Explosive X-point collapse in relativistic magnetically-dominated plasma

TL;DR

This paper develops a relativistic magnetic reconnection model explaining the Crab Nebula's gamma-ray flares, showing that explosive X-point collapse can accelerate particles to produce observed high-energy emissions.

Contribution

It extends classical models of X-point collapse to relativistic regimes, demonstrating how explosive reconnection can produce the extreme particle acceleration observed in astrophysical flares.

Findings

Reconnection rate approaches the speed of light during collapse.

Charge-starved electric fields can exceed local magnetic fields.

Particle spectra include a high-energy component consistent with Crab flares.

Abstract

The extreme properties of the gamma ray flares in the Crab Nebula present a clear challenge to our ideas on the nature of particle acceleration in relativistic astrophysical plasma. It seems highly unlikely that standard mechanisms of stochastic type are at work here and hence the attention of theorists has switched to linear acceleration in magnetic reconnection events. In this series of papers, we attempt to develop a theory of explosive magnetic reconnection in highly-magnetized relativistic plasma which can explain the extreme parameters of the Crab flares. In the first paper, we focus on the properties of the X-point collapse. Using analytical and numerical methods (fluid and particle-in-cell simulations) we extend Syrovatsky's classical model of such collapse to the relativistic regime. We find that the collapse can lead to the reconnection rate approaching the speed of light on macroscopic scales. During the collapse, the plasma particles are accelerated by charge-starved electric fields, which can reach (and even exceed) values of the local magnetic field. The explosive stage of reconnection produces non-thermal power-law tails with slopes that depend on the average magnetization $\sigma$. For sufficiently high magnetizations and vanishing guide field, the non-thermal particle spectrum consists of two components: a low-energy population with soft spectrum, that dominates the number census; and a high-energy population with hard spectrum, that possesses all the properties needed to explain the Crab flares.