  As a fundamental process converting magnetic to plasma energy in high-energy
astrophysical plasmas, relativistic magnetic reconnection is a leading
explanation for the acceleration of particles to the ultrarelativistic energies
necessary to power nonthermal emission (especially X-rays and gamma-rays) in
pulsar magnetospheres and pulsar wind nebulae, coronae and jets of accreting
black holes, and gamma-ray bursts. An important objective of plasma
astrophysics is therefore the characterization of nonthermal particle
acceleration (NTPA) effected by reconnection. Reconnection-powered NTPA has
been demonstrated over a wide range of physical conditions using large
two-dimensional (2D) kinetic simulations. However, its robustness in realistic
3D reconnection -- in particular, whether the 3D relativistic drift-kink
instability (RDKI) disrupts NTPA -- has not been systematically investigated,
although pioneering 3D simulations have observed NTPA in isolated cases. Here
we present the first comprehensive study of NTPA in 3D relativistic
reconnection in collisionless electron-positron plasmas, characterizing NTPA as
the strength of 3D effects is varied systematically via the length in the third
dimension and the strength of the guide magnetic field. We find that, while the
RDKI prominently perturbs 3D reconnecting current sheets, it does not suppress
particle acceleration, even for zero guide field; fully 3D reconnection
robustly and efficiently produces nonthermal power-law particle spectra closely
resembling those obtained in 2D. This finding provides strong support for
reconnection as the key mechanism powering high-energy flares in various
astrophysical systems. We also show that strong guide fields significantly
inhibit NTPA, slowing reconnection and limiting the energy available for plasma
energization, yielding steeper and shorter power-law spectra.
