We study the optical-pump induced ultrafast transient change of the X-ray absorption at the L_3 absorption resonances of the transition metals Ni and Fe in Fe_{0.5}Ni_{0.5} alloy. We find the effect for both elements to occur simultaneously on a femtosecond timescale. This effect may hence be used as a handy cross-correlation scheme providing a time-zero reference for ultrafast optical-pump soft X-ray-probe measurement. The method benefits from a relatively simple experimental setup as the sample itself...