  We report Resonant inelastic x-ray scattering measurements (RIXS) in
YbCu$_2$Si$_2$ at the Yb L$_{3}$ edge under high pressure (up to 22 GPa) and at
low temperatures (down to 7 K) with emphasis on the vicinity of the transition
to a magnetic ordered state. We find a continuous valence change towards the
trivalent state with increasing pressure but with a pronounced change of slope
close to the critical pressure. Even at 22 GPa the Yb$^{+3}$ state is not fully
achieved. The pressure where this feature is observed decreases as the
temperature is reduced to 9 GPa at 7K, a value close to the critical pressure
(\itshape{p\normalfont{$_c$}}\normalfont $\approx$ 7.5 GPa) where magnetic
order occurs. The decrease in the valence with decreasing temperature
previously reported at ambient pressure is confirmed and is found to be
enhanced at higher pressures. We also compare the f electron occupancy between
YbCu$_2$Si$_2$ and its Ce-counterpart, CeCu$_2$Si$_2$.
