Modeling the effect of small-scale magnetic turbulence on the X-ray properties of Pulsar Wind Nebulae

Abstract

Pulsar Wind Nebulae (PWNe) constitute an ideal astrophysical environment to test our current understanding of relativistic plasma processes. It is well known that magnetic fields play a crucial role in their dynamics and emission properties. At present, one of the main issues concerns the level of magnetic turbulence present in these systems, which in the absence of space resolved X-ray polarization measures cannot be directly constrained. In this work we investigate, for the first time using simulated synchrotron maps, the effect of a small scale fluctuating component of the magnetic field on the emission properties in X-ray. We illustrate how to include the effects of a turbulent component in standard emission models for PWNe, and which consequences are expected in terms of net emissivity and depolarization, showing that the X-ray surface brightness maps can provide already some rough constraints. We then apply our analysis to the Crab and Vela nebulae and, by comparing our model with Chandra and Vela data, we found that the typical energies in the turbulent component of the magnetic field are about 1.5 to 3 times the one in the ordered field.

Modeling radio circular polarization in the Crab nebula

“…This implies that the magnetic energy into the turbulent unresolved component of the magnetic field E turb , is about one half of the one into the large scale component E ord (the relation between the two is E turb = 3σ 2 E ord ). This value is smaller than what was found by Bucciantini et al (2017) in modeling the X-ray surface brightness profile of the Crab torus. However, that model assumed a fully toroidal (ordered) large scale field, while in our 3D simulation the turbulent cascade is partially resolved, at least over the larger scales.…”

PHEMTO: the polarimetric high energy modular telescope observatory

“…4 Chandra images of the Vela (left, [79]) and Crab (right, [47]) PWNe for the overall nebula but not for the regions close to the termination shock, where the acceleration takes place. Only resolved X-ray polarization maps would allow the degeneracy between Doppler boosting and magnetic field configuration (orientation, turbulence) closest to the acceleration site to be lifted [12].…”

PHEMTO : Polarimetric High Energy Modular Telescope Observatory

“…Radio polarization can discriminate ordered or disordered magnetic field for the overall nebula but not for the regions close to the termination shock, where the acceleration takes place. Only resolved X-ray polarization maps would allow to lift the degeneracy between Doppler boosting and magnetic field configuration (orientation, turbulence) closest to the acceleration site (Bucciantini et al 2017).…”