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