Flux spectral index between 0.9 mm and 3.2mm computed from the radial profiles extracted with frank (Sect. 3.2, black curve), with shaded areas representing the errors derived from the linear least-squares regression (with the uncertainties for the single fluxes defined as the sum in quadrature of the statistical and calibration errors). The profile is masked at the locations where the flux S/N < 3 for at least one wavelength. Overplotted is the azimuthal average of the map in Fig. 5 (green curve), calculated with bins of 1 beam (~0.″08) and with errorbars computed as for the profiles in Fig. 5, right panel.