The radio halo in the merging cluster A3562

We present new VLA observations at 1.4 GHz confirming the presence of a radio halo at the centre of the cluster A3562, in the core of the Shapley Concentration. We also report a detailed multifrequency radio study of the head tail galaxy J1333-3141, which is completely embedded in the halo emission. The radio halo has an irregular shape, and a maximum linear size of similar to620 kpc, which is among the smallest found in the literature. The source has a steep spectrum, i.e. alpha(843 MHz) (1.4 GHz) similar to2, and its total radio power, P-1.4 GHz similar to 2 x 10(23) WHz(-1), is the lowest known to date. The radio power of the halo and the X-ray parameters of the cluster, such as L-X and kT, nicely fit the correlations found in the literature for the other halo clusters, extending them to low radio powers. We found that the total number of electrons injected in the cluster environment by the head-tail source is enough to feed the halo, if we assume that the galaxy has been radio active over a large fraction of its crossing time. We discuss possible origins of the radio halo in the light of the two-phase model (Brunetti et al. 2001) and propose that the observed scenario is the result of a young source at the beginning of the reacceleration phase.