X-Ray Polarization Detection of Cassiopeia A with IXPE

We report on a $\sim 5\sigma$ detection of polarized 3-6 keV X-ray emission from the supernova remnant Cassiopeia A with Imaging Polarimetry Explorer (IXPE). The overall polarization degree $1.8 \pm 0.3$% is detected by summing over large region, assuming circular symmetry for vectors. measurements imply an average synchrotron component 2.5$%, and close to 5% synchrotron-domimated forward-shock region. These numbers are based assessment thermal non-thermal radiation contributions, which we used detailed spatial-spectral model Chandra data. pixel-by-pixel search provides few tentative detections discrete regions at 3\sigma$ confidence level. Given number pixels, significance {insufficient} claim individual but implies considerable turbulence scales smaller than angular resolution. Cas A's continuum dominated within $\lesssim 10^{17}$ cm forward- reverse shocks. find that i) measured angle corresponds radially-oriented magnetic field, similar what has been inferred radio observations; ii) lower in band ($\sim 5$%). Since shock compression should impose tangential field structure, IXPE results magnetic-fields reoriented 10^{17}$cm shock. If magnetic-field alignment due locally enhanced acceleration near quasi-parallel shocks, preferred suggests size $3\times 10^{16}$ cells radial fields.