GRB 221009A and the Apparently Most Energetic Gamma-Ray Bursts

Abstract

Gamma-ray bursts (GRBs) are often referred to as the most luminous explosions in the Universe, due to their short and highly luminous prompt emission. This apparent luminosity, however, does not reflect the true energy budget of the prompt emission, which is strongly beamed. Accurate estimations of the energy radiated during the prompt phase require taking the geometry of GRB jets into account, which remains poorly known. Nevertheless, one may establish the distribution of well-measured quantities, like Eiso, the GRB isotropic equivalent energy, which encrypts crucial information about GRB jets, with the aim of providing constraints on the jet's radiated energy. In this work, we study the bright end of the GRB isotropic equivalent energy distribution (hereafter called “apparent energy”), using an updated sample of 185 apparently energetic GRBs with Eiso ≥ 1053 erg. This new sample includes GRB 221009A, allowing us to discuss this apparently superenergetic GRB in the context of the general Eiso distribution of long GRBs. We describe the construction of the sample and compare three fits of the Eiso distribution with a simple power law, a cutoff power law, and a broken power law. Our study confirms the existence of a cutoff around Eiso = 4 × 1054 erg, even when GRB 221009A is included in the sample. Based on this finding, we discuss the possible reasons behind the rapid decrease in the number of apparently energetic gamma-ray bursts beyond Eiso = 4 × 1054 erg and the interpretation of GRB 221009A, the most apparently energetic GRB detected to date, in this context.