2017A&A...603A.125V

Aims. We report on a spectral study at radio frequencies of the giant radio halo in A 2142 (z=0.0909), which we performed to explore its nature and origin. The optical and X-ray properties of the cluster suggest that A 2142 is not a major merger and the presence of a giant radio halo is somewhat surprising.
Methods. We performed deep radio observations of A 2142 with the Giant Metrewave Radio Telescope (GMRT) at 608MHz, 322MHz, and 234MHz and with the Very Large Array (VLA) in the 1-2GHz band. We obtained high-quality images at all frequencies in a wide range of resolutions, from the galaxy scale, i.e. ∼5'', up to ∼60'' to image the diffuse cluster-scale emission. The radio halo is well detected at all frequencies and extends out to the most distant cold front in A 2142, about 1 Mpc away from the cluster centre. We studied the spectral index in two regions: the central part of the halo, where the X-ray emission peaks and the two brightest dominant galaxies are located; and a second region, known as the ridge (in the direction of the most distant south-eastern cold front), selected to follow the bright part of the halo and X-ray emission. We complemented our deep observations with a preliminary LOw Frequency ARray (LOFAR) image at 118MHz and with the re-analysis of archival VLA data at 1.4GHz.
Results. The two components of the radio halo show different observational properties. The central brightest part has higher surface brightess and a spectrum whose steepness is similar to those of the known radio halos, i.e. α^1.78GHz_118MHz=1.33±0.08 . The ridge, which fades into the larger scale emission, is broader in size and has considerably lower surface brightess and a moderately steeper spectrum, i.e. α^1.78GHz_118MHz∼1.5. We propose that the brightest part of the radio halo is powered by the central sloshing in A 2142, in a process similar to what has been suggested for mini-halos, or by secondary electrons generated by hadronic collisions in the ICM. On the other hand, the steeper ridge may probe particle re-acceleration by turbulence generated either by stirring the gas and magnetic fields on a larger scale or by less energetic mechanisms, such as continuous infall of galaxy groups or an off-axis (minor) merger.