Hyperaccreting Black Hole as Gamma-Ray Burst Central Engine. II. Temporal evolution of central engine parameters during Prompt and Afterglow Phases

TL;DR

This paper models the temporal evolution of central engine parameters in gamma-ray bursts using analytical solutions for neutrino annihilation and Blandford-Znajek mechanisms, explaining diverse GRB lightcurve features.

Contribution

It provides analytical solutions for the evolution of black hole central engine parameters during GRB prompt and afterglow phases, considering both jet launching mechanisms and their observational signatures.

Findings

BZ jets are more powerful and likely responsible for late-time activity.

Initially non-spinning black holes can launch thermal fireballs and later Poynting-flux jets.

Late-time accretion explains flares, bumps, and plateaus in GRB afterglows.

Abstract

A hyperaccreting stellar-mass black hole has been proposed as the candidate central engine of gamma-ray bursts (GRBs). The rich observations of GRBs by \textit{Fermi} and \textit{Swift} make it possible to constrain the central engine model by comparing the model predications against data. This paper is dedicated to studying the temporal evolution of central engine parameters for both prompt emission and afterglow phases. We consider two jet launching mechanisms, i.e., $\nu\bar{\nu}$ annihilations and the Blandford-Znajek (BZ) processe, and obtain analytical solutions to these two models. We then investigate the black hole central engine parameters, such as the jet power, the dimensionless entropy $\eta$, and the central engine parameter $\mu_0=\eta (1+\sigma_0)$ (where $\sigma_0$ is the initial magnetization of the engine) at the base of the jet. The black hole may be spun up by accretion, or spun down by the BZ process, leaving imprints in GRB lightcurves. Usually, a BZ jet is more powerful and is likely responsible for the late time central engine activities. However, an initially non-spinning black hole central engine may first launch a thermal "fireball" via neutrino annihilations, and then launch a Poynting-flux-dominated jet via the BZ process. Multiple flares, giant bumps, and plateaus in GRB afterglows can be well produced as the result of late time accretion onto the black hole.