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 MHz and 610
MHz. We find this cluster to host a faint ($S_{610} = 5.6 \pm 1.4$ 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, $M_{\rm 500,SZ} = (5.0 \pm 1.2) \times 10^{14} M_\odot$, found to host
a GRH. We measure the GRH at lower significance at 325 MHz ($S_{325} = 10.3 \pm
5.3$ mJy), obtaining a spectral index measurement of $\alpha^{610}_{325} =
1.0^{+0.7}_{-0.9}$. This result is consistent with the mean spectral index of
the population of typical radio halos, $\alpha = 1.2 \pm 0.2$. Adopting the
latter value, we determine a 1.4 GHz radio power of $P_{1.4\text{GHz}} = (1.0
\pm 0.3) \times 10^{24}$ W Hz$^{-1}$, placing this cluster within the scatter
of known scaling relations. Various lines of evidence, including the ICM
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 $v_\perp = 1880 \pm 280$ km s$^{-1}$. We construct a simple merger model to
infer relevant time-scales in the merger. From its location on the $P_{\rm
1.4GHz}{-}L_{\rm X}$ scaling relation, we infer that we observe ACT-CL
J0256.5+0006 approximately 500 Myr before first core crossing.
