Abstract
Supernova remnants (SNRs) and pulsar wind nebulae (PWNs) are among the most significant sources of non-thermal X-rays in the sky, and the best means by which relativistic plasma dynamics and particle acceleration can be investigated. Being strong synchrotron emitters, they are ideal candidates for X-ray polarimetry, and indeed the Crab nebula is up to present the only object where X-ray polarization has been detected with a high level of significance. Future polarimetric measures will likely provide us with crucial information on the level of turbulence that is expected at particle acceleration sites, together with the spatial and temporal coherence of magnetic field geometry, enabling us to set stronger constraints on our acceleration models. PWNs will also allow us to estimate the level of internal dissipation. I will briefly review the current knowledge on the polarization signatures in SNRs and PWNs, and I will illustrate what we can hope to achieve with future missions such as IXPE/XIPE.
Highlights
Pulsar wind nebulae (PWNs) are bubbles of relativistic particles and magnetic fields that form when the relativistic pulsar wind interacts with the ambient medium (interstellar medium (ISM) or supernova remnant (SNR))
In PWNs, as radio emission is dominated by the outer regions of the nebula, where the effects of the interaction with the SNR are stronger, and where Rayleight–Taylor instability operates, radio polarimetry provides at best an estimate of the degree of ordered versus disordered magnetic field
In the Crab nebula, which constitutes a case study for the entire class, the radio polarized fraction is ~16% on average [18,19,20,21] with peaks up to 30%, which is lower than the average optical polarized fraction, which is ~25% [20]
Summary
Pulsar wind nebulae (PWNs) are bubbles of relativistic particles (mostly pairs) and magnetic fields that form when the relativistic pulsar wind interacts with the ambient medium (interstellar medium (ISM) or supernova remnant (SNR)). In PWNs, as radio emission is dominated by the outer regions of the nebula, where the effects of the interaction with the SNR are stronger, and where Rayleight–Taylor instability operates, radio polarimetry provides at best an estimate of the degree of ordered versus disordered magnetic field This means that it cannot be used to probe the conditions in the region close to the termination shock, where most of the variability and the acceleration processes take place. There appears to be a correlation between the orientation of bipolar SNRs and the galactic plane [37,38], and there is further evidence in SNRs from Type II SN of an expansion into a magnetized wind bubble [39] It was found in SN 1006 that the polarized fraction anti-correlates with radio emissions, suggesting that those sites along the shock front that are more likely to accelerate particles have a more turbulent field [38]. In Cas A, for example, high resolution radio observations show evidence of a polarization angle swing at the location of the X-ray rim, which is where particles are accelerated [40]
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