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

TL;DR

This study reports the first detection of X-ray polarization from the neutron star 4U 1820-303, revealing energy-dependent polarization properties and providing insights into the accretion disk and boundary layer structure.

Contribution

It presents the first polarization measurement of 4U 1820-303 in X-rays, combining multi-instrument observations to analyze polarization degree and angle across energies.

Findings

Significant polarization detected above 4 keV.

Polarization degree increases with energy.

Evidence of a 90-degree change in polarization angle around 4 keV.

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.