Incidence of afterglow plateaus in gamma-ray bursts associated with binary neutron star mergers

Gamma-ray bursts (GRBs) have been a great astrophysical mystery since their discovery, and their understanding has progressed as high-energy telescopes progressed. A major breakthrough came with the Swift satellite, which observed GRB X-ray afterglows with unprecedented detail, leading to the definition of a "canonical X-ray afterglow light-curve": an initial steep decay, followed by a shallow ("plateau") phase which then transitions to a typical power-law flux decay. While the latter is in agreement with the theory of synchrotron emission by a relativistic shock, plateaus interpretation requires additional physics. Plateaus occur in ∼ 50% of long GRBs (those associated to the collapse of massive stars), while for short GRBs (SGRBs, associated to binary neutron star (BNS) mergers) the frequency of plateaus is still uncertain. In this Thesis, we analyse the X-ray afterglow light-curve of 85 SGRBs at known redshift detected by Swift between 2005 and 2022, identifying 15 bursts with robust evidence of a plateau ("plateau" fraction of 0.176 < fpl < 0.375). These 15 SGRBs are used to test the magnetar model, one of the leading interpretations for the plateau, which envisions an extra energy injection in the afterglow shock via magnetic dipole radiation by a millisecond-spinning magnetar formed in the BNS merger producing the SGRB. Of these afterglow light-curves, 12/15 were well fit by the magnetar model, implying a "magnetar" fraction 0.141 < fmag < 0.245 in SGRBs. Three were inconsistent with the model and call for further study. Finally, by linking fmag to the fraction of stable NS formed in BNS mergers, we place a constrain on the maximum NS mass (Mmax) implied by the magnetar model. By simulating a population of 10^5 mergers, we conclude that 2.20 < Mmax/M⊙ < 2.31. Our result, consistent with independent estimates of Mmax, awaits confimation by future runs of Advanced LIGO and Virgo, and of the Einstein Telescope which will observe a large population of BNS mergers.