  The Crab Nebula is one of the most efficient accelerators in the Galaxy and
the only galactic source showing direct evidence of PeV particles. In spite of
this, the physical process behind such effective acceleration is still a deep
mystery. While particle acceleration, at least at the highest energies, is
commonly thought to occur at the pulsar wind termination shock, the properties
of the upstream flow are thought to be non-uniform along the shock surface, and
important constraints on the mechanism at work come from exact knowledge of
where along this surface particles are being accelerated. Here we use
axisymmetric relativistic MHD simulations to obtain constraints on the
acceleration site(s) of particles of different energies in the Crab Nebula.
Various scenarios are considered for the injection of particles responsible for
synchrotron radiation in the different frequency bands, radio, optical and
X-rays. The resulting emission properties are compared with available data on
the multi wavelength time variability of the inner nebula. Our main result is
that the X-ray emitting particles are accelerated in the equatorial region of
the pulsar wind. Possible implications on the nature of the acceleration
mechanism are discussed.
