Novel strategies for the observations of fast gamma-ray transients

This thesis presents the development of innovative techniques aimed at improv- ing the detection and observation of gamma-ray transient events, with a focus on Gamma Ray Bursts (GRBs) and the electromagnetic counterparts of grav- itational wave (GW) events. These are some of the most elusive phenomena in the Universe due to their brief duration, random occurrence, and often poor localization. Their study represents the frontier of high energy astrophysics re- search, requiring advanced observational strategies and instrumentation. Any new instrument to be used for the observation of these transients must face the problem of balancing on one hand the angular resolution and on the other the dimensions of the field of view (FoV). Although both space and ground in- struments with large FoVs are better suited for detecting these signals, more sensitive instruments, but with limited FoVs, can lead to far better observations. Adequate pointing strategies for these instruments are therefore of fundamen- tal importance to observe gamma transient phenomena. This thesis tackles this challenge by proposing two novel approaches that aim to increase the number of detected gamma-ray transients with both space and ground based instruments. For space-based observations, the possibility of using the ratios of countings of anti-coincidence shields to estimate the direction of arrival of a transient signal for the proposal of the Galactic Explorer with a Coded Aperture Mask Compton Telescope (GECCO) is investigated. In-depth Monte Carlo simulations showed how GECCO’s thick and large BGO anti-coincidence panels can enable quick and accurate localization of GRBs to allow rapid re-pointing and observation, despite GECCO’s narrow FoV. For ground-based observations, the thesis introduces the Soccer Ball Tes- sellation (SBT) strategy, a novel tiling methodology to optimize follow-up ob- servations of poorly localized events by using Goldberg polyhedra to tile the celestial sphere. This strategy improves the efficiency of searching for transient sources within large error regions (both GRBs and GW counterparts), especially for instruments with small FoVs, such as the Imaging Atmospheric Cherenkov Telescopes (IACT). The SBT strategy is also applicable to extragalactic surveys and promises to improve future observations with the Cherenkov Telescope Ar- ray Observatory (CTAO) by allowing serendipitous observations of gamma-ray transients. The methods developed in this thesis aim to maximize the number of gamma- ray transients (mainly GRBs and GW counterparts) observed by high-performance, small FoV instruments, through improved localization accuracy and coordina- tion between different instruments. These results will have far-reaching implica- tions for astrophysical research, particularly in improving our understanding of GRB emission mechanisms, enabling multi-messenger observations and testing phenomenological models of quantum gravity.