2013A&A...557A.117P

To investigate the nonthermal emission mechanism and their interaction during cluster mergers, we analyze multiple low-frequency radio data for the X-ray luminous massive galaxy cluster MACS J0717.5+3745, located at z=0.5548. Large-scale structure-formation models in the Universe suggest that galaxy clusters grow via constant accretion of gas and the merger of galaxy groups and smaller clusters. Low-frequency radio observations trace these mergers in the form of relics and halos. The dual frequency observations were performed on MACS J0717.5+3745 to investigate the spectral index pattern of the nonthermal emission and its interaction within the intracluster medium (ICM), during merger process. Continuum observations were carried out using GMRT at 0.235 and 0.61GHz on MACS J0717.5+3745 and archival data from the VLA (0.074 and 1.42GHz) and WSRT (0.325GHz) was used to complement the results. Furthermore, to explore the thermal and nonthermal interactions within the ICM and the morphological distribution, Chandra X-ray and HST data were used. A highly complex nonthermal radio emission distribution is seen in the cluster at very low frequencies, with a global spectral index α^0.235_0.61~-1.17±0.37. We have detected a giant radio halo within the cluster system with a linear size of 1.58Mpc and a ``Chair-shaped'' filament structure between the merging subclusters of linear size 853kpc at 0.235GHz. This is the most powerful halo ever observed with P_1.4=9.88x10²⁵W/Hz and an equipartition magnetic field estimate of ∼6.49µG. The bright filament structure is well located in the central merging region of subclusters with enhanced temperature, as shown by Chandra and HST data analysis, further indicating the formation of this structure due to shock waves encountered within the ICM during the merger events.