Spectral index profile in the frequency range 144–1400 MHz across the western lobe of J103025+052430. The regions we used to extract the spectral profile are shown as black boxes overlaid on the lobe in the top left corner and have sizes equal to the beam dimension (6 arcsec = 50 kpc). In the outermost region of the backflow, the spectral index shows a tentative flattening, as discussed in Sect. 4.2.