  The oblique geometry of pulsar wind termination shock ensures that the
Doppler beaming has a strong impact on the shock emission. We illustrate this
using recent relativistic MHD simulations of the Crab Nebula and also show that
the observed size, shape, and distance from the pulsar of the Crab Nebula inner
knot are consistent with its interpretation as a Doppler-boosted emission from
the termination shock. If the electrons responsible for the synchrotron
gamma-rays are accelerated only at the termination shock then their short
life-time ensures that these gamma-rays originate close to the shock and are
also strongly effected by the Doppler beaming. As the result, bulk of the
observed synchrotron gamma-rays of the Crab Nebula around 100 MeV may come from
its inner knot. This hypothesis is consistent with the observed optical flux of
the inner knot provided its optical-gamma spectral index is the same as the
injection spectral index found in the Kennel & Coroniti model of the nebula
spectrum. The observed variability of synchrotron gamma-ray emission can be
caused by the instability of the termination shock discovered in recent
numerical simulations. Given the small size of the knot, it is possible that
the September 2010 gamma-ray flare of the Crab Nebula also came from the knot,
though the actual mechanism remains unclear. The model predicts correlation of
the temporal variability of the synchrotron gamma-ray flux in the Fermi and
AGILE windows with the variability of the unpulsed optical flux from within 1
arcsec of the Crab pulsar.
