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

Résumé fourni par la source

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 H α 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.

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