The X-ray structure and spectrum of the pulsar wind nebula surrounding PSR B0540-69.3

We present the first spatially resolved spectral study of an extragalactic pulsar wind nebula ( PWN), that surrounding the 50 ms pulsar B0540 - 69.3 in the Large Magellanic Cloud, using the ACIS instrument on board the Chandra X-ray Observatory. The spectral properties of the PWN change at a radius of similar to 5", corresponding to a shell detected optically in [ O III]. Within this radius, the spectra are well characterized by a simple power law, whose spectral index varies strongly with radius, so that the effective size of the nebula decreases with increasing photon energy. These X-ray measurements support the idea that the [ O III] shell represents an outer skin surrounding the wind nebula, analogous to that seen in the Crab Nebula. We use the size-energy relation and radial variability of the X-ray spectral index to estimate PWN parameters, including the radial dependence of the magnetic field, and the average ratio of electromagnetic to particle flux.