Non-thermal emission from relativistic MHD simulations of pulsar wind nebulae: from synchrotron to inverse Compton

Abstract

Aims. The goal of this paper is twofold. In the first place we complet e the set of diagnostic tools for synchrotron emitting sourc es presented by Del Zanna et al. (Astron. Astrophys. 453, 621, 2006 ) with the computation of inverse Compton radiation from the same relativistic particles. Moreover we investigate, for the fi rst time, the gamma-ray emission properties of Pulsar Wind Nebulae in the light of the axisymmetric jet-torus scenario. Methods. The proposed method consists in evolving the relativistic MHD equations and the maximum energy of the emitting particles including adiabatic and synchrotron losses along streamlines. The particle energy distribution function is split in t wo components: one corresponding to the radio emitting electrons interpreted as a relic population born at the outburst of the supernova and the other associated with the wind population continuously accelerated at the termination shock and emitting up to the gamma-ray band. The inverse Compton emissivity is calculated using the general Klein-Nishina differential cross-section and three different photon targets for the relativistic particles are considered: the nebular synchrotron photons, photons associated with the far-infrared thermal excess and the cosmic microwave background. Results. When the method is applied to the simulations that better reproduce the optical and X-ray morphology of the Crab Nebula, the overall synchrotron spectrum can only be fitted assuming an excess of injected particles and a steeper power law (E −2.7 ) with respect to previous models. The resulting TeV emission has then the correct shape but is in excess of the data. This is rela ted to the magnetic field structure in the nebula as obtained by the simu lations, in particular the field is strongly compressed near the termination shock but with a lower than expected volume average. The jet-torus structure is found to be clearly visible in high-resolution gammaray synthetic maps too. We also present a preliminary exploration of time variability in the X and gamma-ray bands. We find variations with time-scales of about 2 years in both bands. The variability observed originates from the strongly time-dependent MHD motions inside the nebula.