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. The IXPE observations of 4U 1820−303 were coordinated with Swift X-ray Telescope, Neutron Star Interior Composition Explorer, and Nuclear Spectroscopic Telescope Array 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° in the 4-7 keV energy range, and a polarization degree of 10% ± 2% and a polarization angle of −67° ± 7° 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 ≃90° 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 3σ upper limit of 50% at 7.25 GHz.

First Detection of X-Ray Polarization from the Accreting Neutron Star 4U 1820-303

Alessandro;Thomas D.;Anitra Alessio;Bachetti Matteo;Burderi Luciano;Iaria Rosario;Kajava;Pilia Maura;Sanna Andrea
Conceptualization
;
Martin C.;Marino Alessio;Matt Giorgio;Bongiorno;Del Monte;Possenti Andrea;
2023-01-01

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. The IXPE observations of 4U 1820−303 were coordinated with Swift X-ray Telescope, Neutron Star Interior Composition Explorer, and Nuclear Spectroscopic Telescope Array 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° in the 4-7 keV energy range, and a polarization degree of 10% ± 2% and a polarization angle of −67° ± 7° 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 ≃90° 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 3σ upper limit of 50% at 7.25 GHz.
2023
Low-mass x-ray binary stars; Neutron stars; Polarimetry; Spectropolarimetry; Stellar accretion disks
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11584/426259
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