X-Ray Polarization and Spectral Variations in an Extreme High-synchrotron-peaked Blazar 1ES 1101–232

Abstract We present the first X-ray polarimetric observation of the extreme high-synchrotron-peaked blazar 1ES 1101–232, conducted by the Imaging X-ray Polarimetry Explorer (IXPE). The data analysis incorporates simultaneous and quasi-simultaneous observations from the Swift X-Ray Telescope and NuSTAR. Our results reveal a significant detection of X-ray polarization in the 2–6 keV band at a confidence level (CL) of 6.6 σ , with a polarization degree of Π X = 17.9% ± 2.7% and an electric vector position angle (EVPA) of ψ X = 10 . ° 0 ± 4 . ° 4. An even higher polarization degree of Π X = 38.9% ± 9.1% with an EVPA of ψ X = 13 . ° 9 ± 6 . ° 7 is observed within a narrower time interval, at a CL of 4.3 σ . During the IXPE observational campaign, the X-ray spectrum of 1ES 1101–232 exhibits a clear soft-to-hard spectral evolution in the 0.3–10 keV band, although no significant flux variability is detected. Additionally, a clockwise hysteresis loop is identified in the flux–photon index plane. These findings collectively indicate that the X-ray emission from 1ES 1101–232 originates in a region characterized by a well-ordered magnetic field through synchrotron radiation.