Vorticity and Magnetic Field Generation from Initial Anisotropy in Ultrarelativistic Gamma-Ray Burst Blastwaves

TL;DR

This paper investigates how initial anisotropy in ultrarelativistic gamma-ray burst blastwaves can generate vorticity and magnetic fields, with implications for observed magnetic energy densities in GRB afterglows.

Contribution

It demonstrates that initial angular energy fluctuations produce vorticity in relativistic blastwaves, linking anisotropy to magnetic field generation in GRB afterglows.

Findings

Vorticity is produced by initial anisotropy in blastwaves.

The number of eddy turnovers is about one for marginally nonlinear anisotropy.

Nonlinear anisotropy is necessary to explain observed magnetic energy densities.

Abstract

Because conical segments of quasispherical ultrarelativistic blastwaves are causally disconnected on angular scales larger than the blastwave inverse Lorentz factor, astrophysical blastwaves can sustain initial anisotropy, imprinted by the process that drives the explosion, while they remain relativistic. We show that initial angular energy fluctuations in ultrarelativistic blastwaves imply a production of vorticity in the blastwave, and calculate the vortical energy production rate. In gamma-ray burst (GRB) afterglows, the number of vortical eddy turnovers as the shocked fluid crosses the blastwave shell is about unity for marginally nonlinear anisotropy. Thus the anisotropy must be nonlinear to explain the magnetic energy density inferred in measured GRB spectra.