A Multimission Method for the Reconstruction of Gamma-ray Events on Silicon Tracker Pair Telescopes

gamma-ray astronomy in the energy range from MeV to GeV can provide a unique detection window for gamma-ray bursts and other transient sources, fundamental information on particle acceleration mechanisms, MeV-blazar population studies up to z similar to 4.5, and a full overview of line emission from cosmic-ray interaction. Silicon-based pair tracking telescopes rely on gamma-ray conversion into an electron-positron pair and its tracking using a stack of silicon strips. The method presented in this work is based on a Rauch-Tung-Striebel smoother. Its internal Kalman filter enables keeping multiple hypotheses about particle tracks and implementing statistically meaningful measurement selection among hits on different planes of the tracker. The algorithm can be easily configured to work with different tracker geometries and mass models. It can be used for the exploitation of data from past and current gamma-ray missions as well as to assess the performances of new pair-tracking telescopes. The proposed method has been validated on Astrorivelatore Gamma a Immagini Leggero data and then used to investigate the performances of both e-ASTROGAM and All-Sky-ASTROGAM telescopes. The algorithm efficiency and its accuracy in estimating both the photon direction and energy were evaluated on gamma-ray events simulated at different energies in the range between 30 MeV and 3 GeV. The point-spread function of each tracker was then compared with its angular resolution limit showing both the expected performances of the instrument and the margin of improvement that could be obtained by optimizing the reconstruction method.