[Paper Review] Relativistic Outflows in Gamma-Ray Bursts

M. Á. Aloy, M. Obergaulinger|arXiv (Cornell University)|Jan 8, 2007

Gamma-ray bursts and supernovae1 references3 citations

TL;DR

This paper uses numerical simulations to demonstrate that relativistic outflows in gamma-ray bursts (GRBs) are naturally produced by both collapsars (for long GRBs) and compact binary mergers (for short GRBs), with collimation driven by progenitor dynamics. Key results show that only a fraction of mergers produce observable short GRBs, and Lorentz factors exceed 500 for short GRBs due to low-density environments, while long GRBs reach ~100, depending on neutrino or MHD-driven mechanisms.

ABSTRACT

The possibility that gamma-ray bursts (GRBs) were not isotropic emissions was devised theoretically as a way to ameliorate the huge energetic budget implied by the standard fireball model for these powerful phenomena. However, the mechanism by which after the quasy-isotropic release of a few $10^{50} $erg yields a collimated ejection of plasma could not be satisfactory explained analytically. The reason being that the collimation of an outflow by its progenitor system depends on a very complex and non-linear dynamics. That has made necessary the use of numerical simulations in order to shed some light on the viability of some likely progenitors of GRBs. In this contribution I will review the most relevant features shown by these numerical simulations and how they have been used to validate the collapsar model (for long GRBs) and the model involving the merger of compact binaries (for short GRBs).

Motivation & Objective

• To investigate the viability of collapsar and compact binary merger models as progenitors of long and short GRBs, respectively.
• To understand the physical mechanisms behind the collimation of relativistic outflows in GRB progenitors.
• To determine the conditions under which mergers produce observable short GRBs versus X-ray/UV flashes.
• To quantify the role of neutrino and magnetic field-driven mechanisms in powering relativistic jets.
• To assess the impact of environmental density and progenitor structure on jet Lorentz factors and observable durations.

Proposed method

• Numerical simulations of relativistic hydrodynamics and magnetohydrodynamics (MHD) in axisymmetric, rotating stellar cores and binary merger systems.
• Incorporation of neutrino transport and energy deposition via neutrino-antineutrino pair annihilation in the collapsar model.
• Application of the Blandford-Payne and Blandford-Znajek processes to model magnetic field extraction of rotational energy in black hole systems.
• Use of scale-free MHD simulations to explore jet formation in compact binary mergers, with sensitivity to environmental density.
• Analysis of outflow structure, Lorentz factor evolution, and radial stretching to model observable GRB durations.
• Comparison of simulated light curves and Lorentz factors with observational constraints such as achromatic breaks and superluminal motions.

Experimental results

Research questions

• RQ1 What conditions in the progenitor system lead to the collimation of relativistic outflows in long GRBs?
• RQ2 Why do only a fraction of compact binary mergers produce observable short GRBs, and how does environmental density affect this?
• RQ3 What determines the asymptotic Lorentz factor of GRB outflows, and why are short GRBs typically more relativistic than long GRBs?
• RQ4 Can neutrino-powered and MHD-driven jets coexist, and which mechanism dominates in different GRB types?
• RQ5 How does differential acceleration in the jet lead to observable durations longer than the central engine timescale?

Key findings

• Numerical simulations confirm that collapsars can produce collimated relativistic outflows under general conditions, validating the collapsar model for long GRBs.
• Only mergers in low-density environments produce observable short GRBs; high-density environments lead to X-ray/UV flashes instead.
• Asymptotic Lorentz factors for short GRBs exceed 500–1000, significantly higher than the ~100 seen in long GRBs, due to lower environmental baryon loading.
• The differential acceleration of shells in the jet leads to radial stretching, making the observable duration T > c·t_ce, potentially up to 10 times longer than the engine timescale.
• Both neutrino-powered and MHD-driven mechanisms can operate simultaneously, with MHD dominating in high-Lorentz-factor jets and neutrinos in moderate ones.
• The simulations indicate that not all mergers produce observable GRBs, implying that observed short GRB rates are lower than merger rates, with implications for population synthesis and event rate estimates.