Using the cross-spectral method, we confirm the existence of the X-ray hard lags discovered with cross-correlation function technique during a large flare of Mrk 421 observed with BeppoSAX. For the 0.1–2 versus 2–10 keV light curves, both methods suggest sub-hour hard lags. In the time domain, the degree of hard lag, i.e. the amplitude of the 3.2–10 keV photons lagging the lower energy ones, te...
