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

description: We present measurements of the polarization of X-rays in the $2-8 \thinspace \mathrm{keV}$ band from the nucleus of the radio galaxy Centaurus A (Cen A), using a 100ks 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 $99 \%$ percent confidence (MDP$_{99}$) of $6.5 \%$ using a weighted, spectral model-independent calculation in the $2-8 \thinspace \mathrm{keV}$ band. The polarization angle $ψ$ is consequently unconstrained. Spectral fitting across three orders of magnitude in X-ray energy ($0.3-400 \thinspace \mathrm{keV}$) demonstrates that the SED of Cen A is well described by a simple power law with moderate intrinsic absorption ($N_H \sim 10^{23} \thinspace \mathrm{cm}^{-2}$) and a Fe K$α$ emission line, although a second unabsorbed power law is required to account for the observed spectrum at energies below $2 \thinspace \mathrm{keV}$. 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 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.
description: Accepted for Publication in ApJ, 15 pages