Is the hard X-ray polarization of Cyg X-1 stable?
Abstract
We present results from hard X-ray observations of Cyg X-1 in the hard state, conducted during, and proximate to, the July 2024 balloon flight of XL-Calibur. Temporal and spectral observations from NICER, NuSTAR, and Swift-BAT have been included, complementing XL-Calibur polarimetric observations in the ∼19-64 keV range. We perform joint NICER+NuSTAR+Swift-BAT spectral analyses in the 1.0-195.0 keV range and reference these to the day-by-day polarization analysis. XL-Calibur previously reported (5.0+2.7−3.0)\% polarization degree (PD) and polarization angle (PA) aligned with the jet axis, when integrating over the four observing days. In this paper, we present day-by-day polarimetry results, which indicate variation in the polarization parameters. For July 12-13, the PD is (16.8±4.9)\%, at PA ∼25∘ shifted from the jet, while for the remaining days, the PD and PA are consistent with the integrated values. At the same time, we observed that the reflection fraction parameter (Rf) of spectroscopic model increases by factor of two. These changes place specific requirements on the PD and flux of additional component responsible for them. Such requirements are more likely to be satisfied by geometric changes in corona near the BH than by scattering in the vicinity of the stellar companion.
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BibTeX
@misc{baring2026hard,
title = {{Is the hard X-ray polarization of Cyg X-1 stable?}},
author = {Matthew G. Baring and Jacob Casey and Ephraim Gau and Tomohiro Hakamata and Kun Hu and Daiki Ishi and Fabian Kislat and Mózsi Kiss and Henric Krawczynski and Maitreya Kundu and Haruki Kuramoto and Lindsey Lisalda and Masaki Maekawa and Hironori Matsumoto and Shravan Vengalil Menon and Asca Miyamoto and Takuya Miyazawa and Takashi Okajima and Mark Pearce and Brian F. Rauch and Kentaro Shirahama and Sean Spooner and Hiromitsu Takahashi and Yuusuke Uchida and Varun},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.07032},
eprint = {2611.07032},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.07032},
note = {CC BY-NC-SA 4.0},
}
RIS
TY - UNPB
TI - Is the hard X-ray polarization of Cyg X-1 stable?
AU - Matthew G. Baring
AU - Jacob Casey
AU - Ephraim Gau
AU - Tomohiro Hakamata
AU - Kun Hu
AU - Daiki Ishi
AU - Fabian Kislat
AU - Mózsi Kiss
AU - Henric Krawczynski
AU - Maitreya Kundu
AU - Haruki Kuramoto
AU - Lindsey Lisalda
AU - Masaki Maekawa
AU - Hironori Matsumoto
AU - Shravan Vengalil Menon
AU - Asca Miyamoto
AU - Takuya Miyazawa
AU - Takashi Okajima
AU - Mark Pearce
AU - Brian F. Rauch
AU - Kentaro Shirahama
AU - Sean Spooner
AU - Hiromitsu Takahashi
AU - Yuusuke Uchida
AU - Varun
PY - 2026
DA - 2026/11/04
PB - arXiv
DO - 10.48550/arxiv.2611.07032
UR - https://arxiv.org/abs/2611.07032
AB - We present results from hard X-ray observations of Cyg X-1 in the hard state, conducted during, and proximate to, the July 2024 balloon flight of XL-Calibur. Temporal and spectral observations from NICER, NuSTAR, and Swift-BAT have been included, complementing XL-Calibur polarimetric observations in the ∼19-64 keV range. We perform joint NICER+NuSTAR+Swift-BAT spectral analyses in the 1.0-195.0 keV range and reference these to the day-by-day polarization analysis. XL-Calibur previously reported (5.0^+2.7_−3.0)\% polarization degree (PD) and polarization angle (PA) aligned with the jet axis, when integrating over the four observing days. In this paper, we present day-by-day polarimetry results, which indicate variation in the polarization parameters. For July 12-13, the PD is (16.8±4.9)\%, at PA ∼25^∘ shifted from the jet, while for the remaining days, the PD and PA are consistent with the integrated values. At the same time, we observed that the reflection fraction parameter (R_f) of spectroscopic model increases by factor of two. These changes place specific requirements on the PD and flux of additional component responsible for them. Such requirements are more likely to be satisfied by geometric changes in corona near the BH than by scattering in the vicinity of the stellar companion.
ER -