Analysis of the spectral index trend across the outer and the inner lobes of NGC 6086. Top: NGC 6086 spectral index map between 144 MHz and 675 MHz at low resolution (30 arcsec). Contours show the LOFAR emission and are drawn at (−3, 3)×σ. The dotted black line is the inner lobes region, defined by the 3σ contour of the VLA 4700 MHz and used as a reference. 14 regions across the entire source are selected in the map to look for spectral index trends. The beam is shown in the bottom-left corner and a reference scale in the top-left one. The error map is presented in Fig. A.4. Bottom: spectral index values of the 14 regions. We colour-coded the three main regions of the large-scale diffuse emission detected in the low-resolution images.