Profile of the spectral index β of the dust opacity along the length of the streamer. The approximation β = α − 2 is shown as the orange line, and β, corrected for the deviation from the RJ approximation, is shown as the violet line. This profile has been obtained starting from the maps in Fig. 3, where the central emission had been modeled and subtracted. The centers of the selected apertures in which we derive β are highlighted in white among all the points that sample the manually defined logarithmic spiral in the B6 map as an example (upper inset). Throughout the whole structure, β ~ 1.6. The gray band represents ISM-like β.