  We develop models for time lag between maxima of the source brightness in
different wavelengths during a transient flash of luminosity connected with a
short period of increase of the mass flux onto the central compact object.
  We derive a simple formula for finding the time delay among events in
different wavelengths, valid in general for all disk accreting cosmic sources,
and discuss quantitatively a model for time lag formation in AGNs.
  In close binaries with accretion disks the time lag is connected with effects
of viscosity defining a radial motion of matter in the accretion disk. In AGN
flashes, the falling matter has a low angular momentum, and the time lag is
defined by the free fall time to the gravitating center.
  We show the validity of these models by means of several examples of galactic
and extragalactic accreting sources.
