  Magnetic reconnection is a leading mechanism for dissipating magnetic energy
and accelerating nonthermal particles in Poynting-flux dominated flows. In this
letter, we investigate nonthermal particle acceleration during magnetic
reconnection in a magnetically-dominated ion-electron plasma using fully
kinetic simulations. For an ion-electron plasma with the total magnetization
$\sigma_0=B^2/(4\pi n(m_i+m_e)c^2)$, the magnetization for each species is
$\sigma_i \sim \sigma_0$ and $\sigma_e \sim (m_i/m_e) \sigma_0$, respectively.
We have studied the magnetically dominated regime by varying $\sigma_{e} = 10^3
- 10^5$ with initial ion and electron temperatures $T_i = T_e = 5 - 20 m_ec^2$
and mass ratio $m_i/m_e = 1 - 1836$. The results demonstrate that reconnection
quickly establishes power-law energy distributions for both electrons and ions
within several ($2-3$) light-crossing times. For the cases with periodic
boundary conditions, the power-law index is $1<s<2$ for both electrons and
ions. The hard spectra limit the power-law energies for electrons and ions to
be $\gamma_{be} \sim \sigma_e$ and $\gamma_{bi} \sim \sigma_i$, respectively.
The main acceleration mechanism is a Fermi-like acceleration through the drift
motions of charged particles. When comparing the spectra for electrons and ions
in momentum space, the spectral indices $s_p$ are identical as predicted in
Fermi acceleration. We also find that the bulk flow can carry a significant
amount of energy during the simulations. We discuss the implication of this
study in the context of Poynting-flux dominated jets and pulsar winds
especially the applications for explaining the nonthermal high-energy
emissions.
