Assessment of the High-Energy Spectral Component of Terrestrial Gamma-Ray Flashes

We present the latest results on the observation of Terrestrial Gamma-ray Flashes (TGFs) with high-energy counts (>30 MeV) detected by the MCAL instrument onboard the AGILE satellite. We consider a dataset of 85 short transients detected in the period March - June 2015 and clearly associated to a simultaneous (within 500 microseconds) sferic detected by the WWLLN network. This enables us to build a dataset of confirmed TGFs without the introduction of any additional selection criteria which could bias the results. Twenty events in the sample include at least one count with reconstructed energy larger than 30 MeV, with few photons with energy apparently above 100 MeV. We analyzed in details these high-energy counts taking into account the instrument calibration, the energy reconstruction algorithm and the specific instrument characteristics that are responsible for dead-time and pile-up. In particular, the fine spatial segmentation of the MCAL instrument, consisting of 30 independent detectors each with spectral and spatial capabilities, allows to reconstruct with great details the interaction topology in the instrument active volume, as well as the dead-time conditions at the exact count time. Observations, in terms of energy deposit and spatial distribution, are then compared to the results of Monte Carlo simulations. In particular, we are investigating in detail the effect of pile-up and the possibility that a number of high-energy counts are due to the simultaneous interaction (within 4 microseconds) of a small number of photons with energies in the several tens-of-MeV range. These observations update the view reported in previous works and constrain the energy spectrum and timing structure of TGFs at the source.