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2025ApJ...994...37A - Astrophys. J., 994, 37 (2025/November-3)

XL-Calibur Polarimetry of Cyg X-1 Further Constrains the Origin of Its Hard-state X-Ray Emission.

AWAKI H., BARING M.G., BOSE R., CASEY J., CHUN S., DASGUPTA A., GALCHENKO P., GAU E., GOYA K., HAKAMATA T., HAYASHI T., HEATWOLE S., HU K., ISHI D., ISHIDA M., KISLAT F., KISS M., KLEPPER K., KRAWCZYNSKI H., KURAMOTO H., LISALDA L., MAEDA Y., MATSUMOTO H., MENON S.V., MIYAMOTO A., MIYAMOTO A., MURAKAMI K., OKAJIMA T., PEARCE M., RAUCH B., RODRIGUEZ CAVERO N., SHIRAHAMA K., SPOONER S., TAKAHASHI H., TAMURA K., UCHIDA Y., WIMALASENA K., YOKOTA M. and YOSHIMOTO M.

Abstract (from CDS):

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 ($5.{0}_{-3.0}^{+2.7}$)% (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.

Abstract Copyright: © 2025. The Author(s). Published by the American Astronomical Society.

Journal keyword(s): High energy astrophysics - Black hole physics - Black holes - Stellar mass black holes - X-ray astronomy - X-ray sources - X-ray telescopes - High altitude balloons - High mass x-ray binary stars

Simbad objects: 9

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