NuSTAR Imaging of Pulsar Wind Nebulae MSH 15-52 and the Crab

We present NuSTAR imaging and spectral analysis of the two Pulsar Wind Nebulae (PWNe) MSH 15-52 and the Crab. PWNe are center-filled synchrotron nebulae with complex structures that provide clues to the understanding of particle acceleration and diffusion in relativistic shocks. We show NuSTAR imaging from 3 -- 78 keV of both objects and demonstrate that both nebulae reduce their size with increasing photon energy due to synchrotron burn-off. For the Crab the rate of shrinkage is consistent with theoretical estimates in the plane of the torus, but towards the N-W in the direction of the counter jet, the rate is almost a factor of 2 higher. In both PWNe, we observe the spectral index to steepen with increasing radius as a consequence of synchrotron burn-off, but we will show that for the Crab, the spatially dependent spectrum is more complex that previously assumed, and that the line-of-sight integrated spectrum is better represented by a broken power-law with a break at ~10 keV.