X-ray polarization measurement of the gold standard of radio-quiet active galactic nuclei: NGC 1068
Authors: Marin F. , Marinucci A. , Laurenti M. , Kim D. E. , Barnouin T. , Di Marco A. , Ursini F. , Bianchi S. , Ravi S. , Marshall H. L. , Matt G. , Chen C.-T. , Gianolli V. E. , Ingram A. , Middei R. , Maksym W. P. , Panagiotou C. , Podgorny J. , Puccetti S. , Ratheesh A. , Tombesi F. , Agudo I. , Antonelli L. A. , Bachetti M. , Baldini L. , Baumgartner W. , Bellazzini R. , Bongiorno S. , Bonino R. , Brez A. , Bucciantini N. , Capitanio F. , Castellano S. , Cavazzuti E. , Ciprini S. , Costa E. , De Rosa A. , Del Monte E. , Di Gesu L. , Di Lalla N. , Donnarumma I. , Doroshenko V. , Dovčiak M. , Ehlert S. , Enoto T. , Evangelista Y. , Fabiani S. , Ferrazzoli R. , Garcia J. , Gunji S. , Heyl J. , Iwakiri W. , Jorstad S. , Kaaret P. , Karas V. , Kislat F. , Kitaguchi T. , Kolodziejczak J. , Krawczynski H. , La Monaca F. , Latronico L. , Liodakis I. , Madejski G. , Maldera S. , Manfreda A. , Marscher A. , Massaro F. , Mitsuishi I. , Mizuno T. , Muleri F. , Negro M. , Ng S. , O’Dell S. , Omodei N. , Oppedisano C. , Papitto A. , Pavlov G. , Perri M. , Pesce-Rollins M. , Petrucci P.-O. , Pilia M. , Possenti A. , Poutanen J. , Ramsey B. , Rankin J. , Roberts O. , Romani R. , Sgrò C. , Slane P. , Soffitta P. , Spandre G. , Swartz D. , Tamagawa T. , Tavecchio F. , Taverna R. , Tawara Y. , Tennant A. , Thomas N. , Trois A. , Tsygankov S. , Turolla R. , Vink J. , Weisskopf M. , Wu K. , Xie F. , Zane S. 
Published: May 2024

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.