Geminga's tails: particle diffusion and reacceleration in the pulsar bow shock.

X-ray tails have been observed trailing the Geminga pulsar that provide evidence of a highly-energetic wind with electrons in excess of 100 TeV. Diffusing away from the pulsar,they radiate their energy very quickly by synchrotron and inverse Compton scattering, in particular in the enhanced magnetic field behind the bow shock between the wind and the surrounding medium.Reacceleration also takes place at the bow shock. These diffusion, radiation losses, and shock-acceleration processes have been modelled in 3 dimensions, using non-linear acceleration at the shock in the Bohm limit. The resulting flux map and particle spectra have been confronted to the X-ray data to provide constraints on the diffusion coefficient and the wind characteristics.