XL-Calibur Polarimetry of Cyg X-1 Further Constrains the Origin of Its Hard-state X-Ray Emission
Abstract The balloon-borne hard X-ray polarimetry mission XL-Calibur observed the black hole X-ray binary (BHXRB) Cygnus X-1 (Cyg X-1) during its nearly 6 day long-duration balloon flight from Sweden to Canada in 2024 July. The XL-Calibur observations allowed us to derive the most precise constraints to date of the polarization degree (PD) and polarization angle (PA) of the hard X-ray emission from a BHXRB. XL-Calibur observed Cyg X-1 in the hard state and measured a ∼19–64 keV PD of ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>5</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3.0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>2.7</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> )% (equivalent to an upper limit, at the 99% level, of 11.1%) at a PA of −28° ± 17°, with an 8.7% chance probability of detecting larger PDs than the one observed, given an unpolarized signal. The XL-Calibur results are thus comparable to the 2–8 keV PD and PA found by Imaging X-ray Polarimetry Explorer (IXPE), with a similar agreement between the hard X-ray PA and the radio jet direction. We also discuss the implications of our polarization measurements in the context of models describing the origin of the broadband X-ray and γ -ray emission, to which XL-Calibur provides independent constraints on any proposed emission modeling.
Authors
- M. Kiss (ORCID: https://orcid.org/0000-0001-5191-9306)
- Tomohiro Hakamata (ORCID: https://orcid.org/0000-0002-0987-0278)
- M. Pearce (ORCID: https://orcid.org/0000-0001-7011-7229)
- Fabian Kislat (ORCID: https://orcid.org/0000-0001-7477-0380)
- Sohee Chun (ORCID: https://orcid.org/0009-0002-2488-5272)
- Hisamitsu Awaki (ORCID: https://orcid.org/0000-0001-7204-4350)
- Masayoshi Yokota
- Scott Heatwole
- Takayuki Hayashi (ORCID: https://orcid.org/0000-0001-6665-2499)
- K. Wimalasena (ORCID: https://orcid.org/0000-0003-4434-1766)
- Kun Hu (ORCID: https://orcid.org/0000-0002-9705-7948)
- Hironori Matsumoto (ORCID: https://orcid.org/0000-0002-4637-7781)
- Sean Spooner (ORCID: https://orcid.org/0000-0003-0710-8893)
- Daiki Ishi
- Pavel Galchenko (ORCID: https://orcid.org/0000-0001-6358-5147)
- Yoshitomo Maeda (ORCID: https://orcid.org/0000-0002-9099-5755)
- R. Bose
- Lindsey Lisalda (ORCID: https://orcid.org/0000-0002-5202-1642)
- Keisuke Tamura (ORCID: https://orcid.org/0009-0004-7101-4503)
- Takashi Okajima (ORCID: https://orcid.org/0000-0002-6054-3432)
- B. F. Rauch (ORCID: https://orcid.org/0000-0002-1452-4142)
- Adrika Dasgupta
- H. Krawczynski (ORCID: https://orcid.org/0000-0002-1084-6507)
- Nicole Rodriguez Cavero (ORCID: https://orcid.org/0000-0001-5256-0278)
- Ephraim Gau (ORCID: https://orcid.org/0000-0002-5250-2710)
- H. Takahashi (ORCID: https://orcid.org/0000-0001-6314-5897)
- Matthew G. Baring (ORCID: https://orcid.org/0000-0003-4433-1365)
- Yuusuke Uchida (ORCID: https://orcid.org/0000-0002-7962-4136)
- Kassi Klepper
- Aiko Miyamoto (ORCID: https://orcid.org/0009-0009-2127-8830)
- M. Yoshimoto (ORCID: https://orcid.org/0009-0005-0819-0819)
- Manabu Ishida (ORCID: https://orcid.org/0000-0003-4509-3493)
- Asca Miyamoto (ORCID: https://orcid.org/0009-0008-4216-064X)
- Jacob Casey (ORCID: https://orcid.org/0009-0009-3051-6570)
- Kazuho Goya
- Shravan Vengalil Menon (ORCID: https://orcid.org/0009-0005-0818-7484)
- Kaito Murakami
- Haruki Kuramoto (ORCID: https://orcid.org/0009-0006-9014-2716)
- Kentaro Shirahama
Institutions
- Hiroshima University (JP)
- Goddard Space Flight Center (US)
- University of New Hampshire (US)
- Washington University in St. Louis (US)
- Japan Aerospace Exploration Agency (JP)
- Wallops Flight Facility (US)
- AlbaNova (SE)
- Institute of Space and Astronautical Science (JP)
- Ehime University (JP)
- Tokyo Metropolitan University (JP)
- Rice University (US)
- University of Maryland, Baltimore County (US)
- KTH Royal Institute of Technology (SE)
- The University of Osaka (JP)
Publication Details
- Journal
- The Astrophysical Journal
- Published
- 2025-11-13
- DOI
- https://doi.org/10.3847/1538-4357/ae0f1d
- Citations
- 1
- Primary Topic
- Astrophysical Phenomena and Observations
- Type
- article
- Field-Weighted Citation Impact
- 1.15