Discovering the most elusive radio relic in the sky: Diffuse Shock Acceleration caught in the act?

Abstract

The origin of radio relics is usually explained via diffusive shock acceleration (DSA) or re-acceleration of electrons at/from merger shocks in galaxy clusters. The case of acceleration is challenged by the low predicted efficiency of low-Mach number merger shocks, unable to explain the power observed in most radio relics. In this Letter we present the discovery of a new giant radio relic around the galaxy cluster Abell 2249 ($z=0.0838$) using LOFAR. It is special since it has the lowest surface brightness of all known radio relics. We study its radio and X-ray properties combinig LOFAR data with uGMRT, JVLA and XMM. This object has a total power of $L_{1.4\mathrm{GHz}}=4.1\pm 0.8\times 10^{23}$ W Hz${}^{-1}$ and integrated spectral index $\alpha =1.15\pm 0.23$. We infer for this radio relic a lower bound on the magnetisation of $B\ge 0.4\mu$G, a shock Mach number of $\mathcal{M}\approx 3.79$, and a low acceleration efficiency consistent with DSA. This result suggests that a missing population of relics may become visible thanks to the unprecedented sensitivity of the new generation of radio telescopes.

Letter, 5 pages, 4 figures, accepted for publication on MNRAS Letters

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