2026A&A...707A.154M -
Astronomy and Astrophysics, volume 707A154 (2026/3-1)
IXPE and VLT/FORS2 polarimetry challenge the Seyfert-1.9 classification of MCG-05-23-16.
MARIN F., TAGLIACOZZO D., URSINI F., HUTSEMEKERS D., KOKUBO M., BARNOUIN T., GNARINI A., LAI A.L., SVOBODA J., BIANCHI S., GIANOLLI V.E., GAU E., HU K., KRAWCZYNSKI H., MAKSYM W.P., MARINUCCI A., MARSHALL H., MATT G., MIDDEI R., PETRUCCI P.-O., PUCCETTI S., RODRIGUEZ N., SERAFINELLI R. and TOMBESI F.
Abstract (from CDS):
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.
Abstract Copyright:
© The Authors 2026
Journal keyword(s):
polarization - galaxies: active - galaxies: individual: MCG-05-23-16 - galaxies: Seyfert - X-rays: general
Simbad objects:
9
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