http://arxiv.org/abs/1907.00966
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 the 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. The diffusive shock acceleration (DSA) theory is a commonly adopted scenario to explain the origin of cosmic rays at astrophysical shocks, including in radio relics in galaxy clusters. However, in a few specific cases it has been shown that DSA is not energetically viable to reproduce the luminosity of the relics if particles are accelerated from the thermal pool. Studies based on samples of radio relics are required to further address this limitation of the mechanism. In this paper, we focus on 10 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. We find that in general DSA can not explain the origin of the relics if electrons are accelerated from the thermal pool with an efficiency significantly smaller than 10 percent. Our results show that other mechanisms, such as shock re-acceleration of supra-thermal seed electrons, are required to explain the formation of radio relics.
A. Botteon, G. Brunetti, D. Ryu, et. al.
Tue, 2 Jul 19
68/79
Comments: 10 pages, 9 figures, 3 tables, submitted to A&A
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