Large Array Study

Abstract

Radio relics are elongated sources related to shocks driven by galaxy cluster merger events. Although these objects are highly polarized at GHz frequencies (greater than or similar to 20%), high-resolution studies of their polarization properties are still lacking. We present the first high-resolution and high-sensitivity polarimetry study of the merging galaxy cluster CIZA J2242.8+5301 in the 1-4 GHz frequency band. We use the QU-fitting approach to model the Stokes I, Q, and U emission, obtaining best-fit intrinsic polarization fraction (p(0)), intrinsic polarization angle (chi(0)), rotation measure (RM), and wavelength-dependent depolarization (sigma(RM)) maps of the cluster. Our analysis focuses on the northern relic (RN). For the first time in a radio relic, we observe a decreasing polarization fraction in the downstream region. Our findings are possibly explained by geometrical projections and/or by decreasing of the magnetic field anisotropy toward the cluster center. From the amount of depolarization of the only detected background radio galaxy, we estimate a turbulent magnetic field strength of B-turb similar to 5.6 mu G in the relic. Finally, we observe RM fluctuations of about 30 rad m(-2) around the median value of 140.8 rad m(-2) at the relic position.