Polarization in the inner region of pulsar wind nebulae

Abstract

We present here the first effort to compute synthetic synchrotron polarization maps of Pulsar Wind Nebulae (PWNe). Our goal is to highlight how polarization can be used as an additional diagnostic tool for the flow structure in the inner regions of these nebulae. Recent numerical simulations suggest the presence of flow velocities ∼0.5 c in the surroundings of the termination shock, where most of the high energy emission comes from. We construct polarization maps taking into account relativistic effects like Doppler boosting and position angle swing. The effect of different bulk velocities is clarified with the help of a toy-model consisting of a uniformly emitting torus. We also present a map based on recent numerical simulations of the entire nebula and compare it with presently available data. The comparison with upcoming high resolution observations could provide new insight into the inner structure of the nebula and put constraints on the geometrical properties of the magnetic field.

Infrared imaging and polarimetric observations of the pulsar wind nebula in SNR G21.5-0.9

“…In addition, close to the torus edges, regions that are almost completely depolarised can be found (see Fig. 2 of Bucciantini et al 2005). Simulations also show that the polarisation angle should change across the emitting region.…”

“…The region where the electric vector becomes perpendicular to the symmetry axis depends on the inclination angle of the torus with respect to the sky, and also on the bulk flow speed. Concerning the latter dependence, the position where the polarisation vector becomes perpendicular to the axis moves closer towards the axis with increasing value of the bulk speed (Bucciantini et al 2005).…”

“…For some of the wisps a high degree of linear polarisation, near 0.7, is measured. Recent MHD simulations studying optical polarisation in the inner parts of pulsar wind nebulae (Bucciantini et al 2005, Del Zanna et al 2006, Volpi et al 2009) predict that emission (star) is formed. The PWN expands into the cold SN ejecta, which from the outer side are being heated/compressed by the reverse shock facing inwards in the SN remnant.…”

The Tale of the Two Tails of the Oldish PSR J2055+2539

“…If the pulsar moves supersonically, shocked pulsar wind is expected to flow in an elongated region downstream of the termination shock (basically, the cavity in the interstellar medium (ISM)created by the moving neutron star and its wind), confined by ram pressure. X-ray emission is due to synchrotron emission from the wind particles accelerated at the termination shock, which is typically seen (if angular resolution permits) as the brightest portion of the extended structure (see, e.g., Kargaltsev & Pavlov 2008a), as predicted by MHD simulations (Bucciantini 2002; Van der Swaluw 2003;Bucciantini et al 2005). In order to produce a population of high-energy particles, a highly energetic pulsar is required (Ė greater than ∼10 34 ; see, e.g., Kargaltsev & Pavlov 2008b).…”

Unveiling the magnetic structure of VHE SNRs/PWNe with XIPE, the x-ray imaging-polarimetry explorer

“…Spatially-resolved measurements by XIPE will determine the magnetic field orientation and the level of turbulence in the torus, the jet, and at various distances from the pulsar. This is of special interest because the polarised emission is more sensitive to the plasma dynamics in PWNe than the total synchrotron emission [12]. Knowing how the level of turbulence changes with distance from the shock could test recent MHD scenarios that invoke the conversion of magnetic energy into particle energy inside the radiation region [13].…”