Left figure: low-frequency spectral index map obtained from LOFAR-LBA (45−71 MHz only) and LOFAR-HBA (115−162 MHz) observations, together with VLA map at 325 MHz. All maps were convolved to a resolution of 50′′ (see circle in the lower left corner) and a pixel-by-pixel linear regression was extracted. Pixels where the error were above 3σ are blanked. Contour lines are from the 325 MHz map. Top figure: spectral index 1σ error map.