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

description: We report on a $\sim 5σ$ detection of polarized 3-6 keV X-ray emission from the supernova remnant Cassiopeia A with the Imaging X-ray Polarimetry Explorer (IXPE). The overall polarization degree of $1.8 \pm 0.3$% 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 $\sim 2.5$%, and close to 5% for the X-ray synchrotron-domimated forward-shock region. These numbers are based on an assessment of the thermal and non-thermal 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 $\sim 3σ$ 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 $\lesssim 10^{17}$ 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 ($\sim 5$%). Since shock compression should impose a tangential magnetic field structure, the IXPE results imply that magnetic-fields are reoriented within $\sim 10^{17}$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 $3\times 10^{16}$ cm for cells with radial magnetic fields.
description: IXPE collaboration paper. Accepted by the Astrophysical Journal. This version reflects the version accepted by ApJ. With respect to version 1: new figures added to the Appendix and extension to the text, in particular the discussion section and Appendix