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 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 $\Pi_{\rm X}=17.9\%\pm2.7\%$ and an electric vector position angle (EVPA) of $\psi_{\rm X}=10^\circ.0\pm4^\circ.4$. An even higher polarization degree of $\Pi_{\rm X}=38.9\%\pm9.1\%$ with an EVPA of $\psi_{\rm X}=13^\circ.9\pm6^\circ.7$ 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.