Beaming and rapid variability of high-energy radiation from relativistic pair plasma reconnection

TL;DR

This study uses particle-in-cell simulations to reveal that relativistic pair plasma reconnection produces strongly anisotropic, beamed high-energy radiation with rapid variability, potentially explaining gamma-ray flares in astrophysical sources.

Contribution

First modeling of angular distribution and beaming of particles and radiation in relativistic pair plasma reconnection, highlighting kinetic beaming effects and rapid variability.

Findings

Particles and radiation are strongly anisotropic and beamed.

Synchrotron emission exhibits super-fast variability, as short as one-tenth of the light-crossing time.

Reconnection signatures can explain gamma-ray flares in the Crab Nebula and blazars.

Abstract

We report on the first study of the angular distribution of energetic particles and radiation generated in relativistic collisionless electron-positron pair plasma reconnection, using two-dimensional particle-in-cell simulations. We discover a strong anisotropy of the particles accelerated by reconnection and the associated strong beaming of their radiation. The focusing of particles and radiation increases with their energy; in this sense, this "kinetic beaming" effect differs fundamentally from the relativistic Doppler beaming usually invoked in high-energy astrophysics, in which all photons are focused and boosted achromatically. We also present, for the first time, the modeling of the synchrotron emission as seen by an external observer during the reconnection process. The expected lightcurves comprise several bright symmetric sub-flares emitted by the energetic beam of particles sweeping across the line of sight intermittently, and exhibit super-fast time variability as short as about one tenth of the system light-crossing time. The concentration of the energetic particles into compact regions inside magnetic islands and particle anisotropy explain the rapid variability. This radiative signature of reconnection can account for the brightness and variability of the gamma-ray flares in the Crab Nebula and in blazars.