Polarization of radio relics in galaxy clusters

et al. 2020

A&A

130

Context. Radio relics are diffuse extended synchrotron sources that originate from shock fronts induced by galaxy cluster mergers. The actual particle acceleration mechanism at the shock fronts is still under debate. The galaxy cluster 1RXS J0603.3+4214 hosts one of the most intriguing examples of radio relics, known as the Toothbrush. Aims. To understand the mechanism(s) that accelerate relativistic particles in the intracluster medium (ICM), we investigate the spectral properties of large scale diffuse extended sources in the merging galaxy cluster 1RXS J0603.3+4214. Methods. We present new wideband radio continuum observations made with uGMRT and VLA. Our new observations in combination with previously published data, allowed us to carry out a detailed high spatial resolution spectral and curvature analysis of the known diffuse radio emission sources, over a broad range of frequencies.Results. The integrated spectrum of the Toothbrush follows closely a power law over close to 2 decades in frequency, with a spectral index of −1.16 ± 0.02. We do not find any evidence of spectral steepening below 8 GHz. The subregions of the main Toothbrush also exhibit near-perfect power laws, implying a very regular combination of shock properties across the shock front. Recent numerical simulations show an intriguing similar spectral index, suggesting that the radio spectrum is dominated by the average over the inhomogeneities within the shock, with most of the emission coming from the tail of the Mach number distribution. In contrast to the Toothbrush, the spectrum of the fainter relics show a high frequency steepening. Moreover, also the integrated spectrum of the halo follows a power law from 150 MHz to 3 GHz with a spectral index of −1.16 ± 0.04. We do not find any evidence for spectral curvature, not even in subareas of the halo. This suggest a homogeneous acceleration throughout the cluster volume. Between the "brush" region of the Toothbrush and the halo, the color-color analysis revealed emission that was consistent with an overlap between the two different spectral regions. Conclusions. None of the relic structures, the Toothbrush as a whole or its subregions or the other two fainter relics, show spectral shapes consistent with a single injection of relativistic electrons, such as at a shock, followed by synchrotron aging in a relatively homogeneous environment. Inhomogeneities in some combination of Mach number, magnetic field strengths and projection effects dominate the observed spectral shapes.

Section: Comparison With Numerical Simulationssupporting

confidence: 50%

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

Rajpurohit

Hoeft

et al. 2020

A&A

130

“…We point to recent works (e.g. section 2.1 in Wittor et al 2019) for the numerical details. We took three clusters from the San Pedro-cluster catalogue, which targets the topological study of relics (Wittor et al, in preparation).…”

Section: Enzo Simulationsmentioning

confidence: 93%

Limiting the shock acceleration of cosmic ray protons in the ICM

Wittor

Monthly Notices of the Royal Astronomical Society: Letters

Ryu

et al. 2020

Self Cite

Observations of large-scale radio emissions prove the existence of shock accelerated cosmic ray electrons in galaxy clusters, while the lack of detected γ-rays limits the acceleration of cosmic ray protons in galaxy clusters. This challenges our understanding of how diffusive shock acceleration works. In this work, we couple the most updated recipes for shock acceleration in the intracluster medium to state-of-the-art magnetohydrodynamical simulations of massive galaxy clusters. Furthermore, we use passive tracer particles to follow the evolution of accelerated cosmic rays. We show that when the interplay between magnetic field topology and the feedback from accelerated cosmic rays is taken into account, the latest developments of particle acceleration theory give results that are compatible with observational constraints.

“…As argued in (Rajpurohit et al 2020), the shock surface indeed shows a distribution of Mach numbers, thus a single Mach number derived above can only roughly characterize the shock. Most importantly, the tail of the Mach number distribution toward high values determine the radio spectral index (Wittor et al 2019;Rajpurohit et al 2020).…”

Section: Integrated Spectrummentioning

confidence: 99%

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

Rajpurohit

Hoeft

et al. 2020

A&A

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.