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

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 α58 MHz18.6 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 σRM = 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.

Spectral study of the diffuse synchrotron source in the galaxy cluster Abell 523

“…Out of about 100 radio halos presently confirmed, three show filaments of polarized emission at around 20% at 1.4 GHz (A2255 Govoni et al 2005, MACS J0717.5+3745 Bonafede et al 2009, andA523 Girardi et al 2016). These are likely projected on the cluster center as suggested by recent studies of A2255 and MACS J0717.5+3745 (Pizzo et al 2011;Botteon et al 2020;Rajpurohit et al 2020). Moreover, a 5𝜎 upper limit of 13% on the fractional polarization has been derived for the radio halo in 1E 0657-55.8 by Shimwell et al (2014).…”

“…Detailed spatially-resolved spectral index images have been produced for a number of radio halos, revealing different behaviours. Indications of radial steepening have been found in a few clusters (e.g., Giovannini et al 1993;Feretti et al 2004;Pearce et al 2017), while other radio halos show no clear trend with either uniform (e.g., Vacca et al 2014;Rajpurohit et al 2018;Hoang et al 2019) or complex (e.g., Giacintucci et al 2005;Orrú et al 2007; Kale & Dwarakanath 2010;Shimwell et al 2014;Rajpurohit et al 2020;Botteon et al 2020) distributions. Radio relics are also characterized by steep-spectra (𝛼 1-1.5), and detailed spectral index images show a steepening transversely to their elongation towards the cluster center (e.g., Clarke & Ensslin 2006;de Gasperin et al 2015;Di Gennaro et al 2018).…”

Deep low-frequency radio observations of Abell 2256 I: The filamentary radio relic

“…Recently, it has been found that the integrated spectrum of the Toothbrush (Rajpurohit et al 2020a) and the Sausage relics (Loi et al 2021) both follow a close power-law between 150 MHz to 18.6 GHz and has spectral indices steeper than −1 contrary to previous claims (Stroe et al 2016;Kierdorf et al 2017).…”

“…Galaxy clusters undergoing mergers often show spectacular, megaparsec-scale radio relics and radio halos (see van Weeren et al 2019, for a recent review). The radio spectra of such sources are steep (α ≤ −1.0 where Di Gennaro et al 2021;Rajpurohit et al 2020aRajpurohit et al , 2021a and believed to originate from shock fronts generated in the intracluster medium (ICM) when galaxy clusters merge (Ensslin et al 1998;Roettiger et al 1999). This connection has been established by finding X-ray surface brightness and temperature discontinuities at the location of some relics (see e.g., Sarazin et al 2013;Ogrean et al 2013;Shimwell et al 2015;van Weeren et al 2016;Botteon et al 2016;Thölken et al 2018;Di Gennaro et al 2019).…”

Modelling the Energy Spectra of Radio Relics

“…However, in the quasi-stationary shock scenario, as analysed here, a power-law spectra is expected. Nevertheless, Loi et al [26] and Rajpurohit et al [27] produced the most detailed integrated radio spectra for the sausage relic and toothbrush relic, respectively. These observations cover an even larger frequency range, i.e., up to 18.6 GHz.…”