IXPE and VLT/FORS2 polarimetry challenge the Seyfert-1.9 classification of MCG-05-23-16

¹ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550 11 rue de l’Université 67000 Strasbourg, France
² Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre via della Vasca Navale 84 00146 Roma, Italy
³ Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 19c B5c 4000 Liège, Belgium
⁴ National Astronomical Observatory of Japan, National Institutes of Natural Sciences 2-21-1 Osawa Mitaka Tokyo 181-8588, Japan
⁵ NASA Marshall Space Flight Center Huntsville AL 35812, USA
⁶ Astronomical Institute of the Czech Academy of Sciences Boční II 1401/1 14100 Praha 4, Czech Republic
⁷ Department of Physics and Astronomy, Clemson University Kinard Lab of Physics Clemson SC 29634, USA
⁸ Department of Physics, Washington University St. Louis MO 63130, USA
⁹ McDonnell Center for the Space Sciences, Washington University St. Louis MO 63130, USA
¹⁰ Center for Quantum Leaps, Washington University St. Louis MO 63130, USA
¹¹ ASI – Agenzia Spaziale Italiana Via del Politecnico snc 00133 Roma, Italy
¹² MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology 77 Massachusetts Avenue Cambridge MA 02139, USA
¹³ INAF Osservatorio Astronomico di Roma Via Frascati 33 00078 Monte Porzio Catone RM, Italy
¹⁴ Université Grenoble Alpes, CNRS, IPAG 38000 Grenoble, France
¹⁵ Space Science Data Center, Agenzia Spaziale Italiana Via del Politecnico snc 00133 Roma, Italy
¹⁶ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales Avenida Ejército Libertador 441 Santiago, Chile
¹⁷ Dipartimento di Fisica, Universitá degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1 00133 Roma, Italy
¹⁸ Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, Via della Ricerca Scientifica 1 00133 Roma, Italy

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Received: 19 November 2025
Accepted: 19 January 2026

Abstract

We report the third observation of the Seyfert-1.9 active galactic nucleus (AGN) MCG-05-23-16 with the Imaging X-ray Polarimetry Explorer (IXPE), together with optical spectropolarimetry obtained at the Very Large Telescope (VLT), and combined with archival near-ultraviolet, optical, and near-infrared polarimetric data. We detect no X-ray polarization in the 2–8 keV band, with a 99% confidence upper limit of ≤2.9%, which is further reduced to ≤2.5% when combined with the two previous IXPE observations of the same target. Monte Carlo simulations suggest that equatorial coronal models are disfavored if the AGN is indeed a type 1.9/2 AGN, while coronae coplanar to the accretion disk remain consistent if the source is less inclined than previously assumed. Data from VLT/FORS2 reveal a typical type 2 spectrum in total flux, a broad Hα line in polarized flux, and a polarization degree and angle that depend strongly on wavelength. The polarization angle rotates by nearly 70° across the optical band. Comparison with historical measurements confirms the long-term stability of the polarization spectrum and a ∼90° rotation in the near-ultraviolet. Interpreting the multiwavelength polarization relative to the AGN ionization axis indicates that the main obscurer is not a compact circumnuclear torus, but rather a distant kiloparsec-scale dust lane crossing the galaxy. This result implies that MCG-05-23-16 is, in fact, a type 1 AGN seen through foreground dust. The low X-ray column density becomes consistent with the absence of polarization, provided that the nuclear inclination is low.