First detection of X-ray polarization from the accreting neutron star 4U 1820-303

Abstract

This paper reports the first detection of polarization in the X-rays for atoll-source 4U 1820-303, obtained with the Imaging X-ray Polarimetry Explorer (IXPE) at 99.999% confidence level (CL). Simultaneous polarimetric measurements were also performed in the radio with the Australia Telescope Compact Array (ATCA). The IXPE observations of 4U 1820-303 were coordinated with Swift-XRT, NICER, and NuSTAR aiming to obtain an accurate X-ray spectral model covering a broad energy interval. The source shows a significant polarization above 4 keV, with a polarization degree of 2.0(0.5)% and a polarization angle of -55(7) deg in the 4-7 keV energy range, and a polarization degree of 10(2)% and a polarization angle of -67(7) deg in the 7-8 keV energy bin. This polarization also shows a clear energy trend with polarization degree increasing with energy and a hint for a position-angle change of about 90 deg at 96% CL around 4 keV. The spectro-polarimetric fit indicates that the accretion disk is polarized orthogonally to the hard spectral component, which is presumably produced in the boundary/spreading layer. We do not detect linear polarization from the radio counterpart, with a 99.97% upper limit of 50% at 7.25 GHz.

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• Polarized radiation from the spreading layer of the weakly magnetized neutron stars
• X-ray spectro-polarimetric characterization of GX 340+0 in the horizontal branch: a highly inclined source?
• Characterization of High-polarization Stars and Blazars with DIPOL-1 at Sierra Nevada Observatory
• Constraining the geometry of the dipping atoll 4U 1624-49 with X-ray spectroscopy and polarimetry
• X-ray Dips and Polarization Angle Swings in GX 13+1
• X-ray spectropolarimetric characterization of the Z-source GX 340+0 in the normal branch
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• First Detection of X-ray Polarization and Its Short-term Increase Pre- and Post-Eclipse in HMXB 4U 1700-377
• The NuSTAR view of Ultra-Compact X-ray Binaries
• X-ray polarization of Z-type neutron star low-mass X-ray binaries -- II. Spectropolarimetric analysis