Speaker
Description
4U 1822-371 is the prototype accretion-disc corona neutron-star LMXB. In this geometry, the direct emission from the innermost regions is obscured by the outer disc rim, and the observed X-ray radiation is dominated by scattering and reprocessing. We present the first X-ray polarimetric detection of 4U 1822--371 and the first simultaneous broadband spectro-polarimetric analysis of the source, based on a coordinated campaign with IXPE, XMM-Newton, NuSTAR, and Swift.
The spectral analysis supports the standard ADC scenario: only a fraction of about 1/25 of the intrinsic radiation reaches the observer, consistent with scattering in an extended, optically thin corona with $\tau_{\rm C}\simeq0.05$.
In the 2-8 keV band, IXPE detects a highly significant polarisation signal, with ${\rm PD}=7.9\%\pm0.6\%$ and ${\rm PA}=-24^\circ\pm2^\circ$. The polarisation degree increases with energy, while the polarisation angle remains approximately constant. The orbital-phase-resolved analysis provides a further geometrical constraint: the PD decreases during eclipse, reaching ${\rm PD}=5.5\%\pm1.7\%$, while the PA remains consistent with the phase-averaged value. To our knowledge, 4U 1822--371 is the only high-inclination neutron-star LMXB observed so far with IXPE to show a decrease, rather than an increase, of the PD during eclipse/dipping intervals. This suggests that the eclipse selects a less polarimetrically efficient portion of the extended scattering structure.
These results provide direct polarimetric evidence that 4U 1822--371 is observed in an extreme high-inclination, scattering-dominated regime, where the extended accretion-disc corona shapes both the apparent X-ray luminosity and the observed polarisation properties.