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Scientists used the IXPE X-ray observatory to study polarized X-ray emissions from the active galactic nucleus NGC 4151, measuring a polarization degree of approximately 6-7% above 4 keV with the polarization angle aligned parallel to the radio jet. The research demonstrates that this polarization pattern can be explained by X-ray light reflecting off the accretion disk around the supermassive black hole, illuminated by a compact, lamp-post-like corona positioned close to the black hole, rather than by an extended corona structure as previously proposed.
Why it matters
This finding helps constrain the geometry and physical structure of the immediate environment around supermassive black holes in active galaxies. Understanding how X-rays are produced and scattered in these extreme environments provides crucial insights into black hole accretion processes and tests our models of general relativistic effects near black holes.
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⚠️ Preprint – Noch nicht peer-reviewed
Dieser Artikel wurde noch nicht von unabhängigen Experten begutachtet. Die Ergebnisse sind vorläufig und sollten mit Vorsicht interpretiert werden.
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 $sim 6-7%$ 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 ($h45^circ$ at 3$sigma$), while the disk reflection contributes $sim 20%$ of the 2-8 keV flux. The soft polarized component carries only $sim 1-5%$ of the flux but has a high polarization degree ($>10%$) and a polarization angle around $20^circ$. 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.
Source: Disk reflection as the origin of the X-ray polarization of NGC 4151 with IXPE