X-ray polarization: A view deep inside cosmic ray driven turbulence and particle acceleration in supernova remnants

TL;DR

This paper models highly polarized X-ray synchrotron radiation from young supernova remnants within the framework of diffusive shock acceleration and nonlinear magnetic turbulence, linking polarization measurements to turbulence properties and cosmic ray acceleration.

Contribution

It introduces a model connecting X-ray polarization with magnetic turbulence and cosmic ray acceleration in supernova remnants, providing new insights into turbulence amplification and particle acceleration processes.

Findings

X-ray polarization constrains turbulence amplitude and cosmic ray energies.

Polarization degree and direction depend on shock velocity and ambient density.

Model explains observed polarization in Tycho's SNR and predicts turbulence effects.

Abstract

We show here that highly polarized X-ray synchrotron radiation from young supernova remnants (SNRs) can be modeled within the framework of diffusive shock acceleration (DSA) and nonlinear magnetic turbulence generation.Cosmic ray acceleration by SNR shocks to very high energies requires efficient magnetic turbulence amplification in the shock precursor.As the strong turbulence generated by Bell's instability far upstream from the viscous subshock convects through the subshock, nonlinear dynamical effects on the compressible fluctuations produce a downstream layer filled with strong anisotropic turbulence with predominantly radial magnetic fields.The synchrotron radiation from shock accelerated electrons in the turbulent downstream layer has a high degree of polarization shown to be consistent with recent observations of young SNRs by IXPE.In the case of Tycho's SNR, the measured X-ray radiation constrains the thickness of the energy containing interval and the amplitude of cosmic ray driven magnetic turbulence, as well as the maximal energy of accelerated protons.The preferential direction of the X-ray polarization depends sensitively on the SNR shock velocity and the ambient density.A unique feature of our model is the sensitive dependence of the degree and direction of X-ray polarization on the spatial overlap between regions of amplified magnetic turbulence and TeV electron populations.While this overlap occurs on scales orders of magnitude below the resolution of IXPE,its polarization measurement allows testing of turbulent plasma processes on unprecedented scales.The mechanism of formation of highly polarized X-ray synchrotron radiation in fast shocks with high level of anisotropic turbulent magnetic field preferentially directed along the shock normal may be applied to other systems like shocks produced by black hole jets.