Polarization of reflected X-ray emission from the Sgr A molecular complex: Multiple flares, multiple sources?

The extended X-ray emission observed in the direction of several molecular clouds in the central molecular zone (CMZ) of our Galaxy exhibits spectral and temporal properties consistent with the X-ray echo scenario. This concept postulates that the observed signal is a light-travel-time delayed reflection of a short ( δt < 1.5 yr) and bright ( L X > 10 39 erg s −1 ) flare that was most probably produced a few hundred years ago by Sgr A*. This scenario also predicts a distinct polarization signature for the reflected X-ray continuum, with the polarization vector being perpendicular to the direction toward the primary source and the polarization degree being determined by the scattering angle. We report the results of two deep observations of the currently brightest (in reflected emission) molecular complex Sgr A taken with the Imaging X-ray Polarimetry Explorer (IXPE) in 2022 and 2023. We confirm the previous polarization measurement for a large region encompassing the Sgr A complex with high significance. We also reveal an inconsistent polarization pattern for the brightest reflection region in its center. Specifically, the X-ray polarization from this region is almost perpendicular to the expected direction in the case of Sgr A* illumination, and it shows a smaller degree of polarization compared to the large region. Taken at face value, this could indicate the simultaneous propagation of several illumination fronts throughout the CMZ, with the origin of one of them not being Sgr A*. The primary source could be associated with the Arches stellar cluster or a currently unknown source located closer to the illuminated cloud, potentially lowering the required luminosity of the primary source. Although significantly deeper observations with IXPE would be required to unequivocally distinguish between the scenarios, a combination of high-resolution imaging and micro-calorimetric spectroscopy offers an additional promising path forward.

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Journal
Astronomy and Astrophysics
Published
2026-01-28
DOI
https://doi.org/10.1051/0004-6361/202556768
Primary Topic
Astrophysical Phenomena and Observations
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article
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article

Polarization of reflected X-ray emission from the Sgr A molecular complex: Multiple flares, multiple sources?

Ildar Khabibullin, Chi-yung Ng, Laura Di Gesu, Ioannis Liodakis et al.
Astronomy and Astrophysics
Astrophysical Phenomena and Observations
article

Polarization of reflected X-ray emission from the Sgr A molecular complex: Multiple flares, multiple sources?

Ildar Khabibullin, Chi-yung Ng, Laura Di Gesu, Ioannis Liodakis, C. Chen, F. La Monaca, Abel L. Peirson, Vladimir Karas, Roger M. Smith, Fei Xie, B. Ramsey, Jacco Vink, Fabian Kislat, Silvia Zane, Roberto Taverna, Yuzuru Tawara, T. Kitaguchi, Jeremy Heyl, Jirí Svoboda, Ivan Agudo, Patrick Slane, Ettore Del Monte, Raffaella Bonino, Roberto Turolla, Wataru Iwakiri, Henric Krawczynski, Douglas A. Swartz, Niccolò Di Lalla, Tsunefumi Mizuno, Ikuyuki Mitsuishi, Maura Pilia, Eugene Churazov, John Rankin, Nicholas E. Thomas, Jeffery J. Kolodziejczak, Alessandro Papitto, Niccolo Bucciantini, Javier A. Garcia, Simone Castellano, Pierre-Olivier Petrucci, W. Peter Maksym, Stephen D. Bongiorno, Michal Dovciak, Ajay Ratheesh, Alessio Trois, Wayne H. Baumgartner, Ralph Kraft, Luca Latronico, Teruaki Enoto, Fabio Muleri, Allyn F. Tennant, Ronaldo Bellazzini, Fiamma Capitanio, Chiara Oppedisano, Elisabetta Cavazzuti, Frédéric Marin, Roger W. Romani, Svetlana G. Jorstad, Riccardo Ferrazzoli, Alexey Vikhlinin, George G. Pavlov, William Forman, Shuichi Gunji, Martin C. Weisskopf, Kinwah Wu, Toru Tamagawa, Rashid Sunyaev, Alessandra De Rosa, Francesco Tombesi, Thibault Barnouin, Luca Baldini, Sergey S. Tsygankov, Nicola Omodei, Alessandro Di Marco, Gloria Spandre, Dawoon E. Kim, Francesco Massaro, Sergio Fabiani, Kiyoshi Hayashida, Immacolata Donnarumma, Philip Kaaret, Stefano Ciprini, Alessandro Brez, Steven R. Ehlert, Enrico Costa, Giorgio Matt, Paolo Soffitta, Alan P. Marscher, Stefano Bianchi, Simonetta Puccetti, Melissa Pesce-Rollins, Fabrizio Tavecchio, Herman L. Marshall, Stephen L. O'Dell, Simone Maldera
article en

Abstract

The extended X-ray emission observed in the direction of several molecular clouds in the central molecular zone (CMZ) of our Galaxy exhibits spectral and temporal properties consistent with the X-ray echo scenario. This concept postulates that the observed signal is a light-travel-time delayed reflection of a short ( δt < 1.5 yr) and bright ( L X > 10 39 erg s −1 ) flare that was most probably produced a few hundred years ago by Sgr A*. This scenario also predicts a distinct polarization signature for the reflected X-ray continuum, with the polarization vector being perpendicular to the direction toward the primary source and the polarization degree being determined by the scattering angle. We report the results of two deep observations of the currently brightest (in reflected emission) molecular complex Sgr A taken with the Imaging X-ray Polarimetry Explorer (IXPE) in 2022 and 2023. We confirm the previous polarization measurement for a large region encompassing the Sgr A complex with high significance. We also reveal an inconsistent polarization pattern for the brightest reflection region in its center. Specifically, the X-ray polarization from this region is almost perpendicular to the expected direction in the case of Sgr A* illumination, and it shows a smaller degree of polarization compared to the large region. Taken at face value, this could indicate the simultaneous propagation of several illumination fronts throughout the CMZ, with the origin of one of them not being Sgr A*. The primary source could be associated with the Arches stellar cluster or a currently unknown source located closer to the illuminated cloud, potentially lowering the required luminosity of the primary source. Although significantly deeper observations with IXPE would be required to unequivocally distinguish between the scenarios, a combination of high-resolution imaging and micro-calorimetric spectroscopy offers an additional promising path forward.

Astronomy and AstrophysicsVol. 707
University of Pisa (IT), Boston University (US), Hiroshima University (JP), University of Rome Tor Vergata (IT), Roma Tre University (IT), Physical Research Laboratory (IN), Louisiana State University (US), United States Naval Research Laboratory (US), Marshall Space Flight Center (US), Centre National de la Recherche Scientifique (FR), Goddard Space Flight Center (US), Pennsylvania State University (US), Universities Space Research Association (US), University of Padua (IT), Pioneer (United States) (US), University of British Columbia (CA), Guangxi University (CN), University of Turku (FI), Istituto Nazionale di Fisica Nucleare (IT), St Petersburg University (RU), Washington University in St. Louis (US), Arcetri Astrophysical Observatory (IT), Astronomical Observatory of Rome (IT), Czech Academy of Sciences, Astronomical Institute (CZ), Agenzia Spaziale Italiana (IT), Space Research Institute (RU), Observatoire astronomique de Strasbourg (FR), Brera Astronomical Observatory (IT), Kavli Institute for Particle Astrophysics and Cosmology (US), Istituto Nazionale di Fisica Nucleare, Sezione di Torino (IT), Institute for Space Astrophysics and Planetology (IT), Max Planck Institute for Astrophysics (DE), Istituto Nazionale di Fisica Nucleare, Sezione di Napoli (IT), Istituto Nazionale di Fisica Nucleare, Roma Tor Vergata (IT), Center for Astrophysics Harvard & Smithsonian (US), Istituto Nazionale di Fisica Nucleare, Sezione di Firenze (IT), Astronomical Institute of the Slovak Academy of Sciences (SK), Astronomy and Space (AU), Osservatorio Astronomico di Cagliari (IT), Instituto de Astrofísica de Andalucía (ES), Istituto Nazionale di Fisica Nucleare, Sezione di Pisa (IT), Yamagata University Hospital (JP), University of Florence (IT), University College London (GB), University of Turin (IT), Nagoya University (JP), Massachusetts Institute of Technology (US), University of Tübingen (DE), Ludwig-Maximilians-Universität München (DE), National Institute for Astrophysics (IT), University of Hong Kong (HK), Foundation for Research and Technology Hellas (GR), Université Grenoble Alpes (FR), Netherlands Institute for Radio Astronomy (NL), The University of Osaka (JP)
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