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

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 (\textit{IXPE}), together with optical spectro-polarimetry obtained at the Very Large Telescope (VLT), and combined with archival near-ultraviolet, optical and near-infrared polarimetric data. No X-ray polarization was detected in the 2-8 keV band, with a 99\% confidence upper limit of $\leq$2.9\%, further reduced to $\leq$2.5\% when combined with the two past IXPE observations of the same target. Monte Carlo simulations suggest that equatorial coronal models are disfavored if the AGN is indeed a type-1.9/2 AGN, but coronae coplanar to the accretion disk remain consistent if the source is less inclined than previously assumed. \textit{VLT}/FORS2 data reveal a typical type-2 spectrum in total flux, a broad H$α$ line in polarized flux, and strongly wavelength dependent polarization degree and angle, rotating by nearly 70$^\circ$ across the optical band. Comparison with historical measurements confirms long-term stability of the polarization spectrum and a $\sim$90$^\circ$ rotation in the near-ultraviolet. Interpreting the multi-wavelength polarization relative to the AGN ionization axis indicates that the main obscurer is not a compact circumnuclear torus, but a distant kpc-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.

Figures (11)

• Figure 1: Left panel:IXPE$Q$ (orange crosses) and $U$ (magenta crosses) grouped Stokes spectra of the 3rd IXPE pointing (April 2025) of MCG-05-23-16 are shown with residuals, along with the corresponding best-fitting model. Right panel: contour plot between the polarization degree $P$ and angle $\theta$, summed over the $2-8$ keV energy band of the 3rd IXPE pointing. Purple, pink and orange regions correspond, respectively, to $68\%$, $90\%$ and $99\%$ confidence levels for two parameters of interest.
• Figure 2: $P$, $\theta$ contour plots of the May 2022 observation (dotted contours), May 2022 + November 2022 (dashed contours), May 2022 + November 2022 + April 2025 (solid contours). Contours colors are as in Fig. \ref{['Fig:3rdobs']}.
• Figure 3: NuSTAR, XMM and IXPE I spectra, collected on May 2022, November 2022 and April 2025 of MCG-05-23-16, along with the best-fit model components for each instrument (only for the May 2022 campaign, in which the source was observed by the three instruments simultaneously) and the residuals for the broad-band (2--79 keV) spectral analysis in Sec. \ref{['Results:Broadband']}. NuSTAR photon counting detector modules (FPMA and FPMB) of the same campaign have been grouped for visual clarity, as well as IXPE DUs.
• Figure 4: Same as in Fig. \ref{['Fig:all_nopol']}, but with compTT (assuming a slab coronal configuration) instead of cutoffpl.
• Figure 5: Polarization degree from the MONK simulations as a function of the cosine of the system inclination angle $\cos(\theta_{\rm disk})$ in the case of a spherical lamppost model (left panel), a conical outflows model (middle panel) and a slab corona model (right panel). $\cos(\theta_{\rm disk})=0$ and $\cos(\theta_{\rm disk})=1$ represent the edge-on and face-on views of the source, respectively. The green regions represent the allowed values of the polarization degree: pale green represents the constraint on $P$ found with the broad-band spectro-polarimetric analysis in Sect. \ref{['Results:Broadband_pol']}, while saturated green represents the combination of the constraints on $P$ and the system inclination (Sect. \ref{['Discussion:orientation']}). Since the lamppost and the cone result in $\theta$ perpendicular to the accretion disk axis, the resulting polarization constraint is set to $P<1.1\%$ in the left and middle panels. Conversely, the slab results in $\theta$ parallel to the disk. In that case $P<2.4\%$.