Gamma-Ray Bursts observations with Cherenkov telescopes : H.E.S.S. legacy observations and optimisation of follow-up and detection with the Large-Sized Telescope of CTA

Overview & Synopsis

Since the first detection of a Gamma-Ray burst by the Vela satellites in 1967, they have been studied across the whole light spectrum with detection at different wavelengths. They are also one of the first types of objects detected with multiple messengers by the codetection of gravitational waves and electromagnetic emission, with GW 170817.Cherenkov instruments have searched for a counterpart of GRBs at very high energies (above 100 GeV) for a long time. After more than two decades of effort, the H.E.S.S. telescopes achieved the first detection with GRB 180720B quickly followed by MAGIC with GRB190114C. Since these detections, GRBs emission started to be unveiled at VHE with the detection of two other GRBs.The next generation of Cherenkov telescopes (CTA) is currently being built. GRBs will be one of the key observation targets. LST-1, a prototype of large CTA telescope, is currently under commissioning at La Palma. Thanks to their performance, LST telescopes will be the main contributors of CTA to the detection of GRBs.A system called the bending model is in charge of the online correction of the pointing of telescopes, which is essential to correct systematic errors linked to the deformation of the lightweight structure. I have entirely developed the code, presented here, during my PhD. It takes care of the data acquisition, interacting with subsystem of the telescope, the data quality, the analysis and the determination of a deformation model. I have also worked on the integration of the program into the LST software framework.The observations of GRBs by LST-1 have already started. This was for the occasion to improve the follow-up strategy, especially when the alert comes from instruments with poor localization accuracy, like Fermi/GBM. On the analysis side, I have worked on the first background model for LST but also on the improvement of the sensitivity, either by searching for better selection criteria for the events or testing new reconstruction algorithms based on deep learning created at the LAPP.Finally, H.E.S.S. telescopes, in Namibia, have observed a lot of GRBs since 2004. A catalogue of all these observations and their results is under creation, intending to answer the question of why so many observations were performed without success before the detection of the first GRBs and why the ones detected have been and not the others. I was in charge of the analysis of all the data and the interpretation I made with my colleagues are presented here.