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Published October 1, 2022 | metadata_only
Journal Article

X-Ray Polarization Detection of Cassiopeia A with IXPE

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
De Rosa, Alessandra
Del Monte, Ettore
Di Gesu, Laura
Donnarumma, Immacolata
Doroshenko, Victor
Dovčiak, Michal
Ehlert, Steven R.
Enoto, Teruaki
Evangelista, Yuri
Fabiani, Sergio
García, 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


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.

Additional Information

The Imaging X-ray Polarimetry Explorer (IXPE) is a joint US and Italian mission. The US contribution is supported by the National Aeronautics and Space Administration (NASA) and led and managed by its Marshall Space Flight Center (MSFC), with industry partner Ball Aerospace (contract NNM15AA18C). The Italian contribution is supported by the Italian Space Agency (Agenzia Spaziale Italiana, ASI) through contract ASI-OHBI-2017-12-I.0, agreements ASI-INAF-2017-12-H0 and ASI-INFN-2017.13-H0, and its Space Science Data Center (SSDC), and by the Istituto Nazionale di Astrofisica (INAF) and the Istituto Nazionale di Fisica Nucleare (INFN) in Italy. This research used data products provided by the IXPE Team (MSFC, SSDC, INAF, and INFN) and distributed with additional software tools by the High-Energy Astrophysics Science Archive Research Center (HEASARC), at NASA Goddard Space Flight Center (GSFC). J.V. and D.P. are supported by funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 101004131 (SHARP). The research at Boston University was supported in part by National Science Foundation grant AST-2108622. We thank Dawoon Kim for kindly providing us with his script for making polar plots of polarization degree and angle. Software: ixpeobsim (Baldini et al. 2022).

Additional details

August 22, 2023
August 22, 2023