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

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 large amplitude, 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 the Imaging X-ray Polarimetry Explorer (IXPE) taking into account high-energy electron losses and line-of-sight integration in a spherical remnant. In the case of our model 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 fast shock in a region with high enough density is a favorable place to produce tangential polarization of synchrotron radiation, i.e., a dominantly radial magnetic field. 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.