Limits on X-Ray Polarization at the Core of Centaurus A as Observed with the Imaging X-Ray Polarimetry Explorer
Authors: Ehlert Steven R. , Ferrazzoli Riccardo , Marinucci Andrea , Marshall Herman L. , Middei Riccardo , Pacciani Luigi , Perri Matteo , Petrucci Pierre-Olivier , Puccetti Simonetta , Barnouin Thibault , Bianchi Stefano , Liodakis Ioannis , Madejski Grzegorz , Marin Frédéric , Marscher Alan P. , Matt Giorgio , Poutanen Juri , Wu Kinwah , Agudo Iván , Antonelli Lucio A. , Bachetti Matteo , Baldini Luca , Baumgartner Wayne H. , Bellazzini Ronaldo , Bongiorno Stephen D. , Bonino Raffaella , Brez Alessandro , Bucciantini Niccoló , Capitanio Fiamma , Castellano Simone , Cavazzuti Elisabetta , Ciprini Stefano , Costa Enrico , De Rosa Alessandra , Del Monte Ettore , Di Gesu Laura , Di Lalla Niccoló , Di Marco Alessandro , Donnarumma Immacolata , Doroshenko Victor , Dovčiak Michal , Enoto Teruaki , Evangelista Yuri , Fabiani Sergio , Garcia Javier A. , Gunji Shuichi , Hayashida Kiyoshi , Heyl Jeremy , Iwakiri Wataru , Jorstad Svetlana G. , Karas Vladimir , Kitaguchi Takao , Kolodziejczak Jeffery J. , Krawczynski Henric , La Monaca Fabio , Latronico Luca , Maldera Simone , Manfreda Alberto , Massaro Francesco , Mitsuishi Ikuyuki , Mizuno Tsunefumi , Muleri Fabio , Negro Michela , Ng C.-Y. , O’Dell Stephen L. , Omodei Nicola , Oppedisano Chiara , Papitto Alessandro , Pavlov George G. , Peirson Abel L. , Pesce-Rollins Melissa , Pilia Maura , Possenti Andrea , Ramsey Brian D. , Rankin John , Ratheesh Ajay , Romani Roger W. , Sgrò Carmelo , Slane Patrick , Soffitta Paolo , Spandre Gloria , Tamagawa Toru , Tavecchio Fabrizio , Taverna Roberto , Tawara Yuzuru , Tennant Allyn F. , Thomas Nicholas E. , Tombesi Francesco , Trois Alessio , Tsygankov Sergey , Turolla Roberto , Vink Jacco , Weisskopf Martin C. , Xie Fei , Zane Silvia ,   (IXPE Collaboration), Rodi James , Jourdain Elisabeth , Roques Jean-Pierre 
Published: August 2022

Abstract
We present measurements of the polarization of X-rays in the 2–8 keV band from the nucleus of the radio galaxy Centaurus A (Cen A), using a 100 ks observation from the Imaging X-ray Polarimetry Explorer (IXPE). Nearly simultaneous observations of Cen A were also taken with the Swift, NuSTAR, and INTEGRAL observatories. No statistically significant degree of polarization is detected with IXPE. These observations have a minimum detectable polarization at 99% confidence (MDP99) of 6.5% using a weighted, spectral model-independent calculation in the 2–8 keV band. The polarization angle ψ is consequently unconstrained. Spectral fitting across three orders of magnitude in X-ray energy (0.3–400 keV) demonstrates that the SED of Cen A is well described by a simple power law with moderate intrinsic absorption (N
H ∼ 1023 cm−2) and a Fe Kα emission line, although a second unabsorbed power law is required to account for the observed spectrum at energies below 2 keV. This spectrum suggests that the reprocessing material responsible for this emission line is optically thin and distant from the central black hole. Our upper limits on the X-ray polarization are consistent with the predictions of Compton scattering, although the specific seed photon population responsible for the production of the X-rays cannot be identified. The low polarization degree, variability in the core emission, and the relative lack of variability in the Fe Kα emission line support a picture where electrons are accelerated in a region of highly disordered magnetic fields surrounding the innermost jet.