Chandra Studies of Nonthermal Emission from Supernova Remnants and Pulsar Wind Nebulae

Abstract

While supernova remnants (SNRs) have long been considered prime candidates as sources of cosmic rays, it is only recently that X-ray observations have identified several shell-type SNRs dominated by nonthermal emission, thus revealing shock-accelerated electrons with energies extending far beyond the typical thermal spectrum. Two of these SNRs have been detected as sources of VHE gamma-rays.In other remnants, discrepancies between the shock velocity and the electron temperature point to a strong cosmic ray component that has thrived at the expense of the thermal gas. Modeling of the radio, X-ray, and gamma-ray emission provides constraints on particle acceleration as well as the properties of the medium in which the mechanism prospers. Crab-like pulsar wind nebulae (PWNe) are characterized by a termination shock at which the wind is forced to join the slow expansion of the outer nebula. These shocks also act as sites in which particles are boosted to high energies; the X-ray emission from the Crab Nebula, as well as the inverse Compton radiation observed as VHE gamma-rays, imply electrons with energies in excess of ~100 TeV. Recent X-ray observations have begun to reveal these shock zones in the Crab and other PWNe, and are now allowing us to constrain the nature of pulsar winds as well as the flow conditions in the outer nebulae. Here I present a brief overview of recent studies with the Chandra X-ray Observatory in which the properties of these shock acceleration regions are finally being revealed.