X-Ray Polarization Detection of Cassiopeia A with IXPE

Jacco Vink , Dmitry Prokhorov , Riccardo Ferrazzoli , Patrick Slane , Ping Zhou , Kazunori Asakura , Luca Baldini , Niccoló Bucciantini , Enrico Costa , Alessandro Di Marco , Jeremy Heyl , Frédéric Marin (1) , Tsunefumi Mizuno , C. -Y. Ng , Melissa Pesce-Rollins , Brian D. Ramsey , John Rankin , Ajay Ratheesh , Carmelo Sgró , Paolo Soffitta , Douglas A. Swartz , Toru Tamagawa , Martin C. Weisskopf , Yi-Jung Yang , Ronaldo Bellazzini , Raffaella Bonino , Elisabetta Cavazzuti , Luigi Costamante , Niccoló Di Lalla , Luca Latronico , Simone Maldera , Alberto Manfreda , Francesco Massaro , Ikuyuki Mitsuishi , Nicola Omodei , Chiara Oppedisano , Silvia Zane , Ivan Agudo , Lucio A. Antonelli , Matteo Bachetti , Wayne H. Baumgartner , Stefano Bianchi , Stephen D. Bongiorno , Alessandro Brez , Fiamma Capitanio , Simone Castellano , Stefano Ciprini , Alessandra De Rosa , Ettore del Monte , Laura Di Gesu , Immacolata Donnarumma , Victor Doroshenko , Michal Dovčiak , Steven R. Ehlert , Teruaki Enoto , Yuri Evangelista , Sergio Fabiani , Javier A. Garcia , Shuichi Gunji , Kiyoshi Hayashida , Wataru Iwakiri , Svetlana G. Jorstad , Vladimir Karas , Takao Kitaguchi , Jeffery J. Kolodziejczak , Henric Krawczynski , Fabio La Monaca , Ioannis Liodakis , Andrea Marinucci , Alan P. Marscher , Herman L. Marshall , Giorgio Matt , Fabio Muleri , Stephen L. O'Dell , Alessandro Papitto , George G. Pavlov , Abel L. Peirson , Matteo Perri , Maura Pilia , Andrea Possenti , Juri Poutanen , Simonetta Puccetti , Roger W. Romani , Gloria Spandre , Fabrizio Tavecchio , Roberto Taverna , Yuzuru Tawara , Allyn F. Tennant , Nicolas E. Thomas , Francesco Tombesi , Alessio Trois , Sergey Tsygankov , Roberto Turolla , Kinwah Wu , Fei Xie

1 ObAS - Observatoire astronomique de Strasbourg

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.

Keywords: Supernova remnants; Polarimetry; X-ray astronomy; Shocks; Astrophysics - High Energy Astrophysical Phenomena

Domains: Sciences of the Universe [physics]

Jacco Vink, Dmitry Prokhorov, Riccardo Ferrazzoli, Patrick Slane, Ping Zhou, et al.. X-Ray Polarization Detection of Cassiopeia A with IXPE. The Astrophysical Journal, 2022, 938, ⟨10.3847/1538-4357/ac8b7b⟩. ⟨insu-03822787⟩