2D histograms of the effective slope b_l of the normalized polarization spectrum, as a function of LOS temperature (left panels) and column density (right panels), respectively, colored by the count of data points in each bin. Top panels show results obtained with the ISRF as the only considered radiation source (Setup one), whereas bottom panels show results where the ISRF as well as an offset stellar source are considered (Setup two, see Sect. 5.1 for details). Pearson r, as well as two-tailed p values (p_tt), are calculated from 50 segmented median values in each distribution.