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 ${\Pi }_{\mathrm{X}}=17.9\%\pm 2.7\%$ and an electric vector position angle (EVPA) of ${\psi }_{\mathrm{X}}=10^{\circ }.0\pm 4^{\circ }.4$. An even higher polarization degree of ${\Pi }_{\mathrm{X}}=38.9\%\pm 9.1\%$ 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.

arXiv comment16 pages, 10 figures, 3 tables, accepted for publication in ApJ