X-ray Polarization and Spectral Variations in an Extreme High-Synchrotron-Peaked Blazar 1ES 1101--232

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 Swift-XRT 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$\sigma$, with a polarization degree of ${\mathrm{\Pi }}_{\mathrm{X}}=17.9\mathrm{\%}\pm 2.7\mathrm{\%}$ and an electric vector position angle (EVPA) of ${\psi }_{\mathrm{X}}={10}^{\circ }.0\pm 4^{\circ }.4$. An even higher polarization degree of ${\mathrm{\Pi }}_{\mathrm{X}}=38.9\mathrm{\%}\pm 9.1\mathrm{\%}$ with an EVPA of ${\psi }_{\mathrm{X}}={13}^{\circ }.9\pm 6^{\circ }.7$ is observed within a narrower time interval, at a CL of 4.3$\sigma$. 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.