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 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 \geq 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 .