Spectral index curvature map of Hercules A. The spectral index curvature is calculated as the spectral index between 144 MHz and 1.5 GHz minus the spectral index between 1.5 and 7 GHz. The red regions indicate where the spectrum is steeper at high frequencies than at low frequencies, whereas the blue regions indicate where the spectrum is flatter at high frequencies than at low frequencies. The contours indicate the L-band emission (1.5 GHz), and are drawn at [1, 2, 4, 8, ...] ×   5σ_rms, where σ_rms = 224.2 μJy beam⁻¹.