Marin, FredericChurazov, EugeneKhabibullin, IldarFerrazzoli, RiccardoNegro, Michelaet al2023-01-042023-01-042021http://hdl.handle.net/11603/26534Authors: Fr´ed´eric Marin, Eugene Churazov, Ildar Khabibullin, Riccardo Ferrazzoli, Laura Di Gesu, Thibault Barnouin, Alessandro Di Marco, Riccardo Middei, Alexey Vikhlinin, Enrico Costa, Paolo Soffitta, Fabio Muleri, Rashid Sunyaev, William Forman, Ralph Kraft, Stefano Bianchi, Immacolata Donnarumma, Pierre-Olivier Petrucci, Teruaki Enoto, Iv´an Agudo, Lucio A. Antonelli, Matteo Bachetti, Luca Baldini, Wayne H. Baumgartner, Ronaldo Bellazzini, Stephen D. Bongiorno, Raffaella Bonino, Alessandro Brez, Niccol`o Bucciantini, Fiamma Capitanio, Simone Castellano, Elisabetta Cavazzuti, Chien-Ting Chen, Stefano Ciprini, Alessandra De Rosa, Ettore Del Monte, Niccol`o Di Lalla, Victor Doroshenko, Michal Dovˇciak, Steven R. Ehlert, Yuri Evangelista, Sergio Fabiani, Javier A. Garcia, Shuichi Gunji, Kiyoshi Hayashida, Jeremy Heyl, Adam Ingram, Wataru Iwakiri, Svetlana G. Jorstad, Philip Kaaret, Vladimir Karas, Takao Kitaguchi, Jeffery J. Kolodziejczak, Henric Krawczynski, Fabio La Monaca, Luca Latronico, Ioannis Liodakis, Simone Maldera, Alberto Manfreda, Andrea Marinucci, Alan P. Marscher, Herman L. Marshall, Francesco Massaro, Giorgio Matt, Ikuyuki Mitsuishi, Tsunefumi Mizuno, Michela Negro, C.-Y. Ng, Stephen L. O’Dell, Nicola Omodei, Chiara Oppedisano, Alessandro Papitto, George G. Pavlov, Abel L. Peirson, Matteo Perri, Melissa Pesce-Rollins, Maura Pilia, Andrea Possenti, Juri Poutanen, Simonetta Puccetti, Brian D. Ramsey, John Rankin, Ajay Springer Nature LATEX template X-ray polarimetry of the Galactic center Ratheesh, Oliver J. Roberts, Roger W. Romani, Carmelo Sgr`o, Patrick Slane, Gloria Spandre, Doug Swartz, Toru Tamagawa, Fabrizio Tavecchio, Roberto Taverna, Yuzuru Tawara, Allyn F. Tennant, Nicholas E. Thomas, Francesco Tombesi, Alessio Trois, Sergey S. Tsygankov, Roberto Turolla, Jacco Vink, Martin C. Weisskopf, Kinwah Wu, Fei Xie, and Silvia ZaneThe 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. 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, 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. 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.28 pagesen-USThis work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.Public Domain Mark 1.0Text