Spectral index maps made from the LOFAR, CGPS, and Effelsberg data. Top left: LOFAR in-band spectral index made from the three narrow-band images described in Sect. 2.3 and presented in the appendix. Top right: 143 MHz–408 MHz (LOFAR to CGPS) spectral index. Middle left: 143 MHz–1420 MHz (LOFAR to CGPS) spectral index. Middle right: 143 MHz–2695 MHz (LOFAR to Effelsberg) spectral index map. Bottom left: spectral index from CGPS and Effelsberg data (1420 MHz–2725 MHz). Bottom right: spectral index map combining all frequencies, as presented in Fig. 4.