SEDs from the correlations of surface brightness values over the main filament. Panels a–d show the band-to-band correlations for the northern (red) and southern (blue) parts of the filaments. The points correspond to an oversampling by a factor of 2 relative to the data resolution. Panel e shows the resulting SEDs and modified black-body fits to the combined data for three fixed values of the spectral index β. The result for a fit with free β are shown below the results of the fixed-β fits.