Disk reflection as the origin of the X-ray polarization of NGC 4151 with IXPE
Abstract
We present an X-ray spectro-polarimetric study of the nearby type-1 active galactic nucleus NGC 4151 using two long IXPE observations obtained in 2022 and 2024, supported by simultaneous XMM-Newton and NuSTAR spectroscopy. IXPE measures a polarization degree of ∼ 6−7% above 4 keV, with a polarization angle parallel to the radio jet, and a distinct low-energy component with a different angle, indicating at least two polarized components in the 2−8 keV band. Previous work interpreted the hard X-ray polarization as evidence for a radially extended slab-like corona. Here we test an alternative scenario in which the observed polarization is produced predominantly by relativistic reflection from an accretion disk illuminated by a compact, lamp-post-like corona. Using recently developed models, we fit the IXPE Stokes spectra with a lamp-post plus distant-torus geometry, including partial-covering absorption and an additional soft polarized power-law component. We find that the data require a low coronal height (h<9 Rg at 3σ) and a relatively large torus opening angle (>45∘ at 3σ), while the disk reflection contributes ∼ 20% of the 2-8 keV flux. The soft polarized component carries only ∼ 1−5% of the flux but has a high polarization degree (>10%) and a polarization angle around 20∘. The same configuration provides acceptable fits to the 0.4−79 keV XMM-Newton and NuSTAR spectra, demonstrating that disk reprocessing by a compact corona can simultaneously account for both the polarization and broadband spectral properties of NGC 4151.
Versions
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BibTeX
@misc{kammoun2026disk,
title = {{Disk reflection as the origin of the X-ray polarization of NGC 4151 with IXPE}},
author = {E. Kammoun and M. Dovčiak and J. Podgorný and I. E. Papadakis and V. Binas-Valavanis and S. Bianchi and V. E. Gianolli and F. Ursini and J. A. García},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2606.08862},
eprint = {2606.08862},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.08862},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Disk reflection as the origin of the X-ray polarization of NGC 4151 with IXPE
AU - E. Kammoun
AU - M. Dovčiak
AU - J. Podgorný
AU - I. E. Papadakis
AU - V. Binas-Valavanis
AU - S. Bianchi
AU - V. E. Gianolli
AU - F. Ursini
AU - J. A. García
PY - 2026
DA - 2026/09/15
PB - arXiv
DO - 10.48550/arxiv.2606.08862
UR - https://arxiv.org/abs/2606.08862
AB - We present an X-ray spectro-polarimetric study of the nearby type-1 active galactic nucleus NGC 4151 using two long IXPE observations obtained in 2022 and 2024, supported by simultaneous XMM-Newton and NuSTAR spectroscopy. IXPE measures a polarization degree of ∼ 6−7% above 4 keV, with a polarization angle parallel to the radio jet, and a distinct low-energy component with a different angle, indicating at least two polarized components in the 2−8 keV band. Previous work interpreted the hard X-ray polarization as evidence for a radially extended slab-like corona. Here we test an alternative scenario in which the observed polarization is produced predominantly by relativistic reflection from an accretion disk illuminated by a compact, lamp-post-like corona. Using recently developed models, we fit the IXPE Stokes spectra with a lamp-post plus distant-torus geometry, including partial-covering absorption and an additional soft polarized power-law component. We find that the data require a low coronal height (h<9 R_g at 3σ) and a relatively large torus opening angle (>45^∘ at 3σ), while the disk reflection contributes ∼ 20% of the 2-8 keV flux. The soft polarized component carries only ∼ 1−5% of the flux but has a high polarization degree (>10%) and a polarization angle around 20^∘. The same configuration provides acceptable fits to the 0.4−79 keV XMM-Newton and NuSTAR spectra, demonstrating that disk reprocessing by a compact corona can simultaneously account for both the polarization and broadband spectral properties of NGC 4151.
ER -