Discovery of High X-Ray Polarization from the Neutron Star Low-mass X-Ray Binary Cyg X-2 on the Horizontal Branch - UTU Research Portal

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Discovery of High X-Ray Polarization from the Neutron Star Low-mass X-Ray Binary Cyg X-2 on the Horizontal Branch

Authors: Gnarini, Andrea; Ravi, Swati; Kaaret, Philip; Bobrikova, Anna; Poutanen, Juri; Forsblom, Sofia V.; Ursini, Francesco; Baglio, Maria Cristina; Bianchi, Stefano; Capitanio, Fiamma; Cocchi, Massimo; Teodori, María Alejandra Díaz; Fabiani, Sergio; Farinelli, Ruben; Matt, Giorgio; Ng, Mason; Salganik, Alexander; Soffitta, Paolo; Tarana, Antonella; Zane, Silvia

Publisher: Institute of Physics Publishing

Publication year: 2026

Journal: Astrophysical Journal

Article number: 299

Volume: 997

Issue: 2

ISSN: 0004-637X

eISSN: 1538-4357

DOI: https://doi.org/10.3847/1538-4357/ae2ad0

Publication's open availability at the time of reporting: Open Access

Publication channel's open availability : Open Access publication channel

Web address : https://iopscience.iop.org/article/10.3847/1538-4357/ae2ad0

Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/515502144

Self-archived copy's licence: CC BY

Self-archived copy's version: Publisher`s PDF

Abstract

We present results from simultaneous X-ray polarimetric and spectroscopic observations of the bright neutron star low-mass X-ray binary Cyg X-2, performed by the Imaging X-ray Polarimetry Explorer (IXPE) and the Nuclear Spectroscopic Telescope Array. IXPE detected significant polarization (15σ) from the source in the 2–8 keV energy band with an average polarization degree (PD) of 4.5% ± 0.3% and a polarization angle (PA) of 128° ± 2° as the source moved along the horizontal branch of its Z-track. The PD increases with energy reaching 9.9% ± 2.8% in the 7–8 keV band, with no evidence for energy-dependent variation in the PA. The PA is roughly consistent with previous measurements obtained during the normal and flaring branches and also with the known radio jet axis. From spectropolarimetric analysis, the main contribution to the polarized radiation is due to Comptonized photons, but the polarization is higher than predicted in typical spreading layer geometries. The observed high polarization may be due to a combination of a highly polarized reflected component and a moderately polarized spreading layer on the neutron star surface or produced by electron scattering in an equatorial wind.

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Funding information in the publication:
A.G., S.B., G.M., and F.U. acknowledge financial support by the Italian Space Agency (Agenzia Spaziale Italiana (ASI)) through the contract ASI-INAF-2022-19-HH.0. F.C. acknowledges financial support by the Istituto Nazionale di Astrofisica (INAF) grant 1.05.23.05.06: “Spin and Geometry in accreting X-ray binaries: The first multi frequency spectropolarimetric campaign.” S.F. and A.T. acknowledge financial support by the INAF grant 1.05.24.02.04: “A multi frequency spectropolarimetric campaign to explore spin and geometry in Low Mass X-ray Binaries.” S.F., G.M., and P.S. have also been supported by the project PRIN 2022—2022LWPEXW—“An X-ray view of compact objects in polarized light,” CUP C53D23001180006. M.N. is a Fonds de Recherche du Québec—Nature et Technologies (FRQNT) postdoctoral fellow. A.S. acknowledges the support of the Jenny and Antti Wihuri Foundation (grant No. 00240331). This work reports observations obtained with the Imaging X-ray Polarimetry Explorer (IXPE), a joint US (NASA) and Italian (ASI) mission, led by the Marshall Space Flight Center (MSFC). The research uses data products provided by the IXPE Science Operations Center (MSFC), using algorithms developed by the IXPE Collaboration (MSFC, Istituto Nazionale di Astrofisica—INAF, Istituto Nazionale di Fisica Nucleare—INFN, ASI Space Science Data Center—SSDC), and distributed by the High-Energy Astrophysics Science Archive Research Center (HEASARC). This research has made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by NASA. Data analysis was performed using the NuSTAR Data Analysis Software (NuSTARDAS), jointly developed by the ASI Science Data Center (SSDC, Italy) and the California Institute of Technology (USA).