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. 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.

X-Ray Polarimetry of the Dipping Accreting Neutron Star 4U 1624–49

“…The values of kT in and kT 0 are similar to that of other NS-LMXBs observed with IXPE (e.g., Di Marco et al 2023a;Ursini et al 2023b;Capitanio et al 2023;Farinelli et al 2023;Rankin et al 2024). There is a wide uncertainty in the normalization of the diskbb Component, likely due to the fact most of its emission is below the IXPE band.…”

“…The PD does show marginal significance for an increase with energy as also observed for other NS-LMXBs. A notable example is 4U 1820−303, which shows a strong increase in polarization with energy (Di Marco et al 2023a). The PD strongly depends on both the shape of the Comptonizing region and the inclination of the source with respect to the line of sight (e.g., Gnarini et al 2022).…”

X-Ray Polarized View of the Accretion Geometry in the X-Ray Binary Circinus X-1

“…In the intermediate HRs, the two components are mixed. Such a two-component model, composed of, e.g., a multicolor blackbody from the accretion disk and a Comptonized component, is the obvious candidate to explain two components in the polarization and so has been proposed for the other IXPE observations of weakly magnetized accreting NSs (Ursini et al 2023a;Di Marco et al 2023b;Capitanio et al 2023;Cocchi et al 2023;Farinelli et al 2023). From simple geometrical considerations, and in the absence of relativistic effects, each component can be expected to be polarized either parallel or orthogonal to its symmetry axis.…”

The Imaging X-ray Polarimetry Explorer (IXPE) and New Directions for the Future

“…Among these, Cyg X-2 [53], XTE J1702-462 [54], GX5-1 [55], and Sco X-1 [56] are called "Z sources" because of the characteristic "Z" shape in the color-color diagram. For "Atoll", the observed sources were GS 1826-238 [57], GX 9 + 9 [58], and 4U1830-303 [59].…”