Local Kelvin-Helmholtz instability and synchrotron modulation in Pulsar Wind Nebulae

¹ Astronomy Department, University of California at Berkeley, 601 Campbell Hall, Berkeley, CA 94720-3411, USA e-mail: niccolo@astron.berkeley.edu
² Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, L.go E. Fermi 2, 50125 Firenze, Italy

Received: 7 November 2005
Accepted: 14 March 2006

Abstract

We present here a series of numerical simulations of the development of Kelvin-Helmholtz instability in a relativistically hot plasma. The physical parameters in the unperturbed state are chosen to be representative of local conditions encountered in Pulsar Wind Nebulae (PWNe), with a main magnetic field perpendicular to a mildly relativistic shear layers. By using a numerical code for Relativistic MHD, we investigate the effect of an additional magnetic field component aligned with the shear velocity, and we follow the evolution of the instability to the saturation and turbulent regimes. Based on the resulting flow structure, we then compute synchrotron maps in order to evaluate the signature of Kelvin-Helmholtz instability on the emission and we investigate how the time scale and the amplitude of the synchrotron modulations depend on shear velocity and magnetic field. Finally we compare our results to the observed variable features in the Crab Nebula. We show that the Kelvin-Helmholtz instability cannot account for the wisps variability, but it might be responsible for the time dependent filamentary structure observed in the main torus.