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

¹ Universitäts-Sternwarte, Fakultät fuer Physik, Ludwig-Maximilians-Universität Muenchen, Scheinerstr. 1, 81679 Muenchen, Germany
² Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
³ Space Research Institute (IKI), Profsoyuznaya 84/32, Moscow 117997, Russia
⁴ INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
⁵ NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
⁶ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
⁷ Astronomical Institute of the Czech Academy of Sciences, Boční II 1401/1, 14100 Praha 4, Czech Republic
⁸ Center for Astrophysics, Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
⁹ Science and Technology Institute, Universities Space Research Association, Huntsville, AL 35805, USA
¹⁰ Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
¹¹ Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
¹² Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
¹³ Department of Physics and Astronomy, 20014 University of Turku, Finland
¹⁴ Instituto de Astrofísica de Andalucía – CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹⁵ INAF Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone (RM), Italy
¹⁶ Space Science Data Center, Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
¹⁷ Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
¹⁸ Dipartimento di Fisica, Università di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
¹⁹ Naval Research Laboratory, 4555 Overlook Ave. SW, Washington, DC 20375, USA
²⁰ Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
²¹ Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
²² INAF Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
²³ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via Sansone 1, 50019 Sesto Fiorentino (FI), Italy
²⁴ Istituto Nazionale di Fisica Nucleare, Sezione di Firenze, Via Sansone 1, 50019 Sesto Fiorentino (FI), Italy
²⁵ Department of Physics and Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA 94305, USA
²⁶ Institut fuer Astronomie und Astrophysik, Universität Tuebingen, Sand 1, 72076 Tuebingen, Germany
²⁷ RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
²⁸ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
²⁹ Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata-shi 990-8560, Japan
³⁰ Osaka University, 1-1 Yamadaoka, Suita, Osaka 565-0871, Japan
³¹ University of British Columbia, Vancouver, BC V6T 1Z4, Canada
³² International Center for Hadron Astrophysics, Chiba University, Chiba 263-8522, Japan
³³ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
³⁴ Department of Astrophysics, St. Petersburg State University, Universitetsky pr. 28, Petrodvoretz, 198504 St. Petersburg, Russia
³⁵ Department of Physics and Astronomy and Space Science Center, University of New Hampshire, Durham, NH 03824, USA
³⁶ Physics Department and McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, MO 63130, USA
³⁷ Dipartimento di Fisica, Università degli Studi di Roma “Tor Ver-gata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
³⁸ Institute of Astrophysics, Foundation for Research and Technology – Hellas, Voutes, 7110, Heraklion, Greece
³⁹ Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Strada Comunale Cinthia, 80126 Napoli, Italy
⁴⁰ MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
⁴¹ Graduate School of Science, Division of Particle and Astrophysical Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan
⁴² Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁴³ Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA
⁴⁴ Department of Physics, University of Hong Kong, Pokfulam, Hong Kong
⁴⁵ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16801, USA
⁴⁶ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁴⁷ INAF Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
⁴⁸ INAF Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
⁴⁹ Physical Research Laboratory, Thaltej, Ahmedabad, Gujarat 380009, India
⁵⁰ Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Via Marzolo 8, 35131 Padova, Italy
⁵¹ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
⁵² Anton Pannekoek Institute for Astronomy & GRAPPA, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁵³ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, China

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Received: 6 August 2025
Accepted: 16 January 2026

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 > 10³⁹ 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.