Using the polarization properties of double radio relics to probe the turbulent compression scenario

Abstract

Context. Radio relics are Mpc-size synchrotron sources located in the outskirts of some merging galaxy clusters. Binary-merging systems with favorable orientation may host two almost symmetric relics, named double radio relics. Aims. Double radio relics are seen preferentially edge-on and, thus, constitute a privileged sample for statistical studies. Their polarization and Faraday rotation properties give direct access to the relics' origin and magnetic fields. Methods. In this paper, we present a polarization and Rotation Measure (RM) synthesis study of four clusters hosting double radio relics, namely 8C 0212+703, Abell 3365, PLCK G287.0+32.9, previously missing polarization studies, and ZwCl 2341+0000, for which conflicting results have been reported. We used 1-2 GHz Karl G. Jansky Very Large Array observations. We also provide an updated compilation of known double radio relics with important observed quantities. We studied their polarization and Faraday rotation properties at 1.4 GHz and we searched for correlations between fractional polarization and physical resolution, distance from the cluster center, and shock Mach number. Results. The weak correlations found between these quantities are well reproduced by state-of-the-art magneto-hydrodynamical simulations of radio relics, confirming that merger shock waves propagate in a turbulent medium with tangled magnetic fields. Both external and internal Faraday depolarization should play a fundamental role in determining the polarization properties of radio relics at 1.4 GHz. Although the number of double radio relics with RM information is still low, their Faraday rotation properties (i.e., restframe RM and RM dispersion below 40 rad m −2 and non-Gaussian RM distribution) can be explained in the scenario in which shock waves with Mach numbers larger than 2.5 propagate along the plane of the sky and compress the turbulent intra-cluster medium.

Polarized accretion shocks from the cosmic web

“…While few in number, the findings from studies of cluster relics agree well with the findings in this work. The excess between the stacked connected clusters has a spectral index in the range of −0.85 ± 0.1 to −1.05 ± 0.15, consistent with typical values of ∼ −1 from relic compilations (10,12) (and references therein). In another compilation of relics (11), the minimum reported integrated flux density of a known relic at 1.4 GHz was 2.4 millijansky (mJy) (with a mean of the reported relics being tens of millijanskys).…”

“…The results seen from stacking single clusters fit well in line with the theory, although this emission has not been observed before at these faint levels, at these distances from the clusters, or in low-mass clusters. From a compilation of known cluster relics (11), relics tend to be found at distances around 0.5 to 2 Mpc from the cluster centers; in double relic systems, the average distance is 1.03 Mpc (12). They also tend to be strongly polarized (20 to 30%) (10-12), while the central part of the clusters may be filled with diffuse emission from radio halos, which, to date, have not been detected in polarized intensity due to the high levels of turbulence in the central regions of the clusters causing depolarization.…”

“…There have been numerous detections and studies of diffuse radio emission in clusters (10)(11)(12). This emission consists of different categories such as radio halos, which are centrally located largescale diffuse sources that roughly follow the intracluster medium mass distribution and are generally produced through secondorder Fermi acceleration (Fermi-II) (13,14) and/or secondary electrons for cosmic ray acceleration (15)(16)(17).…”