A closer look at these bursts (Figure and) allows to identify additional features. Indeed, one notices that the envelope of these maser pulses show an asymmetric profile, with a faster rise than decay, a characteristics that is not explained by a simple Bloch-Maxwell equations of the radiation feedback theory. Moreover, the frequency offset of the signal with respect to the carrier frequency is not constant over time, which can be seen by superimposing time traces of different bursts and comparing the periodicity of the signals. Mere visual inspection of such a superposition, depicted in Fig. for the first fives maser pulses of Figure , shows a shorter period during the first maser pulse than during the next ones. This phenomenon can be ascribed to the presence of the DDF, the manifestations of which have already been described above, further investigated by numerical simulations in the following.