Spectral index image of the radio halo in the cluster Abell 520, which hosts the famous bow shock

Context. Synchrotron-radio emission has been detected from an increasing number of galaxy clusters. Spectral index images are a powerful tool for investigating the origin and nature of these sources, and their connection with the dynamical state of the cluster. Aims: The aim of this work is to investigate the spectral index distribution of the radio halo in the galaxy cluster A520, known to be a complex system from an optical, radio, and X-ray point of view. Methods: We present deep Very Large Array observations in total intensity at 325 and 1400 MHz. We produced and analyzed spectral index images of the radio halo in this frequency range at a resolution of 39'' and 60'' and determined possible correlations with the thermal properties of the cluster. Results: We found an integrated radio-halo spectral index α3251400 ~ 1.12. No strong radial steepening is present and the spectral index distribution is intrinsically complex with fluctuations only partially due to measurement errors. The radio-halo-integrated spectral index and the cluster temperature follow the general trend observed in other galaxy clusters, although a strong point-to-point correlation between the spectral index and the thermal gas temperature has not been observed. Conclusions: The complex morphology in the spectral index image of the radio halo in A520 agrees with the primary models for radio-halo formation. The flatness of the radial profile suggests that the merger is still ongoing and is uniformly and continuously (re-) accelerating the population of relativistic electrons responsible of the radio emission even at large (~1 Mpc) distances from the cluster center.