2016MNRAS.459.4240K

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) x 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) x 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. We construct a simple merger model to infer relevant time-scales in the merger. From its location on the P_1.4 GHz_–LX_ scaling relation, we infer that we observe ACT-CL J0256.5+0006 just before first core crossing.