GRB Prompt Emission Spectral Curvature in the MeV Regime

Gamma-ray Bursts (GRBs) emit relativistic jets that host extreme conditions unobtainable on Earth, which is why studying these jets would be studying physics at its limits. The early flash of gamma-rays produced by a GRB, the prompt emission, is mostly non-thermal, and thought to be of synchrotron origin. The prompt emission spectra in the keV regime has been traditionally fit with the Band function, an empirical smoothly broken power law. However, the values of the Band function's parameters are sometimes incompatible with synchrotron emission. In addition, when the Band function is fit to data in the keV regime (using data from Fermi Gamma-ray Burst Monitor; GBM), and extrapolated into the MeV regime, the high-energy power-law overestimates the data (from Fermi Large Area Telescope; LAT). This discrepancy can be explained by (i) calibration issues between the two instruments, (ii) a more complex spectrum in the MeV regime, and/or (iii) the use of an inappropriate model in the keV regime. The prompt emission MeV spectrum is particularly important for interpreting the physics of GRB jets; for instance, in the synchrotron scenario, the MeV spectral slope informs on the distribution of accelerated electrons in the jet, which subsequently informs on the acceleration mechanisms at play. Studies showed that the Band function does not adequately capture the shape of the prompt emission spectrum in some GRBs, which justifies the use of other physically motivated models, such as the "multicomponent model", which uses a mixture of thermal emission (assumed to be photospheric emission from the GRB jet), and non-thermal emission from the accelerated particles within the jet. In this presentation, I explore the curvature in the prompt emission MeV spectrum of bright GRBs detected by GBM and LAT, using these physically motivated models and fine time-resolved spectroscopy for tracking the fast evolution of the GRB jets.