## 4 Discussion  
When a metallic thin film is excited by an ultrashort optical pulse, the electronic population close to the Fermi level is modified, causing a change in the X-ray absorption spectra on femtosecond timescales. This was first measured at the Ni \(L_{3}\)-edge by [PERSON] et al. [22], who suggested that optical pumping of the sample can induce a redshift of the absorption edge during the first hundreds of femtoseconds. The effect was theoretically underpinned by [PERSON] et al. [24], who explained the observed phenomenon as resulting from an optical excitation-induced electron depletion close to the Fermi energy. Furthermore, their calculations suggested that, in addition to the pure absorption spectra, the MCD should also be affected on sub-picosecond timescales. While subsequent experiments remained limited to the \(L\)-edges of transition metals [17], very recent work by [PERSON] et al. [34] and [PERSON] et al. [30] unveiled that similar effects can also be observed at the \(M\)-edges of Fe, Co, and Ni. Furthermore, both studies demonstrated that, depending on the probing energy, a delay between the magnetic and pure charge dynamics (up to 100 fs) can exist. As detailed in [PERSON]'s study [34], this might hint at a pump-induced redshift of the MCD spectra. To test this hypothesis, we calculated the \(\frac{\Delta H(\Delta E)}{\mu_{0}}\) and \(\Delta MCD(\Delta E)\) (using the static data shown in Figure 2b), for different redshift values \(\Delta E\) ranging from 0.05 eV up to 0.2 eV (see lines in Figure 4c,d). On top of these computed curves (black to gray lines), we have plotted our experimental data using color symbols for different time delays. When considering absorption changes as a function of energy, our experimental results are in reasonable agreement with the redshift predictions (Figure 4c). The maximum change in absorption is obtained using \(E=65.7\) eV after 200 fs and corresponds to \(\Delta E_{\text{max}}=-0.15\) eV with respect to the static spectrum. Note that this is in good agreement with the aforementioned \(L_{3}\)-edge measurements of [PERSON] et al., who reported an XAS shift of \(-0.13\) eV after 200 fs [22], as well as with the calculation presented in the [PERSON] et al. study [34]. All other probed energies give rise to less dramatic changes, in qualitative agreement with our calculations. However, from a quantitative perspective, the observed magnitudes of \(\frac{\Delta H(\Delta E)}{\mu_{0}}\) show slight deviations from the simple rigid edge shift, which hints at more subtle transient modifications of the absorption spectrum [34]. A comparison of the calculated \(\Delta MCD(\Delta E)\) with our experimental data, Figure 4d, also provides support for a shift of the MCD. Note that since the MCD is inferior to 0, a negative value of \(\Delta MCD(\Delta E)\) corresponds to an increase in magnetization. Therefore, we can interpret the observed increase in the magnetization shortly after \(t=0\) (\(E=65.7\) eV, magenta data in panel (b)), as well as the small delays between \(t=0\) and the onset of the demagnetization process (\(E=64.6\) eV and, to a lesser extent, \(E=66.3\) eV) as resulting from a redshift of the MCD. This increase in the magnetization competes with the usual ultrafast demagnetization, and is visible only briefly after \(t=0\). Note that this explanation could provide additional insight into a recent study where an increase of the Ni magnetization on early timescales, in an FeNi alloy, has been attributed to optically induced spin transfer effects [44]. Furthermore, it is in agreement with the theoretical calculation of [PERSON] et al. [24]. For \(t>100\) fs the magnetization quenching dominates, irrespective of the energy. For probing energies where the aforementioned electronic redistribution and ultrafast demagnetization channels do not compete (66.7 end 68 eV), we do not find any lag between the onset of the electronic and magnetic dynamics (Figure 3a,b).