Pulsars steadily dissipate their rotational energy via relativistic winds.Confinement of these outflows generates luminous pulsar wind nebulae, seen across the electromagnetic spectrum in synchrotron and inverse Compton emission, and in optical emission lines when they shock the surrounding medium. These sources act as important probes of relativistic shocks, particle acceleration and of interstellar gas. We review the many recent advances in the study of pulsar wind nebulae, with particular focus on the evolutionary stages through which these objects progress as they expand into their surroundings, and on morphological structures within these nebulae which directly trace the physical processes of particle acceleration and outflow. We conclude by considering some exciting new probes of pulsar wind nebulae, including the study of TeV gamma-ray emission from these sources, and observations of pulsar winds in close binary systems.

We present evidence for cosmic ray acceleration at the forward shock in Tycho's supernova remnant (SNR) from three X-ray observables: (1) the proximity of the contact discontinuity to the forward shock, or blast wave, (2) the morphology of the emission from the rim of Tycho, and (3) the spectral nature of the rim emission. We determine the locations of the blast wave (BW), contact discontinuity (CD), and reverse shock (RS) around the rim of Tycho's supernova remnant using a principal component analysis and other methods applied to new Chandra data. The azimuthal-angle-averaged radius of the BW is 251 ′′ . For the CD and RS we find average radii of 241 ′′ and 183 ′′ , respectively. Taking account of projection effects, we find ratios of 1 : 0.93 : 0.70 (BW : CD : RS). We show these values to be inconsistent with adiabatic hydrodynamical models of SNR evolution. The CD:BW ratio can be explained if cosmic ray acceleration of ions is occurring at the forward shock. The RS:BW ratio, as well as the strong Fe Kα emission from the Tycho ejecta, imply that the RS is not accelerating cosmic rays. We also extract radial profiles from ∼34% of the rim of Tycho and compare them to models of surface brightness profiles behind the BW for a purely thermal plasma with an adiabatic shock. The observed morphology of the rim is much more strongly peaked than predicted by the model, indicating that such thermal -2emission is implausible here. Spectral analysis also implies that the rim emission is non-thermal in nature, lending further support to the idea that Tycho's forward shock is accelerating cosmic rays.

G347.3-0.5 (RX J1713.7-3946) is a member of the new class of shell-type Galactic supernova remnants (SNRs) that feature non-thermal components to their X-ray emission. We have analyzed the X-ray spectrum of this SNR over a broad energy range (0.5 to 30 keV) using archived data from observations made with two satellites, the Röntgensatellit (ROSAT ) and the Advanced Satellite for Cosmology and Astrophysics (ASCA), along with data from our own observations made with the Rossi X-ray Timing Explorer (RXTE ). Using a combination of the models EQUIL and SRCUT to fit thermal and non-thermal emission, respectively, from this SNR, we find evidence for a modest thermal component to G347.3-0.5's diffuse emission with a corresponding energy of kT ≈ 1.4 keV. We also obtain an estimate of 70 TeV for the maximum energy of the cosmicray electrons that have been accelerated by this SNR.

We present results from the first light observations of the Cassiopeia A supernova remnant (SNR) by the Chandra X-Ray Observatory. Based on representative spectra from four selected regions, we investigate the processes of nucleosynthesis and mixing in Cas A. We make the first unequivocal identification of iron-rich ejecta produced by explosive silicon burning in a young Galactic SNR. Elsewhere in the remnant, we see silicon-rich ejecta from explosive oxygen burning. The Fe-rich ejecta lie outside the Si-rich material, indicating that bulk motions were extensive and energetic enough in Cas A to cause a spatial inversion of a significant portion of the supernova core. It is likely that this inversion was caused by "Fe"-rich ejecta emerging in plumes from the rising bubbles in the neutrino-driven convection layer during the supernova explosion. In addition, the radioactive decay energy from 56Ni may have contributed to the subsequent evolution of the material. We have also discovered faint, well-defined filaments with featureless X-ray spectra that are possibly sites of cosmic-ray acceleration in Cas A.

A pulsar wind nebula inside a supernova remnant provides a unique insight into the properties of the central neutron star, the relativistic wind powered by its loss of rotational energy, its progenitor supernova, and the surrounding environment. In this paper, we present a new semi-analytic model for the evolution of such a pulsar wind nebula. This model couples the dynamical and radiative evolution of the pulsar wind nebulae, traces the evolution of the pulsar wind nebulae throughout the lifetime of the supernova remnant produced by the progenitor explosion, and predicts both the dynamical (e.g. radius and expansion velocity) and radiative (radio to TeV γ-ray spectrum) properties of the pulsar wind nebula during this period. As a result, it is uniquely qualified for using the observed properties of a pulsar wind nebula in order to constrain the physical characteristics of the neutron star, pulsar wind, progenitor supernova, and surrounding interstellar medium. We also discuss the expected evolution for a particular set of these parameters, and show that it reproduced the large spectral break observed in radio and X-ray observations of many young pulsar wind nebulae, and the low break frequency, low radio luminosity and high TeV γ-ray luminosity, and high magnetization observed for several older pulsar wind nebulae. The predicted spectrum of this pulsar wind nebula also contains spectral features during different phases of its evolution detectable with new radio and γ-ray observing facilities such as the Extended Very Large Array and the Fermi Gamma-ray Space Telescope. Finally, this model has implications for determining if pulsar wind nebulae can inject a sufficient number of energetic electrons and positrons into the surrounding interstellar medium to explain the recent measurements of the cosmic ray positron fraction by PAMELA and the cosmic ray lepton spectrum by ATIC and HESS.