Limits on X-Ray Polarization at the Core of Centaurus A as Observed with the Imaging X-Ray Polarimetry Explorer
Steven R. Ehlert, Riccardo Ferrazzoli, Andrea Marinucci, Herman L. Marshall, R. Middei, L. Pacciani, M. Perri, Pierre-Olivier Petrucci, Simonetta Puccetti, Thibault Barnouin, S. Bianchi, Ioannis Liodakis, G. Madejski, Frédéric Marin, Alan P. Marscher, G. Matt, Juri Poutanen, Kinwah Wu, I. Agudo, L. A. Antonelli, Matteo Bachetti, L. Baldini, W. H. Baumgartner, R. Bellazzini, Stephen D. Bongiorno, R. Bonino, A. Brez, N. Bucciantini, Fiamma Capitanio, Simone Castellano, E. Cavazzuti, S. Ciprini, E. Costa, Alessandra De Rosa, E. Del Monte, Laura Di Gesu, Niccolò Di Lalla, Alessandro Di Marco, I. Donnarumma, Victor Doroshenko, Michal Dovčiak, Teruaki Enoto, Y. Evangelista, Sergio Fabiani, Javier A. García, Shuichi Gunji, Kiyoshi Hayashida, Jeremy Heyl, W. Iwakiri, Svetlana G. Jorstad, V. Karas, Takao Kitaguchi, Jeffery J. Kolodziejczak, H. Krawczynski, Fabio La Monaca, L. Latronico, S. Maldera, Alberto Manfreda, F. Massaro, Ikuyuki Mitsuishi, Tsunefumi Mizuno, Fabio Muleri, Michela Negro, C.‐Y. Ng, Stephen L. O’Dell, N. Omodei, C. Oppedisano, Alessandro Papitto, George G. Pavlov, Abel L. Peirson, M. Pesce-Rollins, M. Pilia, Andrea Possenti, Brian D. Ramsey, John Rankin, Ajay Ratheesh, Roger W. Romani, C. Sgró, Patrick Slane, P. Soffitta, G. Spandre, Toru Tamagawa, F. Tavecchio, Roberto Taverna, Yuzuru Tawara, Allyn F. Tennant, Nicholas E. Thomas, Francesco Tombesi, A. Trois, Sergey S. Tsygankov, Roberto Turolla, Jacco Vink, Martin C. Weisskopf, Fei Xie, Silvia Zane, J. Rodi, E. Jourdain, J. P. Roques
Abstract
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 (MDP 99 ) 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 ∼ 10 23 cm −2 ) and a Fe K α 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 K α emission line support a picture where electrons are accelerated in a region of highly disordered magnetic fields surrounding the innermost jet.