Left panel: LOFAR 144-MHz image with overlaid all boxes used to measure the flux densities as listed in Table 3. The red and blue boxes have been used to measure the flux density of the entire source and the newly detected extended emission, respectively, using the 20-arcsec resolution images. The green and magenta boxes were used to measure the flux density of the double lobes and the filament F1, respectively, using the 7-arcsec resolution images. Yellow circles (1–6) and cyan boxes (1–11) were used to compute the spectral profiles shown in Fig. 8 and described in Sect. 4.3. Right panel: radio spectrum of the radio galaxy NGC 507. Green filled triangles show the double-lobe flux density measurements presented in this paper (green box in the left panel and see Table 3), while black circles show the flux densities listed by Murgia et al. (2011) and the value reported in the NTI catalogue at 150 MHz. The blue solid line represents the best spectral age fit of the main lobes, whose parameters are listed in the top-right corner and further described in Sect. 6. Blue filled triangles show the flux density of the newly detected extended emission (blue box in the left panel) and the grey dashed line shows the respective best spectral age fit, whose parameters are listed in the bottom of the panel and further described in Sect. 6. The core flux density measurements are shown as empty circles (in red is the measurement at 675 MHz presented in this work and in black are those published by Murgia et al. 2011) and are fitted with a power-law function as shown by the dotted line.