HST-COS spectra o C IV region of PG 1126-041 taken between 2014 and 2015. Top panel: four spectra are plotted overlapped after being normalized and matched at 1595 − 1605 Å to show both changes in emission and absorption. Bottom three panels: zoomed-in view of the 1600 − 1640 Å region, comparing different couples of epochs in velocity scale, after normalization using a second-order polynomial fit. Three velocity systems are marked, as well as three likely C IV doublets in system II. The strong absorption line at 1608 Å is due to Galactic Fe II and was removed prior to the measurements, so it does not appear in the bottom three panels. All spectra in this figure have been smoothed with a boxcar filter with a width of 31 pixels (corresponding to less than 0.4 Å or 75 km s⁻¹) for visualization purposes.