Spectral index (α, see Sect. 3.2) against the flux-to-intensity ratio for the radio continuum sources detected in both frequency bands and inside the primary beam of both images (sources listed as “iC/iK” in Table B.1). The gray shaded region depicts the spectral index regime associated with thermal free–free emission (i.e., in the range from − 0.1 to + 2). Black dots correspond to sources detected in both bands (see spectral indices in Table 3), blue upward-pointing triangles correspond to sources detected only in the K band (i.e., lower limits), and red downward-pointing triangles correspond to sources detected only in the C band (i.e., upper limits).