Polarimétrie des sursauts gamma avec l'essaim de CubeSats COMCUBE-S

Gamma-Ray Bursts (GRBs) are the most violent electromagnetic explosions in the universe, making them visible at cosmological distances. Their intensity makes them promising candidates for use as standard candles, but our incomplete understanding of jet physics limits this possibility. Constraining these mechanisms is essential not only for their use in cosmology but also for studying their formation conditions and the nature of the central engine. Despite current uncertainties in the measurements, gamma-ray polarimetry is a promising tool for constraining GRB models. Compton telescopes are very well suited to this type of measurement. The COMCUBE-S mission is a European collaboration that will use a swarm of CubeSats, each carrying a Compton polarimeter, to measure GRB prompt emission polarisation. This work was carried out in the context of the proposal submitted to ESA, for which I was responsible for estimating the mission's performance. Selected by ESA, it aims to achieve a full-sky coverage and real-time alerts. The primary scientific objective of COMCUBE-S is to distinguish between relativistic GRB jet models. Simulations were performed using jet models such as Synchrotron Ordered (SO), which predicts intrinsically high polarisation (Poynting-Flux-Dominated, PFD), and Kinetic-Energy-Dominated (KED) models, such as Synchrotron Random (SR), Compton Drag (CD), and Photospheric Jet (PJ), which generally exhibit low polarisation unless viewed off-axis. This led to considering the detection of at least 60 GRBs with a Minimum Detectable Polarisation (MDP) of at most 30 % as the main scientific objective of the mission to distinguish between the different jet models. Simulations predict that COMCUBE-S, in its baseline configuration involving 27 satellites in a 500 km equatorial orbit, could detect around 675 GRBs per year, surpassing the detection capabilities of all other missions. GRBs could be observed at redshifts above 9. Polarimetric performance is estimated at around 36 GRBs per year with an MDP of at most 30 %, which would fulfil the main scientific requirement within a two-year mission of nominal operation in orbit. In addition, COMCUBE-S is expected to perform time-resolved polarisation measurements on 12 GRBs, to study polarisation angle variations. A prototype of Compton polarimeter was tested during a transatlantic stratospheric balloon flight in June 2024. This confirmed that the instrument functioned properly in conditions similar to those experienced in low-Earth orbit. During the flight, electron precipitation events and a solar flare were detected by the gamma-ray detectors. Simulations of the background also validated the functioning of the prototype. COMCUBE-S is currently in feasibility study phase (Phase A) at ESA, and the full constellation is expected to launch in 2033. This mission will provide new insights into the physics of gamma-ray bursts, transient events in general, and multimessenger astronomy.