NGC 6086 spectral index maps at a resolution of 14 arcsec. The beam is shown in the bottom-left corner, a reference scale is in the top-left one, and the black ‘×’ marker is used to underline the location of the host galaxy. Left panel: spectral index map between 144 and 400 MHz overlaid with 3σ LOFAR contours. The black dotted line region is the 4700 MHz 3σ contour and is used as a reference for the inner lobes. Right panel: spectral index map between 1400 MHz and 4700 MHz, with VLA 4700 MHz 3σ contours overlaid. The associated error maps at low and high frequencies are shown in Fig. A.3.