Time lag in transient cosmic accreting sources

Abstract

Models are developed 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. A simple formula is derived for finding the time delay among events in different wavelengths, valid in general for all disk accreting cosmic sources. 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. The validity of these models is shown by means of several examples of galactic and extragalactic accreting sources.