Discovery of High X-Ray Polarization from the Neutron Star Low-mass X-Ray Binary Cyg X-2 on the Horizontal Branch
Year: 2026
Type: article
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 (15 σ ) 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.
Source: The Astrophysical Journal
Authors: Andrea Gnarini, Swati Ravi, Philip Kaaret, Anna Bobrikova, Juri Poutanen, Sofia V. Forsblom, Francesco Ursini, Maria Cristina Baglio, Stefano Bianchi, Fiamma Capitanio, Massimo Cocchi, María Alejandra Díaz Teodori, Sergio Fabiani, Ruben Farinelli, Giorgio Matt, Mason Ng, Alexander Salganik, Paolo Soffitta, Antonella Tarana, Silvia Zane
Institutions: Roma Tre University, Marshall Space Flight Center, Massachusetts Institute of Technology, University of Turku, Brera Astronomical Observatory, National Institute for Astrophysics, Institute for Space Astrophysics and Planetology, Osservatorio Astronomico di Cagliari, Nordic Optical Telescope, Osservatorio di Astrofisica e Scienza dello Spazio, Trottier Space Institute at McGill, McGill University, University College London
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $4,499
Funders: National Aeronautics and Space Administration, California Institute of Technology, Agenzia Spaziale Italiana, Istituto Nazionale di Astrofisica, Istituto Nazionale di Fisica Nucleare, Jet Propulsion Laboratory, Marshall Space Flight Center, Fonds de recherche du Québec – Nature et technologies, Jenny ja Antti Wihurin Rahasto
Awards: G236304463, G2633759738, G5023948312
OpenAlex ID: W7125964420