2025ApJ...984L..26S

We report the discovery of a radio megahalo in the merging cluster PLCKG287.0+32.9, based on upgraded Giant Metrewave Radio telescope (uGMRT) observations at frequencies 300-850 MHz. The sensitive radio observations provide a new window to study the complex physics occurring in this system. Apart from significant detections of the known diffuse radio emission in the cluster, we detect the central diffuse emission to a much larger extent of ∼3.2 Mpc, reaching the R₅₀₀ of the cluster. The radial surface brightness profile shows a distinct flattening beyond ∼0.5R₅₀₀, dividing the emission into inner and outer components. This outer envelope shows a steep spectral index (≲-1.5) and emissivity ∼20 times lower than the inner component, confirming the megahalo characteristics. The radial profile of the spectral index also distinguishes the steep spectrum megahalo emission. Our observational results align with recent numerical simulations, showing megahalo emission oriented along the merger axis and the reacceleration of electrons driven by late-stage merger-induced turbulence. This is the first detection of a radio megahalo at a frequency higher than the LOFAR 144 MHz, opening the possibilities for more discoveries and spectral studies to understand their origin.