X-Ray Polarization Detection of Cassiopeia A with IXPE
Authors: Vink Jacco , Prokhorov Dmitry , Ferrazzoli Riccardo , Slane Patrick , Zhou Ping , Asakura Kazunori , Baldini Luca , Bucciantini Niccoló , Costa Enrico , Di Marco Alessandro , Heyl Jeremy , Marin Frédéric , Mizuno Tsunefumi , Ng C.-Y. , Pesce-Rollins Melissa , Ramsey Brian D. , Rankin John , Ratheesh Ajay , Sgró Carmelo , Soffitta Paolo , Swartz Douglas A. , Tamagawa Toru , Weisskopf Martin C. , Yang Yi-Jung , Bellazzini Ronaldo , Bonino Raffaella , Cavazzuti Elisabetta , Costamante Luigi , Di Lalla Niccoló , Latronico Luca , Maldera Simone , Manfreda Alberto , Massaro Francesco , Mitsuishi Ikuyuki , Omodei Nicola , Oppedisano Chiara , Zane Silvia , Agudo Ivan , Antonelli Lucio A. , Bachetti Matteo , Baumgartner Wayne H. , Bianchi Stefano , Bongiorno Stephen D. , Brez Alessandro , Capitanio Fiamma , Castellano Simone , Ciprini Stefano , Rosa Alessandra De , Del Monte Ettore , Gesu Laura Di , Donnarumma Immacolata , Doroshenko Victor , Dovčiak Michal , Ehlert Steven R. , Enoto Teruaki , Evangelista Yuri , Fabiani Sergio , Garcia Javier A. , Gunji Shuichi , Hayashida Kiyoshi , Iwakiri Wataru , Jorstad Svetlana G. , Karas Vladimir , Kitaguchi Takao , Kolodziejczak Jeffery J. , Krawczynski Henric , La Monaca Fabio , Liodakis Ioannis , Marinucci Andrea , Marscher Alan P. , Marshall Herman L. , Matt Giorgio , Muleri Fabio , O’Dell Stephen L. , Papitto Alessandro , Pavlov George G. , Peirson Abel L. , Perri Matteo , Pilia Maura , Possenti Andrea , Poutanen Juri , Puccetti Simonetta , Romani Roger W. , Spandre Gloria , Tavecchio Fabrizio , Taverna Roberto , Tawara Yuzuru , Tennant Allyn F. , Thomas Nicolas E. , Tombesi Francesco , Trois Alessio , Tsygankov Sergey , Turolla Roberto , Wu Kinwah , Xie Fei 
Published: October 2022

Abstract
We report on a ∼5σ detection of polarized 3–6 keV X-ray emission from the supernova remnant Cassiopeia A (Cas A) with the Imaging X-ray Polarimetry Explorer (IXPE). The overall polarization degree of 1.8% ± 0.3% is detected by summing over a large region, assuming circular symmetry for the polarization vectors. The measurements imply an average polarization degree for the synchrotron component of ∼2.5%, and close to 5% for the X-ray synchrotron-dominated forward shock region. These numbers are based on an assessment of the thermal and nonthermal radiation contributions, for which we used a detailed spatial-spectral model based on Chandra X-ray data. A pixel-by-pixel search for polarization provides a few tentative detections from discrete regions at the ∼ 3σ confidence level. Given the number of pixels, the significance is insufficient to claim a detection for individual pixels, but implies considerable turbulence on scales smaller than the angular resolution. Cas A’s X-ray continuum emission is dominated by synchrotron radiation from regions within ≲1017 cm of the forward and reverse shocks. We find that (i) the measured polarization angle corresponds to a radially oriented magnetic field, similar to what has been inferred from radio observations; (ii) the X-ray polarization degree is lower than in the radio band (∼5%). Since shock compression should impose a tangential magnetic-field structure, the IXPE results imply that magnetic fields are reoriented within ∼1017 cm of the shock. If the magnetic-field alignment is due to locally enhanced acceleration near quasi-parallel shocks, the preferred X-ray polarization angle suggests a size of 3 × 1016 cm for cells with radial magnetic fields.