Tiny Electromagnetic Explosions

Abstract

This paper considers electromagnetic transients of a modest total energy (${\cal E} \gtrsim 10^{40}$ erg) and small initial size (${\cal R} \gtrsim 10^{-1}$ cm). They could be produced during collisions between relativistic field structures (e.g. macroscopic magnetic dipoles) that formed around, or before, cosmic electroweak symmetry breaking. The outflowing energy has a dominant electromagnetic component; a subdominant thermal component (temperature $> 1$ GeV) supplies inertia in the form of residual $e^\pm$. A thin shell forms that expands subluminally, attaining a Lorentz factor $\sim 10^{6-7}$ before decelerating. Drag is supplied by the reflection of an ambient magnetic field, and by deflection of ambient free electrons. Emission of low-frequency (GHz-THz) superluminal waves takes place through three channels: i) reflection of the ambient magnetic field; ii) direct linear conversion of the embedded magnetic field into a superluminal mode; and iii) excitation outside the shell by corrugation of its surface. The escaping electromagnetic pulse is very narrow (a few wavelengths) and so the width of the detected transient is dominated by propagation effects. GHz radio transients are emitted from i) the dark matter halos of galaxies and ii) the near-horizon regions of supermassive black holes that formed by direct gas collapse and now accrete slowly. Brighter and much narrower 0.01-1 THz pulses are predicted at a rate at least comparable to fast radio bursts, experiencing weaker scattering and absorption. The same explosions also accelerate protons up to $\sim 10^{19}$ eV and heavier nuclei up to $10^{20-21}$ eV.