Discovery of a radio relic in the low mass, merging galaxy cluster PLCK G200.9-28.2

Radio relics at the peripheries of galaxy clusters are tracers of the elusive cluster merger shocks. We report the discovery of a single radio relic in the galaxy cluster PLCK G200.9-28.2 (z = 0.22, M₅₀₀ = 2.7 ± 0.2 × 10¹⁴ M_⊙) using the Giant Metrewave Radio Telescope at 235 and 610 MHz and the Karl G. Jansky Very Large Array at 1500 MHz. The relic has a size of ∼1 × 0.28 Mpc, an arc-like morphology and is located at 0.9 Mpc from the X-ray brightness peak in the cluster. The integrated spectral index of the relic is 1.21 ± 0.15. The spectral index map between 235 and 610 MHz shows steepening from the outer to the inner edge of the relic in line with the expectation from a cluster merger shock. Under the assumption of diffusive shock acceleration, the radio spectral index implies a Mach number of 3.3 ± 1.8 for the shock. The analysis of archival XMM-Newton data shows that PLCK G200.9-28.2 consists of a northern brighter sub-cluster, and a southern sub-cluster in a state of merger. This cluster has the lowest mass amongst the clusters hosting single radio relics. The position of the Planck Sunyaev-Ze'ldovich effect in this cluster is offset by 700 kpc from the X-ray peak in the direction of the radio relic, suggests a physical origin for the offset. Such large offsets in low mass clusters can be a useful tool to select disturbed clusters and to study the state of merger.