  We present a novel, highly versatile, and self-referenced arrival time
monitor for measuring the femtosecond time delay between a hard X-ray pulse
from a free-electron laser and an optical laser pulse, measured directly on the
same sample used for pump-probe experiments. Two chirped and picosecond long
optical supercontinuum pulses traverse the sample with a mutually fixed time
delay of 970 fs, while a femtosecond X-ray pulse arrives at an instant in
between both pulses. Behind the sample the supercontinuum pulses are temporally
overlapped to yield near-perfect destructive interference in the absence of the
X-ray pulse. Stimulation of the sample with an X-ray pulse delivers non-zero
contributions at certain optical wavelengths, which serve as a measure of the
relative arrival time of the X-ray pulse with an accuracy of better than 25 fs.
We find an excellent agreement of our monitor with the existing timing
diagnostics at the SACLA XFEL with a Pearson correlation value of 0.98. We
demonstrate a high sensitivity to measure X-ray pulses with pulse energies as
low as 30 $\mu$J. Using a free-flowing liquid jet as interaction sample ensures
the full replacement of the sample volume for each X-ray/optical event, thus
enabling its utility even at MHz repetition rate XFEL sources.

Optics;Instrumentation and Detectors