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arXiv · 2509.14494

First Detection of X-ray Polarization and Its Short-term Increase Pre- and Post-Eclipse in HMXB 4U 1700-377

Kaito Ninoyu·Keisuke Uchiyama·Shinya Yamada·Ryota Hayakawa·Shunji Kitamoto·Nao Kominato·Takayoshi Kohmura·Misaki Mizumoto·Yuusuke Uchida·Toru Tamagawa·Ryota Tomaru·Seoru Ito
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Abstract

We report the first statistically significant detection of X-ray polarization from the high-mass X-ray binary (HMXB) 4U 1700-377, observed using the Imaging X-ray Polarimetry Explorer (IXPE). A polarization degree exceeding 10% was detected above 5 keV, placing it among the highest polarization observed in HMXBs to date. The observation was conducted over a full orbital period of the binary system, during which several sporadic and instantaneous flares were detected. We identify a clear correlation between the polarization degree and orbital phase, with the highest polarization occurring just before and after the eclipse, reaching over 20% for a few tens of ks. These results suggest that the scattering medium responsible for the observed polarization is spatially localized between the compact object and the O-type companion star, likely created by large-scale inhomogeneities in the stellar wind and its interaction with the compact star's emission. We also explore the roles of disk winds and orbital reflection in the observed polarization variability. While both mechanisms contribute to the polarization, the substantial increase in polarization before and after the eclipse cannot be fully explained by these models alone, suggesting that the involvement of additional factors. The properties of the X-ray polarization observed by IXPE provide new insights into the accretion processes, X-ray emission, and wind structure in 4U 1700-377, advancing our understanding of their complex environments and the nature of the compact objects within.

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Kaito Ninoyu, Keisuke Uchiyama, Shinya Yamada, Ryota Hayakawa, Shunji Kitamoto, Nao Kominato, Takayoshi Kohmura, Misaki Mizumoto, Yuusuke Uchida, Toru Tamagawa, Ryota Tomaru, Seoru Ito. 2025-09-18. First Detection of X-ray Polarization and Its Short-term Increase Pre- and Post-Eclipse in HMXB 4U 1700-377. https://doi.org/10.1093/pasj%2Fpsaf060

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