X-ray Polarization of NGC 4151 Explained by Disk Reflection from Compact Corona
A new study using NASA's IXPE space telescope, supported by XMM-Newton and NuSTAR observations, finds that the X-ray polarization of the active galactic nucleus NGC 4151 can be explained by relativistic reflection off its accretion disk illuminated by a compact, lamp-post-like corona. This challenges a previous interpretation that attributed the hard X-ray polarization to a radially extended, slab-like corona. The findings help constrain the geometry of the corona and accretion disk around the supermassive black hole at the center of NGC 4151.
Researchers have conducted a detailed X-ray spectro-polarimetric analysis of NGC 4151, a nearby type-1 active galactic nucleus, using two long IXPE observations from 2022 and 2024, supplemented by simultaneous XMM-Newton and NuSTAR data. IXPE measured a polarization degree of roughly 6–7% above 4 keV, with the polarization angle aligned parallel to the radio jet, along with a distinct lower-energy polarized component at a different angle, suggesting at least two separate polarized emission components in the 2–8 keV band. While prior work interpreted this hard X-ray polarization as evidence for an extended slab-like corona, the new study tests an alternative model in which a compact, lamp-post corona sitting close to the black hole illuminates the accretion disk, producing the observed polarization through relativistic reflection. The best-fit model requires a low coronal height of less than 9 gravitational radii (at 3-sigma confidence) and a relatively large torus opening angle exceeding 45 degrees, with disk reflection contributing roughly 20% of the 2–8 keV flux. A soft polarized power-law component, carrying only 1–5% of the flux but with a polarization degree above 10%, is also needed to fully describe the data. The same configuration simultaneously fits the broadband 0.4–79 keV spectra from XMM-Newton and NuSTAR, demonstrating internal consistency. The paper has been submitted to The Astrophysical Journal and is currently a preprint.
What's missing
The lamp-post model assumes a specific, idealized coronal geometry; more complex or extended corona configurations are not fully ruled out. As a preprint, the results have not yet undergone peer review.
What different sources said
- arXiv astro-phCenter
X-ray polarization of reflected thermal emission
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