On the role of a cavity in the hypernova ejecta of GRB 190114C

Abstract: Within the binary-driven hypernova I (BdHN I) scenario, the gamma-ray burst GRB190114C originates in a binary system composed of a massive carbon-oxygen core (CO$<em>{core}$), and a binary neutron star (NS) companion. As the CO$</em>{core}$ undergoes a supernova explosion with the creation of a new neutron star ($\nu$NS), hypercritical accretion occurs onto the companion binary neutron star until it exceeds the critical mass for gravitational collapse. The formation of a black hole (BH) captures $10<sup>{57}$ baryons by enclosing them within its horizon, and thus a cavity of approximately $10<sup>{11}$ cm is formed around it with initial density $10<sup>{-7}$ g/cm$<sup>3$. A further depletion of baryons in the cavity originates from the expansion of the electron-positron-photon ($e<sup>{+}e<sup>{-}\gamma$) plasma formed at the collapse, reaching a density of $10<sup>{-14}$ g/cm$<sup>3$ by the end of the interaction. It is demonstrated here using an analytical model complemented by a hydrodynamical numerical simulation that part of the $e<sup>{+}e<sup>{-}\gamma$ plasma is reflected off the walls of the cavity. The consequent outflow and its observed properties are shown to coincide with the featureless emission occurring in a time interval of duration $t_{rf}$, measured in the rest frame of the source, between $11$ and $20$ s of the GBM observation. Moreover, similar features of the GRB light curve were previously observed in GRB 090926A and GRB 130427A, all belonging to the BdHN I class. This interpretation supports the general conceptual framework presented in Ruffini et al. (2019) and guarantees that a low baryon density is reached in the cavity, a necessary condition for the operation of the "inner engine" of the GRB presented in an accompanying article (Ruffini & Moradi 2019).