Mass loading of pulsar winds
  The dynamics of relativistic magnetized mass loaded outflows carrying toroidal magnetic field is analyzed in the context of Pulsar Wind Nebulae (PWNs). Mass loading is very efficient in slowing down super-relativistic magnetized flows and weakening of relativistic shocks. We suggest that weakening of relativistic reverse shocks by mass loading in PWNs is responsible for the low radiative efficiencies of the majority of the PWNs. Mass loading may also result in a shock transition near the fast magnetosonic point; this is unlikely to happen in majority of PWNs. The evolution of magnetized mass loaded flows beyond the reverse shock is complicated: after initial deceleration to the minimal velocity required to transport the magnetic flux, the mass loaded flows have to {\it accelerate}. In order to be able to expand to infinity, magnetized flows should either become time dependent or destroy the toroidal magnetic flux by developing internal instabilities. Destruction of the magnetic flux initiated by mass loading may allow for the flow to slow down to sub-relativistic velocities and resolve the $\sigma$ paradox of the pulsar wind nebula. 
Maxim Lyutikov (McGill University, MIT, CITA National Fellow)