Left panel: 610 MHz GMRT image of RXJ1720.1+2638 with its contour levels in white starting at 3σ, where σ = 45 μJy beam⁻¹ spaced by a factor of two. The first three contour levels from the high-resolution LOFAR image of Fig. 9 are overlaid in grey. The beam are 5′′ × 5′′ and 14′′ × 9′′ for GMRT and LOFAR, respectively, and are shown in the bottom left corner of the image. Right panel: spectral index map between the 610 MHz GMRT and 144 MHz LOFAR images of RXJ1720.1+2638. The contour levels, overlaid in black, are from the LOFAR image obtained for the spectral analysis (uniform weighting, uv-cut, see main text), not shown in the paper. Pixels below 3σ are blanked. The error map is shown in Fig. A.3.