A giant radio halo in a low-mass SZ-selected galaxy cluster: ACT-CL J0256.5+0006

We present the detection of a giant radio halo (GRH) in the Sunyaev-Zel'dovich (SZ)-selected merging galaxy cluster ACT-CL J0256.5+0006 (z = 0.363), observed with the Giant Metrewave Radio Telescope at 325 and 610 MHz. We find this cluster to host a faint (S₆₁₀ = 5.6 ± 1.4 mJy) radio halo with an angular extent of 2.6 arcmin, corresponding to 0.8 Mpc at the cluster redshift, qualifying it as a GRH. J0256 is one of the lowest mass systems, M_{500, SZ} = (5.0 ± 1.2) × 10¹⁴ M_⊙, found to host a GRH. We measure the GRH at lower significance at 325 MHz (S₃₂₅ = 10.3 ± 5.3 mJy), obtaining a spectral index measurement of α ^{610}_{325} = 1.0^{+0.7}_{-0.9}. This result is consistent with the mean spectral index of the population of typical radio haloes, α = 1.2 ± 0.2. Adopting the latter value, we determine a 1.4 GHz radio power of P_{1.4 GHz} = (1.0 ± 0.3) × 10²⁴ W Hz^-1, placing this cluster within the scatter of known scaling relations. Various lines of evidence, including the intracluster medium morphology, suggest that ACT-CL J0256.5+0006 is composed of two subclusters. We determine a merger mass ratio of 7:4, and a line-of-sight velocity difference of v_⊥ = 1880 ± 210 km s^-1. We construct a simple merger model to infer relevant time-scales in the merger. From its location on the P_{1.4 GHz}-L_X scaling relation, we infer that we observe ACT-CL J0256.5+0006 just before first core crossing.