| Authors : | Ramananda Santra (NCRA-TIFR), Ruta Kale (NCRA-TIFR), Simona Giacintucci (NRL, US), Daniele Dallacasa (University of Bologna and INAF-IRA), Tiziana Venturi (INAF-IRA), Rossella Cassano (INAF-IRA), Gianfranco Brunetti (INAF-IRA) |
| Abstract : | Diffuse radio emissions from galaxy clusters termed as radio halos and relics, not associated with individual galaxies, are the tracers of the different energetic processes (mergers and shock), happening over a Gyr timescale in the Intracluster medium (ICM). Here we present the first detailed analysis of the radio halo, the prototypical ultra-steep spectrum radio halo in the cluster Abell 521, using the Upgraded Giant Metrewave Radio Telescope (uGMRT). This cluster is experiencing a complex merger and hosts a giant radio halo and a relic. Radio observations over 300 - 800 MHz combined with archival Chandra and XMM Newton data, allow us to study the physical mechanisms active in this system. Our deep and sensitive images have confirmed the presence of a second relic situated diametrically opposite to the previous one. We have also detected a faint extension, a length of ~ 1 Mpc, of the previous relic at both 400 and 650 MHz images. The integrated radio emission of the entire halo is well described with a power law of a spectral index = -1.86 ± 0.12, with a radial steepening at the outer regions. Resolved spectral index map also reveals some small-scale fluctuations over the entire radio halo region, but is consistent with the integrated spectral index value. Despite a complex elongated structure, the radio halo is well correlated morphologically with the Chandra and XMM-Newton surface brightness. Our analysis has shown that the northern part of the radio halo correlates well with the X-ray emission, compared to the southern one. A sublinear correlation between the thermal and non-thermal components, along with the radial spectral steepening favors the re-acceleration of some pre-existing relativistic electrons via turbulence injected through the mergers, for the origin of the radio halo. |
