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Abstract

This thesis is devoted to the study of extremely energetic short-timescale
astrophysical events, Gamma-ray bursts (GRBs). GRBs exhibit broad-band
bright non-thermal emission, which was analysed using two major experiments:
the High Altitude Water Cherenkov observatory (HAWC) and the
High Energy Stereoscopic System (H.E.S.S.). The two experiments are in
many respects complementary for the observation of very high energy (VHE)
gamma-ray emission from GRBs, and in this work the respective advantages
were exploited to maximise the sensitivity to VHE signals. After the analysis
of several tens of GRBs observed using H.E.S.S. until 2017, where no significant
emission was detected, improvements in the observation strategy of
H.E.S.S. allowed the detection of GRB 180729B and GRB 190829A. These
detections are presented in context with multi-wavelength data, proposing
plausible emission mechanisms, thus concluding a decade-long search for
these elusive phenomena at VHE. In the second part, novel methods to improve
the accuracy of the HAWC detector simulation are presented, including
better modelling of the detector efficiencies and electronics. A model that
accounts for the detector response and the GRB flux evolution has been
developed to estimate the optimal integration time for VHE searches with
HAWC. Thanks to these improvements, it is possible to exploit the wide field
of view and high duty cycle of HAWC for the search of VHE emission in
several tens of GRBs. Preliminary evidence for emission is found in one of
the GRBs studied, and upper limits are obtained for all the GRBs analysed
and placed in context of the X-ray properties of these events. Finally, the
limits and detections presented in this work are placed within the framework
of the current understanding of GRBs and prospects for future and present
VHE gamma-ray detectors are presented.