  Rotation-powered radio pulsars are born with inferred initial rotation
periods of order 300 ms (some as short as 20 ms) in core-collapse supernovae.
In the traditional picture, this fast rotation is the result of conservation of
angular momentum during the collapse of a rotating stellar core. This leads to
the inevitable conclusion that pulsar spin is directly correlated with the
rotation of the progenitor star. So far, however, stellar theory has not been
able to explain the distribution of pulsar spins, suggesting that the birth
rotation is either too slow or too fast. Here we report a robust instability of
the stalled accretion shock in core-collapse supernovae that is able to
generate a strong rotational flow in the vicinity of the accreting
proto-neutron star. Sufficient angular momentum is deposited on the
proto-neutron star to generate a final spin period consistent with
observations, even beginning with spherically symmetrical initial conditions.
This provides a new mechanism for the generation of neutron star spin and
weakens, if not breaks, the assumed correlation between the rotational periods
of supernova progenitor cores and pulsar spin.
