GRB 171205A: Hypernova and Newborn Neutron Star

TL;DR

This paper models the core-collapse of a binary system leading to a hypernova and a newborn neutron star, explaining the observed gamma-ray burst and supernova features, and inferring the explosion timing from the neutron star's evolution.

Contribution

It introduces a detailed model of a binary-driven hypernova, linking neutron star formation, spin evolution, and electromagnetic emissions to observed GRB and supernova data.

Findings

The neutron star spins up to 47 ms period, powering afterglow emission.

The supernova explosion occurred within 7.36 hours before the GRB.

The model explains low-luminosity GRB characteristics through neutron star spin dynamics.

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.