In magnetized astrophysical outflows , the dissipation of field energy into particle energy via magnetic reconnection is often invoked to explain the observed non-thermal signatures .
By means of two- and three-dimensional particle-in-cell simulations , we investigate anti-parallel reconnection in magnetically-dominated electron-positron plasmas .
Our simulations extend to unprecedentedly long temporal and spatial scales , so we can capture the asymptotic state of the system beyond the initial transients , and without any artificial limitation by the boundary conditions .
At late times , the reconnection layer is organized into a chain of large magnetic islands connected by thin X-lines .
The plasmoid instability further fragments each X-line into a series of smaller islands , separated by X-points .
At the X-points , the particles become unmagnetized and they get accelerated along the reconnection electric field .
We provide definitive evidence that the late-time particle spectrum integrated over the whole reconnection region is a power-law , whose slope is harder than -2 for magnetizations \sigma \gtrsim 10 .
Efficient particle acceleration to non-thermal energies is a generic by-product of the long-term evolution of relativistic reconnection in both two and three dimensions .
In three dimensions , the drift-kink mode corrugates the reconnection layer at early times , but the long-term evolution is controlled by the plasmoid instability , that facilitates efficient particle acceleration , in analogy to the two-dimensional physics .
Our findings have important implications for the generation of hard photon spectra in pulsar winds and relativistic astrophysical jets .