  In the framework of the model that uses black holes endowed with
electromagnetic structure (EMBH) as the energy source, we study how an
elementary spike appears to the detectors. We consider the simplest possible
case of a pulse produced by a pure $e^+e^-$ pair-electro-magnetic plasma, the
PEM pulse, in the absence of any baryonic matter. The resulting time profiles
show a {\em Fast-Rise-Exponential-Decay} shape, followed by a power-law tail.
This is obtained without any special fitting procedure, but only by fixing the
energetics of the process taking place in a given EMBH of selected mass,
varying in the range from 10 to $10^3$ $M_\odot$ and considering the
relativistic effects to be expected in an electron-positron plasma gradually
reaching transparency. Special attention is given to the contributions from all
regimes with Lorentz $\gamma$ factor varying from $\gamma=1$ to $\gamma=10^4$
in a few hundreds of the PEM pulse travel time. Although the main goal of this
paper is to obtain the elementary spike intensity as a function of the arrival
time, and its observed duration, some qualitative considerations are also
presented regarding the expected spectrum and on its departure from the thermal
one. The results of this paper will be comparable, when data will become
available, with a subfamily of particularly short GRBs not followed by any
afterglow. They can also be propedeutical to the study of longer bursts in
presence of baryonic matter currently observed in GRBs.
