2026ApJ...997..362H

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 6.6σ, with a polarization degree of Π_X = 17.9% ± 2.7% and an electric vector position angle (EVPA) of ψ_X = 10$\mathop{.}\limits^{\unicode{x000b0}}$0 ± 4$\mathop{.}\limits^{\unicode{x000b0}}$4. An even higher polarization degree of Π_X = 38.9% ± 9.1% with an EVPA of ψ_X = 13$\mathop{.}\limits^{\unicode{x000b0}}$9 ± 6$\mathop{.}\limits^{\unicode{x000b0}}$7 is observed within a narrower time interval, at a CL of 4.3σ. 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.