[Paper Review] Electromagnetic Outflows and GRBs

Maxim Lyutikov, R. D. Blandford|arXiv (Cornell University)|Oct 30, 2002

Gamma-ray bursts and supernovae3 citations

TL;DR

This paper proposes that gamma-ray bursts (GRBs) originate from relativistic, electromagnetically-dominated outflows powered by spinning, highly magnetized neutron stars or black holes. The outflow forms a collimated, force-free magnetic bubble that accelerates pairs via electromagnetic instabilities at ~10^16 cm, producing γ-ray emission, while energy transfer to the circumstellar medium drives the afterglow via a non-spherical relativistic blast wave with L_Ω ∝ θ^{-2}.

ABSTRACT

We study the dynamics of relativistic electromagnetic explosions as a possible mechanism for the production of Gamma-Ray Bursts. We propose that a rotating relativistic stellar-mass progenitor loses much of its spin energy in the form of an electromagnetically-dominated outflow. After the flow becomes optically thin, it forms a relativistically expanding, non-spherically symmetric magnetic bubble - a ''cold fireball''. We analyze the structure and dynamics of such a cavity in the force-free approximation. During relativistic expansion, most of the magnetic energy in the bubble is concentrated in a thin shell near its surface (contact discontinuity). We suggest that either the polar current or the shell currents become unstable to electromagnetic instabilities at a radius $\sim10^{16}$ cm. This leads to acceleration of pairs and causes the $γ$-ray emission. At a radius $\sim10^{17}$ cm, the momentum contained in the electromagnetic shell will have been largely transferred to the surrounding blast wave propagating into the circumstellar medium. Particles accelerated at the fluid shock may combine with electromagnetic field from the electromagnetic shell to produce the afterglow emission.

Motivation & Objective

• To address shortcomings in baryonic fireball models, such as inefficient energy conversion and fine-tuning of baryon content.
• To explore the feasibility of electromagnetic energy transport as the primary mechanism for GRB production.
• To explain the observed burst energy distribution and variability without relying on internal shocks or baryonic jets.
• To predict observable signatures of a Poynting-flux dominated outflow, including afterglow light curves and polarization features.
• To demonstrate that electromagnetic energy can remain coherent from source to emission site, enabling efficient, intermittent dissipation.

Proposed method

• Model the dynamics of a relativistic, force-free electromagnetic bubble expanding into the circumstellar medium.
• Use the force-free approximation to analyze the structure of a non-spherically symmetric magnetic cavity driven by a rotating, magnetized compact object.
• Calculate the Poynting flux and electromagnetic power from a unipolar inductor model with EMF ~10^22 V and current ~10^20 A.
• Analyze instability of surface and shell currents at r ~ 10^16 cm, leading to turbulence and particle acceleration.
• Simulate the relativistic blast wave evolution using a relativistic Kompaneets approximation to derive the energy distribution L_Ω ∝ θ^{-2}.
• Link γ-ray emission to electromagnetic instabilities in the shell and afterglow emission to shock interaction with the swept-up medium.

Experimental results

Research questions

• RQ1 Can relativistic electromagnetic outflows explain the high efficiency and collimation of GRBs without requiring baryonic jets?
• RQ2 What mechanisms enable efficient particle acceleration in a cold, electromagnetic-dominated outflow?
• RQ3 How does the geometry of the electromagnetic driver influence the afterglow light curve and the observed energy distribution?
• RQ4 Can electromagnetic instabilities in the shell or polar current lead to the observed short-timescale variability in GRBs?
• RQ5 What observational signatures, such as polarization or achromatic breaks, distinguish this model from standard fireball scenarios?

Key findings

• The electromagnetic outflow forms a cold, force-free magnetic bubble that expands relativistically, with most magnetic energy concentrated in a thin shell at the surface.
• At r ~ 10^16 cm, shell or polar currents become unstable to electromagnetic instabilities, leading to efficient pair acceleration and γ-ray emission.
• The afterglow is driven by a non-spherically symmetric relativistic blast wave with energy per steradian scaling as L_Ω ∝ θ^{-2}, consistent with observations.
• The model predicts that all bursts are observable regardless of viewing angle, with fluence depending strongly on angle, reducing the incidence of orphan afterglows.
• The Lorentz factor of the blast wave is expected to be Γ >> θ^{-1}, with an achromatic break when Γ ≈ θ^{-1}, detectable in light curves.
• The model naturally explains the narrow energy distribution of GRBs and the intermittent nature of variability through localized electromagnetic turbulence.