Hyper-accreting black hole as GRB central engine. I: Baryon loading in GRB jets

TL;DR

This paper investigates baryon loading in GRB jets from hyper-accreting black holes, comparing neutrino-driven and magnetically dominated mechanisms, and finds magnetically dominated jets are generally cleaner and better fit observational data.

Contribution

It provides a detailed analysis of baryon loading processes in different GRB jet models, highlighting the conditions under which jets are baryon-poor and aligning models with recent empirical relations.

Findings

More luminous jets tend to be baryon-poor.

Neutrino-driven jets are generally dirtier than magnetic jets.

Magnetically dominated jets better match observed GRB properties.

Abstract

A hyper-accreting stellar-mass black hole has been long speculated as the best candidate of central engine of gamma-ray bursts (GRBs). Recent rich observations of GRBs by space missions such as Swift and Fermi pose new constraints on GRB central engine models. In this paper, we study the baryon loading processes of a GRB jet launched from a black hole central engine. We consider a relativistic jet powered by $\nu \bar\nu$-annihilation or by the Blandford-Znajek (BZ) mechanism. We consider baryon loading from a neutrino-driven wind from a neutrino-cooling-dominated accretion flow. For a magnetically dominated BZ jet, we consider neutron-drifting from the magnetic wall surrounding the jet and subsequent positron capture and proton-neutron inelastic collisions. The minumim baryon loads in both types of jet are calculated. We find that in both cases, a more luminous jet tends to be more baryon poor. A neutrino-driven "fireball" is typically "dirtier" than a magnetically dominated jet, while a magnetically dominated jet can be much cleaner. Both models have the right scaling to interpret the empirical $\Gamma-L_{\rm iso}$ relation discovered recently. Since some neutrino-driven jets have too much baryon loading as compared with the data, we suggest that at least a good fraction of GRBs should have a magnetically dominated central engine.