Axially symmetric relativistic MHD simulations of Pulsar Wind Nebulae in Supernova Remnants

Abstract

Abstract. The structure and the evolution of Pulsar Wind Nebulae (PWNe) are studied by means of two-dimensional axisymmetric relativistic magnetohydrodynamic (RMHD) simulations. After the first imaging of the Crab Nebula with Chandra, a growing number of objects has been found to show in the X-rays spatial features such as rings and jets, that clearly cannot be accounted for within the standard framework of one-dimensional semi-analytical models. The most promising explanation suggested so far is based on the combined effects of the latitude dependence of the pulsar wind energy flux, shaping the wind termination shock and naturally providing a higher equatorial emission, and of the wind magnetization, likely responsible for the jet collimation by hoop stresses downstream of the shock. This scenario is investigated here by following the evolution of a PWN interacting with the confining Supernova Remnant (SNR), from the free expansion to the beginning of the reverberation phase. Our results confirm the oblate shape of the wind termination shock and the formation of a polar jet with supersonic velocities (v ≈ 0.5−0.7c) for high enough values of the equatorial wind magnetization parameter (σ > ∼ 0.01).

The termination shock of a magnetar wind: a possible origin of gamma-ray burst X-ray afterglow emission

“…The nearly overlapping of the curves with σ varying from 0 to 3 indicates that the dynamic evolutions of both the shocks are insensitive to the degree of magnetization of the wind. It is not surprising to obtain such a result, which actually has been know for a long time for ordinary pulsar wind nebulae (e.g., Emmering & Chevalier 1987;Bucciantini et al 2003;Del Zanna et al 2004). …”

Pulsar-Wind Nebulae

“…This general conjecture has been confirmed by axisymmetric MHD simulations (Komissarov and Lyubarsky 2004;Del Zanna et al 2004Volpi et al 2008;Bucciantini et al 2011). These simulations were able to explain also nontrivial features of the fine structure, such as a mysterious knot, which is located within 1" from the Crab pulsar (Hester et al 1995).…”

Dissipation in pulsar winds

“…Del Zanna et al (2004) concentrate on the case in which σ is independent of angle, with a current sheet in the equatorial plane, as in the case of the "aligned" split monopole. On the other hand, Lyubarsky (2003, 2004) are motivated by the scenario where low magnetisation is achieved by complete dissipation of the oscillating component of the magnetic field in the "striped wind" of the oblique split-monopole solution (Coroniti, 1990; Fig.…”