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

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Table 2.

Best-fit parameters from the broadband spectral analysis of MCG-05-23-16 using NuSTAR (3–79 keV), XMM-Newton (2–10 keV), and IXPE I (2–8 keV) combined datasets.

Parameter	Best fitting value
N_H, Gal [cm⁻²]	7.8 × 10²⁰*
N_H, z (May/November 2022) [cm⁻²]	( $1 . 66_{- 0.04}^{+ 0.08}$ $Mathematical equation: $ 1.66_{-0.04}^{+0.08} $$ ) ×10²²
N_H, z (April 2025) [cm⁻²]	( $1 . 48_{- 0.03}^{+ 0.05}$ $Mathematical equation: $ 1.48_{-0.03}^{+0.05} $$ ) ×10²²

`zpl`
Γ (May/November 2022)	$1 . 88_{- 0.01}^{+ 0.02}$ $Mathematical equation: $ 1.88^{+0.02}_{-0.01} $$
Γ (April 2025)	$1 . 75_{- 0.01}^{+ 0.02}$ $Mathematical equation: $ 1.75^{+0.02}_{-0.01} $$
E_C [keV]	116 ± 5
N (May 2022)	(3.4 ± 0.1) ×10⁻²
N (November 2022)	(3.5 ± 0.1) ×10⁻²
N (April 2025)	(3.1 ± 0.1) ×10⁻²

`relxill`
θ_incl [deg]	35 ± 3
a	0.998*
R_in [R_G]	1.24*
log ξ	3.3 ± 0.1
N (May 2022)	1.2 ± 0.1 × 10⁻⁴
N (November 2022)	7 ± 1 × 10⁻⁵
N (April 2025)	(4 ± 1) ×10⁻⁵

`xillver`
A_Fe	$0 . 9_{- 0.06}^{+ 0.04}$ $Mathematical equation: $ 0.9_{-0.06}^{+0.04} $$
N (May 2022)	(3.3 ± 0.2) ×10⁻⁴
N (November 2022)	(4.3 ± 0.2)×10⁻⁴
N (April 2025)	$3 . 6_{- 0.2}^{+ 0.3} \times 10^{- 4}$ $Mathematical equation: $ 3.6_{-0.2}^{+0.3}\times10^{-4} $$

`vashift` [km/s]
NuSTAR (May 2022)	$5 . 7_{- 1.1}^{+ 0.9} \times 10^{3}$ $Mathematical equation: $ 5.7_{-1.1}^{+0.9}\times 10^3 $$
NuSTAR (November 2022)	(4.8 ± 0.7)×10³
XMM (May 2022)	(5.0 ± 0.4)×10³
XMM (April 2025)	//

Cross-calibration constants
NuSTAR/FPMB	1.005 ± 0.003
XMM	0.697 ± 0.002
IXPE/DU1	0.822 ± 0.003
IXPE/DU2	0.856 ± 0.003
IXPE/DU3	0.797 ± 0.003

IXPEF_2 − 10 [erg cm ⁻²s⁻¹]
May 2022	(8.84 − 8.90)×10⁻¹¹
November 2022	(8.64 − 8.69)×10⁻¹¹
April 2025	(8.58 − 8.64)×10⁻¹¹

IXPEL_2 − 10 [erg s⁻¹]
May 2022	(1.41 − 1.42)×10⁴³
November 2022	(1.38 − 1.39)×10⁴³
April 2025	(1.44 − 1.45)×10⁴³

R (May 2022)	0.17
R (April 2025)	0.16

χ²/dof	2673/2396

Notes. Errors are quoted at the 68% confidence level for one parameter of interest. Parameters without quoted errors and marked with an asterisk (*) were fixed during the fit.

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