Relativistic magnetic explosions

TL;DR

This paper investigates the dynamics of relativistic magnetic explosions in astrophysics, revealing conditions for their formation and providing analytical solutions for their expansion, highlighting differences from fluid explosions due to magnetic flux conservation.

Contribution

It introduces a threshold condition for relativistic magnetic explosions and offers an analytical solution to the expansion of force-free plasma into vacuum.

Findings

Relativistic magnetic explosions require high initial over-pressure and rapid external pressure decrease.

Subcritical cases result in quiet expansion without relativistic outflows.

An analytical solution to Prendergast's problem is derived for force-free plasma expansion.

Abstract

Many explosive astrophysical events, like magnetars' bursts and flares, are magnetically driven. We consider dynamics of such magnetic explosions - relativistic expansion of highly magnetized and highly magnetically over-pressurized clouds. The corresponding dynamics is qualitatively different from fluid explosions due to the topological constraint of the conservation of the magnetic flux. Using analytical, relativistic MHD as well as force-free calculations, we find that the creation of a relativistically expanding, causally disconnected flow obeys a threshold condition: it requires sufficiently high initial over-pressure and sufficiently quick decrease of the pressure in the external medium (the pre-explosion wind). In the subcritical case the magnetic cloud just "puffs-up" and quietly expands with the pre-flare wind. We also find a compact analytical solution to the Prendergast's problem - expansion of force-free plasma into vacuum.