  During magnetically dominated relativistic reconnection, inflowing plasma
depletes the initial relativistic pressure at the x-line and collisionless
plasma heating inside the diffusion region is insufficient to overcome this
loss. The resulting pressure drop causes a collapse at the x-line, essentially
a localization mechanism of the diffusion region necessary for fast
reconnection. The extension of this low-pressure region further explains the
bursty nature of anti-parallel reconnection because a once opened outflow
exhaust can also collapse, which repeatedly triggers secondary tearing islands.
However, a stable single x-line reconnection can be achieved when an external
guide field exists, since the reconnecting magnetic field component rotates out
of the reconnection plane at outflows, providing additional magnetic pressure
to sustain the opened exhausts.
