  A change in the character of maser generation in a two-level system is found
when the initial population inversion exceeds some threshold value proportional
to the square root of the total number of atoms. Above this threshold, the
number of photons begins to grow exponentially with time and the pulse with
short leading edge and broadened trailing edge is generated. In this work, we
attempt to explain the nature of this threshold. Coherent pulse duration
estimated by its half-width increases significantly with increasing inversion,
if all other parameters are fixed and the absorption is neglected. The
inclusion of the energy loss of photons leads to the fact that the duration of
coherent pulse is almost constant with increasing inversion, at least well away
from the threshold. If there is exist a recovery mechanism for the population
inversion, the pulsating mode of stimulated emission generation becomes
possible. The integral radiation intensity at this may be increased several
times. This approach can be used for analysis of the cosmic radiation that
might help explain a great variety of pulsating radiation sources in space.
