Relics in galaxy clusters at high radio frequencies

Abstract

Aims. We investigated the magnetic properties of radio relics located at the peripheries of galaxy clusters at high radio frequencies, where the emission is expected to be free of Faraday depolarization. The degree of polarization is a measure of the magnetic field compression and, hence, the Mach number. Polarization observations can also be used to confirm relic candidates. Methods. We observed three radio relics in galaxy clusters and one radio relic candidate at 4.85 and 8.35 GHz in total emission and linearly polarized emission with the Effelsberg 100-m telescope. In addition, we observed one radio relic candidate in X-rays with the Chandra telescope. We derived maps of polarization angle, polarization degree, and Faraday rotation measures. Results. The radio spectra of the integrated emission below 8.35 GHz can be well fitted by single power laws for all four relics. The flat spectra (spectral indices of 0.9 and 1.0) for the so-called Sausage relic in cluster CIZA J2242+53 and the so-called Toothbrush relic in cluster 1RXS 06+42 indicate that models describing the origin of relics have to include effects beyond the assumptions of diffuse shock acceleration. The spectra of the radio relics in ZwCl 0008+52 and in Abell 1612 are steep, as expected from weak shocks (Mach number ≈2.4). Polarization observations of radio relics offer a method of measuring the strength and geometry of the shock front. We find polarization degrees of more than 50% in the two prominent Mpc-sized radio relics, the Sausage and the Toothbrush, which are among the highest percentages of linear polarization detected in any extragalactic radio source to date. This is remarkable because the large beam size of the Effelsberg single-dish telescope corresponds to linear extensions of about 300 kpc at 8.35 GHz at the distances of the relics. The high degree of polarization indicates that the magnetic field vectors are almost perfectly aligned along the relic structure, as expected for shock fronts that are observed edge-on. The polarization degrees correspond to Mach numbers of >2.2. Polarized emission is also detected in the radio relics in ZwCl 0008+52 and, for the first time, in Abell 1612. The smaller sizes and lower degrees of polarizations of the latter relics indicate a weaker shock and/or an inclination between the relic and the sky plane. Abell 1612 shows a complex X-ray surface brightness distribution, indicating a recent major merger and supporting the classification of the radio emission as a radio relic. In our cluster sample, no wavelength-dependent Faraday depolarization is detected between 4.85 GHz and 8.35 GHz, except for one component of the Toothbrush relic. Faraday depolarization between 1.38 GHz and 8.35 GHz varies with distance from the center of the host cluster 1RXS 06+42, which can be explained by a decrease in electron density and/or in strength of a turbulent (or tangled) magnetic field. Faraday rotation measures show large-scale gradients along the relics, which cannot be explained by variations in the Milk...

New mysteries and challenges from the Toothbrush relic: wideband observations from 550 MHz to 8 GHz

“…We compare our flux density measurements with values obtained with single dish observations that do not resolve out flux of extended sources. At 4.8 GHz, we measure a flux density of 83±8 mJy which is higher than the measurement with Effelsberg at 4.85 GHz, namely 68 ± 5 mJy (Kierdorf et al 2017). However, they noted that their value is possibly too low because of the insufficient size and low quality of the radio map.…”

“…Out of these five mentioned relics, a spectral steepening has been detected between 2-5 GHz for three relics, namely the Sausage relic, the relic in A2256, and Bullet cluster. In contrast to this, some authors reported no steepening in the integrated spectrum for the Sausage relic (Kierdorf et al 2017;Loi et al 2017). Fig.…”

“…They highlighted a steepening of the integrated spectrum beyond 2 GHz (from α = −1.00 to −1.45). In contrast, the studies conducted with high frequency single dish observations at 4.8 GHz and 8.3 GHz have not shown significant evidence of spectral steepening (Kierdorf et al 2017). Notes.…”

Shock Acceleration Model for the Toothbrush Radio Relic

“…The squares at low frequencies are the observational data given in Table A1 of Stroe et al (2016). The two squares at 4.85 and 8.35 GHz are fluxes in Table 5 of Kierdorf et al (2017), multiplied by a factor of 0.71. The error bars are given in the same tables.…”

The origin of radio relics in galaxy clusters: a combined $\gamma$ and radio wavelength analysis.

“…This argument seems to strongly contend the DSA scenario for the origin of the relativistic electron population necessary to produce RR emission. In addition, (35) recently found that the spectral index measured in several relics is not what is expected from the DSA theory, where…”