When the Milky Way hosted a quasar
Authors: Marin Frederic , Churazov Eugene , Khabibullin Ildar , Ferrazzoli Riccardo , Gesu Laura Di , Barnouin Thibault , Marco Alessandro Di , Middei Riccardo , Vikhlinin Alexey , Costa Enrico , Soffitta Paolo , Muleri Fabio , Sunyaev Rashid , Forman William , Kraft Ralph , Bianchi Stefano , Donnarumma Immacolata , Petrucci Pierre-Olivier , Enoto Teruaki , Agudo Iván , Antonelli Lucio , Bachetti Matteo , Baldini Luca , Baumgartner Wayne , Bellazzini Ronaldo , Bongiorno Stephen , Bonino Raffaella , Brez Alessandro , Bucciantini Niccolo , Capitanio Fiamma , Castellano Simone , Cavazzuti Elisabetta , Chen Chien-Ting , Ciprini Stefano , De Rosa Alessandra , Monte Ettore Del , Lalla Niccolo Di , Doroshenko Victor , Dovciak Michal , Ehlert Steven , Evangelista Yuri , Fabiani Sergio , Garcia Javier , Gunji Shuichi , Hayashida Kiyoshi , Heyl Jeremy , Ingram Adam , Iwakiri Wataru , Jorstad Svetlana , Kaaret Philip , Karas Vladimir , Kitaguchi Takao , Kolodziejczak Jeffery , Krawczynski Henric , Monaca Fabio La , Latronico Luca , Liodakis Ioannis , Maldera Simone , Manfreda Alberto , Marinucci Andrea , Marscher Alan , Marshall Herman , Massaro Francesco , Matt Giorgio , Mitsuishi Ikuyuki , Mizuno Tsunefumi , Negro Michela , Ng C.-Y. , O'Dell Stephen , Omodei Nicola , Oppedisano Chiara , Papitto Alessandro , Pavlov George , Peirson Abel , Perri Matteo , Pesce-Rollins Melissa , Pilia Maura , Possenti Andrea , Poutanen Juri , Puccetti Simonetta , Ramsey Brian , Rankin John , Ratheesh Ajay , Roberts Oliver , Romani Roger , Sgro Carmelo , Slane Patrick , Spandre Gloria , Swartz Doug , Tamagawa Toru , Tavecchio Fabrizio , Taverna Roberto , Tawara Yuzuru , Tennant Allyn , Thomas Nicholas , Tombesi Francesco , Trois Alessio , Tsygankov Sergey , Turolla Roberto , Vink Jacco , Weisskopf Martin , Wu Kinwah , Xie Fei , Zane Silvia 
Published: November 2022

Abstract
The Galactic Center harbors diffuse X-ray emission co-spatial with giant molecular clouds and featuring hard X-ray spectra with a prominent fluorescent iron line at 6.4 keV [1–3]. These spectral properties are characteristic of the reflected emission from a neutral gas illuminated by X-rays. However, there are no persistent X-ray sources in that region that are bright enough to provide the required illumination level. Moreover, the observed diffuse emission is variable on time scales of years [4, 5], implying that the primary source of X-rays must be variable, too. The most exciting and far-reaching scenario is that a very powerful flare from the central supermassive black hole of the Milky Way, Sgr A*, provided the required flux of X-ray photons, when for a period shorter than a few years it became at least five orders of magnitude brighter than is typically observed today. Even if the primary emission was unpolarized, the reflected emission should be polarized in the direction perpendicular to the scattering plane with the degree of polarization being set by the scattering angle. Therefore, by measuring X-ray polarization one can infer the direction towards the primary source and, simultaneously, the mutual positions in space of the source and the cloud [6]. Here, we report the detection of X-ray polarization from the Galactic Center region obtained by the recently launched Imaging X-ray Polarimetry Explorer (IXPE) mission. For a large region where Chandra and XMM-Newton identify spectral signatures of the reflection, IXPE measures a polarization degree of 31% ± 11% with a polarization angle -48° ± 11° for the scattered continuum. The latter corroborates the conjecture that Sgr A* is the primary source of illuminating photons, while the former implies that some 200 years ago our Galactic Center hosted an extremely bright active galactic nucleus, on par with the ones found in Seyfert galaxies, though only for a short time.