Numerical simulations of mass loading in the tails of Bow Shock Pulsar Wind Nebulae

When a pulsar is moving through a partially ionized medium, a fraction of neutral Hydrogen atoms penetrate inside the pulsar wind and can be photo-ionized by the nebula UV radiation. The resulting protons remains attached to the magnetic field of the light leptonic pulsar wind enhancing its inertia and changing the flow dynamics of the wind. We present here the first numerical simulations of such effect in the tails of bow shock nebulae. We produce a set of different models representative of pulsars moving in the interstellar medium with different velocities, from highly subsonic to supersonic, by means of 2D hydrodynamic relativistic simulations. We compare the different tail morphologies with results from theoretical models of mass loading in bow shocks. As predicted by analytical models we observe a fast sideways expansion of the tail with the formation of secondary shocks in the ISM. This effect could be at the origin of the head-and-shoulder morphology observed in many BSPWNe.