  The HERMES (High Energy Rapid Modular Ensemble of Satellites) Pathfinder
mission aims to develop a constellation of nanosatellites to study astronomical
transient sources, such as gamma-ray bursts, in the X and soft $\gamma$ energy
range, exploiting a novel inorganic scintillator. This study presents the
results obtained describing, with an empirical model, the unusually intense and
long-lasting residual emission of the GAGG:Ce scintillating crystal after
irradiating it with high energy protons (70 MeV) and ultraviolet light ($\sim$
300 nm). From the model so derived, the consequences of this residual
luminescence for the detector performance in operational conditions has been
analyzed. It was demonstrated that the current generated by the residual
emission peaks at 1-2 pA, thus ascertaining the complete compatibility of this
detector with the HERMES Pathfinder nanosatellites.
