Left: Spectral index map between 366 MHz and 672 MHz for the radio halo in El-Gordo at 15″ × 15″ resolution in color scale. The contours from the 672 MHz image at 0.1, 0.2, 0.4, 0.8, and 1.6 mJy beam⁻¹ are overlaid. Right: Spectral index error map. High errors at the edges are expected due to the low S/N at the outskirts. The numbered regions used for analysis are shown in the map. The beam is shown in the top left corner in both panels.