Are Reconnection X-points Important for Nonthermal Particle Acceleration in Relativistic Magnetic Reconnection?

There is strong evidence suggesting that reconnection is an efficient process for producing energetic particles various plasma systems. Reconnection x-points (regions with electric field larger than the magnetic field for a vanishing guide field) have been suggested to be important for particle injection that boosts particle energy to the lower energy bound of the power-law energy distribution in relativistic magnetic reconnection. We carry out particle-in-cell kinetic simulations and analyses to elucidate the roles of x-points among different injection mechanisms during nonthermal particle acceleration in relativistic magnetic reconnection. We show that for a vanishing guide field, the x-points can only host particles for a short time, leading to a limited energy gain insufficient for particle injection. Meanwhile, most energy gain for particle injection and nonthermal particle acceleration are through regions outside of the x-points. Interestingly, we also find that some energy gain already occurs before particles interact with the x-points. By evolving a test-particle component in the PIC simulation that does not "see" the x-point electric field, we show that particles can still be efficiently accelerated. Meanwhile, the mechanisms associated with electric field perpendicular to the magnetic field outside the x-points dominates the injection process. We conclude that X-points are not important for particle injection or nonthermal particle acceleration during relativistic magnetic reconnection.