X-ray polarization measurement of the gold standard of radio-quiet active galactic nuclei: NGC 1068
Frédéric Marin, Andrea Marinucci, M. Laurenti, Dawoon E. Kim, Thibault Barnouin, Alessandro Di Marco, F. Ursini, S. Bianchi, Swati Ravi, Herman L. Marshall, G. Matt, Chien‐Ting Chen, V. E. Gianolli, Adam Ingram, R. Middei, W. Peter Maksym, C. Panagiotou, Jakub Podgorný, S. Puccetti, A. Ratheesh, Francesco Tombesi, 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, S. Castellano, E. Cavazzuti, S. Ciprini, E. Costa, Alessandra De Rosa, E. Del Monte, L. Di Gesu, N. Di Lalla, I. Donnarumma, V. Doroshenko, M. Dovčiak, Steven R. Ehlert, Teruaki Enoto, Y. Evangelista, Sergio Fabiani, Riccardo Ferrazzoli, Javier A. García, Shuichi Gunji, Jeremy Heyl, W. Iwakiri, Svetlana G. Jorstad, P. Kaaret, V. Karas, Fabian Kislat, T. Kitaguchi, Jeffery J. Kolodziejczak, H. Krawczynski, Fabio La Monaca, L. Latronico, Ioannis Liodakis, G. Madejski, S. Maldera, Alberto Manfreda, Alan P. Marscher, F. Massaro, I. Mitsuishi, T. Mizuno, Fabio Muleri, Michela Negro, S. Ng, Stephen L. O’Dell, N. Omodei, C. Oppedisano, A. Papitto, George G. Pavlov, M. Perri, M. Pesce-Rollins, Pierre-Olivier Petrucci, M. Pilia, Andrea Possenti, Juri Poutanen, Brian D. Ramsey, J. Rankin, O. J. Roberts, Roger W. Romani, C. Sgró, Patrick Slane, P. Soffitta, G. Spandre, Douglas A. Swartz, T. Tamagawa, F. Tavecchio, Roberto Taverna, Y. Tawara, Allyn F. Tennant, Nicholas E. Thomas, A. Trois, Sergey S. Tsygankov
Abstract
Context. NGC 1068 is the most observed radio-quiet active galactic nucleus (AGN) in polarimetry, yet its high-energy polarization has never been probed before due to a lack of dedicated polarimeters. Aims. Using the first X-ray polarimeter sensitive enough to measure the polarization of AGNs, we want to probe the orientation and geometric arrangement of (sub)parsec-scale matter around the X-ray source. Methods. We used the Imaging X-ray Polarimetry Explorer (IXPE) satellite to measure, for the first time, the 2–8 keV polarization of NGC 1068. We pointed IXPE at the target for a net exposure time of 1.15 Ms, in addition to using two Chandra snapshots of ∼10 ks each in order to account for the potential impact of several ultraluminous X-ray sources (ULXs) within IXPE’s field of view. Results. We measured a 2–8 keV polarization degree of 12.4% ± 3.6% and an electric vector polarization angle of 101° ± 8° at a 68% confidence level. If we exclude the spectral region containing bright Fe K lines and other soft X-ray lines where depolarization occurs, the polarization fraction rises to 21.3% ± 6.7% in the 3.5–6.0 keV band, with a similar polarization angle. The observed polarization angle is found to be perpendicular to the parsec-scale radio jet. Using a combined Chandra and IXPE analysis plus multiwavelength constraints, we estimated that the circumnuclear “torus” may sustain a half-opening angle of 50–55° (from the vertical axis of the system). Conclusions. Thanks to IXPE, we have measured the X-ray polarization of NGC 1068 and found comparable results, both in terms of the polarization angle orientation with respect to the radio jet and the torus half-opening angle, to the X-ray polarimetric measurement achieved for the other archetypal Compton-thick AGN: the Circinus galaxy. Probing the geometric arrangement of parsec-scale matter in extragalactic objects is now feasible thanks to X-ray polarimetry.