Magnetic Rayleigh-Taylor instability in pulsar wind nebulae

We present two-dimensional numerical simulations of the development of the Rayleigh-Taylor (RT) instability at the interface between an expanding Pulsar Wind Nebula (PWN) and the surrounding swept-up shell of supernova ejecta. The simulations include the presence of a magnetic field which is expected to be close to equipartition in the PWN, and are performed in a proper special relativistic MHD regime. The RT instability in the presence of a magnetic field is revised in the framework of the PWN expansion, and a stability criterion is derived. Simulations show that even a very weak magnetic field can suppress the formation of RT fingers. This has important implications in the correct understanding of the local physical conditions at the interface between the PWN and the ejecta. The results suggest that efficient radiative cooling in the ejecta shell is required to explain the filamentary network observed in the Crab Nebula as due to RT instability.