Color-scale image of the spectral index distribution between 150 MHz and 330 MHz (a) and the associated error map (b), computed from primary-beam-corrected images with the same uv range and beam of 24″ × 18″. The spectral index was calculated in each pixel where the surface brightness is above the 3σ level in both images. Overlaid are the 330 MHz contours, spaced by a factor of 2 from 3σ = 3 mJy/beam. The white box marks the region occupied by the inner double source (Fig. 2a). The white cross marks the optical peak (Fig. 1).