Intrinsic colour of quasars as a function of redshift for (u − z), (u − i), (g − r) and (g − i), top to bottom. The black points give the intercept of the (mX − mY) vs. NH i relation in each bin of redshift (see Eq. (E.7)). The triangles give the intrinsic colour of quasars computed with the composite spectrum (see Eq. (E.8)).
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