μ(v) magnification profiles of C IV (in red) computed from spectra of images B and C of J1138, simultaneously recorded. The μ(v) profiles are binned into 20 spectral elements, with the uncertainties shown in green. The superimposed line profiles from image B (in blue) and C (in black) are continuum-subtracted, corrected by the M(C IV) factor, and arbitrarily rescaled. Two values of the factor M(C IV) are used: M(C IV) = 0.90 (left panel) and 1.05 (right panel). The zero-velocity corresponds to the C IVλ1549 wavelength at the source redshift.