Mapping the circumnuclear regions of the Circinus galaxy with the imaging X-ray polarimetry explorer

Mapping the circumnuclear regions of the Circinus galaxy with the imaging X-ray polarimetry explorer Ursini, F.; Marinucci, A.; Matt, G.; Bianchi, S.; Marin, F.; Marshall, H.L.; Middei, R.; Poutanen, J.; Rogantini, D.; Rosa, A. De; Gesu, L. Di; García, J.A.; Ingram, A.; Kim, D.E.; Krawczynski, H.; Puccetti, S.; Soffitta, P.; Svoboda, J.; Tombesi, F.; Weisskopf, M.C.; Barnouin, T.; Perri, M.; Podgorny, J.; Ratheesh, A.; Zaino, A.; Agudo, I.; Antonelli, L.A.; Bachetti, M.; Baldini, L.; Baumgartner, W.H.; Bellazzini, R.; Bongiorno, S.D.; Bonino, R.; Brez, A.; Bucciantini, N.; Capitanio, F.; Castellano, S.; Cavazzuti, E.; Ciprini, S.; Costa, E.; Monte, E. Del; Lalla, N. Di; Marco, A. Di; Donnarumma, I.; Doroshenko, V.; Dovčiak, M.; Ehlert, S.R.; Enoto, T.; Evangelista, Y.; Fabiani, S.; Ferrazzoli, R.; Gunji, S.; Heyl, J.; Iwakiri, W.; Jorstad, S.G.; Karas, V.; Kitaguchi, T.; Kolodziejczak, J.J.; Monaca, F. La; Latronico, L.; Liodakis, I.; Maldera, S.; Manfreda, A.; Marscher, Alan P.; Mitsuishi, I.; Mizuno, T.; Muleri, F.; Ng, C.Y.; O'Dell, S.L.; Omodei, N.; Oppedisano, C.; Papitto, A.; Pavlov, G.G.; Peirson, A.L.; Pesce-Rollins, M.; Petrucci, P.-O.; Pilia, M.; Possenti, A.; Ramsey, B.D.; Rankin, J.; Romani, R.W.; Sgrò, C.; Slane, P.; Spandre, G.; Tamagawa, T.; Tavecchio, F.; Taverna, R.; Tawara, Y.; Tennant, A.F.; Thomas, N.E.; Trois, A.; Tsygankov, S.S.; Turolla, R.; Vink, J.; Wu, K.; Xie, F.; Zane, S. We report on the Imaging X-ray Polarimetry Explorer (IXPE) observation of the closest and X-ray brightest Compton-thick active galactic nucleus (AGN), the Circinus galaxy. We find the source to be significantly polarized in the 2--6 keV band. From previous studies, the X-ray spectrum is known to be dominated by reflection components, both neutral (torus) and ionized (ionization cones). Our analysis indicates that the polarization degree is 28 ± 7 per cent (at 68 per cent confidence level) for the neutral reflector, with a polarization angle of 18° ± 5°, roughly perpendicular to the radio jet. The polarization of the ionized reflection is unconstrained. A comparison with Monte Carlo simulations of the polarization expected from the torus shows that the neutral reflector is consistent with being an equatorial torus with a half-opening angle of 45°-55°. This is the first X-ray polarization detection in a Seyfert galaxy, demonstrating the power of X-ray polarimetry in probing the geometry of the circumnuclear regions of AGNs, and confirming the basic predictions of standard Unification Models.