GRB 171205A: Hypernova and Newborn Neutron Star

Abstract

GRB 171205A is a low-luminosity, long-duration gamma-ray burst (GRB) associated with SN 2017iuk, a broad-line type Ic supernova (SN). It is consistent with being formed in the core-collapse of a widely separated binary, which we have called the binary-driven hypernova (BdHN) of type III. The core-collapse of the CO star forms a newborn NS ($\nu$NS) and the SN explosion. Fallback accretion transfers mass and angular momentum to the $\nu$NS, here assumed to be born non-rotating. The accretion energy injected into the expanding stellar layers powers the prompt emission. The multiwavelength power-law afterglow is explained by the synchrotron radiation of electrons in the SN ejecta, powered by energy injected by the spinning $\nu$NS. We calculate the amount of mass and angular momentum gained by the $\nu$NS, as well as the $\nu$NS rotational evolution. The $\nu$NS spins up to a period of $47$ ms, then releases its rotational energy powering the synchrotron emission of the afterglow. The paucity of the $\nu$NS spin explains the low-luminosity characteristic and that the optical emission of the SN from the nickel radioactive decay outshines the optical emission from the synchrotron radiation. From the $\nu$NS evolution, we infer that the SN explosion had to occur at most $7.36$ h before the GRB trigger. Therefore, for the first time, the analysis of the GRB data leads to the time of occurrence of the CO core-collapse leading to the SN explosion and the electromagnetic emission of the GRB event.