Volume 634, February 2020
|Number of page(s)||12|
|Section||Cosmology (including clusters of galaxies)|
|Published online||10 February 2020|
Shock acceleration efficiency in radio relics
Dipartimento di Fisica e Astronomia, Università di Bologna, via P. Gobetti 93/2, 40129 Bologna, Italy
2 INAF – IRA, via P. Gobetti 101, 40129 Bologna, Italy
3 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
4 Department of Physics, School of Natural Sciences, UNIST, Ulsan 44919, Korea
Accepted: 22 December 2019
Context. Radio relics in galaxy clusters are giant diffuse synchrotron sources powered in cluster outskirts by merger shocks. Although the relic–shock connection has been consolidated in recent years by a number of observations, the details of the mechanisms leading to the formation of relativistic particles in this environment are still not well understood.
Aims. The diffusive shock acceleration (DSA) theory is a commonly adopted scenario to explain the origin of cosmic rays at astrophysical shocks, including those in radio relics in galaxy clusters. However, in a few specific cases it has been shown that the energy dissipated by cluster shocks is not enough to reproduce the luminosity of the relics via DSA of thermal particles. Studies based on samples of radio relics are required to further address this limitation of the mechanism.
Methods. In this paper, we focus on ten well-studied radio relics with underlying shocks observed in the X-rays and calculate the electron acceleration efficiency of these shocks that is necessary to reproduce the observed radio luminosity of the relics.
Results. We find that in general the standard DSA cannot explain the origin of the relics if electrons are accelerated from the thermal pool with an efficiency significantly smaller than 10%. Our results show that other mechanisms, such as shock re-acceleration of supra-thermal seed electrons or a modification of standard DSA, are required to explain the formation of radio relics.
Key words: acceleration of particles / radiation mechanisms: non-thermal / radiation mechanisms: thermal / galaxies: clusters: intracluster medium / galaxies: clusters: general / shock waves
© ESO 2020
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