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Decoding Cygnus X-2: The critical role of reflection in IXPE data

Lookup NU author(s): Dr Adam IngramORCiD

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Abstract

© 2026 Elsevier B.V. We present a spectro-polarimetric re-analysis of the first IXPE observation of Cygnus X-2 which we determine to be mainly in the normal branch, from quasi-simultaneous observations with NuSTAR, NICER, and INTEGRAL. We measure the hard X-ray polarization angle and find it to be consistent with the previously measured position angle of the radio jet. Leveraging NuSTAR’s detection of both the relativistic Fe K emission line and the Compton hump, we constrain the flux contribution of the reflected emission from the inner accretion disk to be 10% of the total X-ray flux in the IXPE energy band. Unlike previous studies that modeled only the Fe K emission line, we fit the full-band reflection spectrum using a fully relativistic disk model. A polarization degree of approximately 20% for the reflection component could explain the hard X-ray polarization data from IXPE. We also discuss the inferred disk inclination angle derived from our spectro-polarimetric modeling.

Publication metadata

Author(s): Liu H, Jiang J, Ingram A, Bambi C, Fabian AC, Farinelli R, Ludlam R, Degenaar N, Podgorny J, Santangelo A, Steiner JF, Young AJ, Zhang Z

Publication type: Article

Publication status: Published

Journal: Journal of High Energy Astrophysics

Year: 2026

Volume: 51

Print publication date: 01/03/2026

Online publication date: 12/01/2026

Acceptance date: 04/01/2026

ISSN (print): 2214-4048

ISSN (electronic): 2214-4056

Publisher: Elsevier BV

URL: https://doi.org/10.1016/j.jheap.2026.100548

DOI: 10.1016/j.jheap.2026.100548

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