  We use Monte Carlo/Fokker-Planck simulations to study the X-ray time lags.
Our results show that soft lags will be observed as long as the decay of the
flare is dominated by radiative cooling, even when acceleration and cooling
timescales are similar. Hard lags can be produced in presence of a competitive
achromatic particle energy loss mechanism if the acceleration process operates
on a timescale such that particles are slowly moved towards higher energy while
the flare evolves. In this type of scenario, the {\gamma} -ray/X-ray quadratic
relation is also reproduced.
