Validation of the ASIM MXGS performance using cosmic Gamma-Ray Bursts

The Atmosphere-Space Interactions Monitor (ASIM) is a mission of the European Space Agency launched in April 2018 and hosted onboard the International Space Station (ISS). ASIM is dedicated to study the physics of Transient Luminous Events (TLEs) and Terrestrial Gamma-ray Flashes (TGFs) and their relation to lightning. TGFs are X- and Gamma-ray flashes associated to lightning discharges, with average duration of few tens of microseconds and energies up to 40 MeV. ASIM detects TGFs by means of the Modular X- and Gamma-ray Sensor (MXGS). So far, the MXGS performance (efficiency, effective area) have been evaluated by Monte Carlo simulations only, while energy calibration is monitored using built-in radioactive sources and background lines. TGFs are local events, very rarely observed by more then one spacecraft simultaneously, therefore it is difficult to use them to validate the MXGS performance. Goal of this study is to use cosmic Gamma-ray Bursts (GRBs) simultaneously detected by ASIM and other spacecraft as calibration sources to validate the spectral performance of MXGS. GRBs are the brightest explosions in the universe, associated to the collapse of massive stars or the merger of compact objects, involving at least one neutron star, at cosmological distances. During the period from June 2018 to December 2021, 12 GRBs were detected by ASIM and by one or more other spacecraft. Here we use data from the Konus-WIND mission and from the Fermi Gamma Burst Monitor (GBM), both considered as benchmarks in the field of GRB analysis. We cross-correlate the light curves of the three instruments, and we perform simultaneous spectral analysis using the forward-folding approach. In some cases, we find good consistency between the detectors, indicating an overall validation of the MXGS performance. In other cases, we identified discrepancies, possibly due to absorption from structures of the ISS, currently under investigation. In this presentation, we show our data sample, the methodology used and the preliminary joint spectral analysis results. This work is relevant because it will provide an independent assessment of the MXGS performance, with clear implications for ASIM TGF results.