  The extreme properties of the gamma ray flares in the Crab Nebula present a
clear challenge to our ideas on the nature of particle acceleration in
relativistic astrophysical plasma. It seems highly unlikely that standard
mechanisms of stochastic type are at work here and hence the attention of
theorists has switched to linear acceleration in magnetic reconnection events.
In this series of papers, we attempt to develop a theory of explosive magnetic
reconnection in highly-magnetized relativistic plasma which can explain the
extreme parameters of the Crab flares. In the first paper, we focus on the
properties of the X-point collapse. Using analytical and numerical methods
(fluid and particle-in-cell simulations) we extend Syrovatsky's classical model
of such collapse to the relativistic regime. We find that the collapse can lead
to the reconnection rate approaching the speed of light on macroscopic scales.
During the collapse, the plasma particles are accelerated by charge-starved
electric fields, which can reach (and even exceed) values of the local magnetic
field. The explosive stage of reconnection produces non-thermal power-law tails
with slopes that depend on the average magnetization $\sigma$. For sufficiently
high magnetizations and vanishing guide field, the non-thermal particle
spectrum consists of two components: a low-energy population with soft
spectrum, that dominates the number census; and a high-energy population with
hard spectrum, that possesses all the properties needed to explain the Crab
flares.
