Disk Reflection as the Origin of the X-Ray Polarization of NGC4151 with IXPE

Abstract We present an X-ray spectropolarimetric 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 slablike 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 3 σ ) and a relatively large torus opening angle (>45° at 3 σ ), 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.