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. There is strong degeneracy between the Comptonized and reflection components. Given that the Comptonized component is not expected to be highly polarized, a polarization degree of approximately 20% for the reflection component could explain the X-ray polarization data from IXPE. We also discuss the disk inclination angle inferred from our spectro-polarimetric modeling, as well as other possible explanations for the data.
@article{arxiv.2601.01669,
title = {Decoding Cygnus X-2: The Critical Role of Reflection in IXPE Data},
author = {Honghui Liu and Jiachen Jiang and Adam Ingram and Cosimo Bambi and Andrew C. Fabian and Ruben Farinelli and Renee Ludlam and Nathalie Degenaar and Jakub Podgorny and Andrea Santangelo and James F. Steiner and Andrew J. Young and Zuobin Zhang},
journal= {arXiv preprint arXiv:2601.01669},
year = {2026}
}
Comments
18 pages, 9 figures, accepted for publication in JHEAp