On the Short GRB GeV emission from a Kerr Black hole

TL;DR

This paper investigates the origin of GeV emission in short gamma-ray bursts, linking it to black hole formation and deriving properties of the black hole based on observed luminosity decay patterns.

Contribution

It establishes that GeV emission originates at black hole formation, with specific power-law decay, and uses this to infer black hole mass and spin in short GRBs.

Findings

GeV emission starts at black hole formation.

Luminosity decay follows a power-law with index around -1.2 to -1.3.

GeV emission indicates black hole creation in short GRBs.

Abstract

It has recently become clear that in both short and long gamma-ray bursts (GRBs) it coexists a sequence of different events, each characterized by specific physical processes and corresponding values of the Lorentz gamma factors. The ultra-relativistic prompt emission (UPE) phase, with Lorentz factor $\Gamma\leq10^4$, is followed by a mildly relativistic plateau-afterglow phase with $\Gamma\lesssim2$. The GeV radiation, with $\Gamma\lesssim50$, coexists with the above two phases. It is shown that: a) the GeV radiation originates at the onset of the formation of a black hole (BH), b) its luminosity follows specific power-law dependence when measured in the rest frame of the source with a decay index $\gamma=-1.29\pm0.06$ in the case of the short GRBs, and $\gamma=-1.20\pm0.04$ in the case of the long GRBs, c) these energetics requirements are used to derive the mass and spin of the BH originating this extended GeV emission. We present these conceptual results for the case of short GRBs in this article and give the extended analysis for long GRBs in a companion article. A direct astrophysical application of these results is that the merger of binary neutron stars leading to BH formation emits GeV radiation: the GeV emission is a necessary and sufficient condition to indicate the creation of a BH in S-GRBs.