Letter to the Editor

A perfect power-law spectrum even at the highest frequencies: The Toothbrush relic^⋆

¹ Dipartimento di Fisica e Astronomia, Universitát di Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
e-mail: kamlesh.rajpurohit@unibo.it
² INAF-Istituto di Radio Astronomia, Via Gobetti 101, 40129 Bologna, Italy
³ Thüringer Landessternwarte (TLS), Sternwarte 5, 07778 Tautenburg, Germany
⁴ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁵ INAF-Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, CA, Italy
⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁷ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 12 August 2020
Accepted: 19 September 2020

Abstract

Radio relics trace shock fronts generated in the intracluster medium (ICM) during cluster mergers. The particle acceleration mechanism at the shock fronts is not yet completely understood. We observed the Toothbrush relic with the Effelsberg and Sardinia Radio Telescope at 14.25 GHz and 18.6 GHz, respectively. Unlike previously claimed, the integrated spectrum of the relic closely follows a power law over almost three orders of magnitude in frequency, with a spectral index of α_{58 MHz}^{18.6 GHz} = −1.16 ± 0.03. Our finding is consistent with a power-law injection spectrum, as predicted by diffusive shock acceleration theory. The result suggests that there is only little magnetic field strength evolution downstream of the shock. From the lack of spectral steepening, we find that either the Sunyaev–Zeldovich decrement produced by the pressure jump is less extended than ∼600 kpc along the line of sight or, conversely, that the relic is located far behind in the cluster. For the first time, we detect linearly polarized emission from the “brush” at 18.6 GHz. Compared to 8.3 GHz, the degree of polarization across the brush increases at 18.6 GHz, suggesting a strong Faraday depolarization toward lower frequencies. The observed depolarization is consistent with an intervening magnetized screen that arises from the dense ICM containing turbulent magnetic fields. The depolarization, corresponding to a standard deviation of the rotation measures as high as σ_RM = 212 ± 23 rad m⁻², suggests that the brush is located in or behind the ICM. Our findings indicate that the Toothbrush relic can be consistently explained by the standard scenario for relic formation.