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

TL;DR

This study presents the first X-ray polarization measurements of the EHSP blazar 1ES 1101--232 using IXPE, complemented by simultaneous Swift-XRT and NuSTAR data. Through PCUBE and SpecPol analyses, the authors detect significant polarization in the 2–6 keV band (Π ≈ 18%) with an EVPA around 10°, and even higher polarization during a narrow interval, implying a well-ordered magnetic field in the X-ray emission region and synchrotron origin. The X-ray spectrum shows soft-to-hard evolution with clockwise hysteresis, consistent with shock-driven particle acceleration and energy-stratified electron populations. Comparisons with other IXPE EHSPs suggest a common polarization behavior in EHSP jets, though broader multiwavelength polarimetry is needed to fully constrain jet magnetohydrodynamics and particle acceleration sites.

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

Figures (10)

• Figure 1: Broadband X-ray spectrum of 1ES 1101--232 in the 0.3--79 keV band, which was jointly fitted using the contemporaneous from IXPE (orange, blue and green points), Swift-XRT (black points) and NuSTAR (red and violet points).
• Figure 2: Normalized Stokes parameters $q$ and $u$ (sub-figure [a]) and 2-D polarization contours between $\Pi_{\rm X}$ and $\psi_{\rm X}$ (sub-figure [b]) of 1ES 1101--232 in the 2--6 keV band. In sub-figure (a), the orange, blue, green, and black points represent the results of DU1, DU2, DU3, and the combination of the three DUs, respectively, measured through the PCUBE algorithm within ixpeobssim. The red point represents the result of the combination of the three DUs, estimated via SpecPol fit within Xspec. In sub-figure (b), the black cross marks the best-fit values of $\Pi_{\rm X}$ and $\psi_{\rm X}$ and the red, blue and green contours refer to the results at the 68%, 90% and 99% CLs, respectively, derived through the SpecPol fit. The violet shaded areas in both sub-figures represent the MDP$_{99}$ value of the IXPE observation for 1ES 1101--232.
• Figure 3: SpecPol fit result of 1ES 1101--232 in the 2--6 keV band. The panels from left to right represent the fits to the IXPE Stokes parameter $I$, $Q$ and $U$ with the associated residuals. In the $I$ spectra panel, the triangles indicate the background spectra for different DUs.
• Figure 4: $P_{\rm Null}$ of $\chi^{2}$ test on the variation of the normalized Stokes parameters $q$ (red points) and $u$ (blue points) in logarithmic scale as a function of number of time bins, assuming that the X-ray polarization of 1ES 1101--232 remained constant druing the IXPE pointing epoch. The cyan shaded area represent the region where $P_{\rm Null}>1\%$. The dotted and dash-dot lines represent the statistical significance where the null hypothesis can be rejected at the 2$\sigma$ (90.45%) and 3$\sigma$ (99.73%) CLs.
• Figure 5: Polarization light curves of 1ES 1101--232 in the 2--6 keV band. In each sub-figure, the top, middle and bottom panels represent light curves of $\Pi_{\rm X}$, $\psi_{\rm X}$ and $\Gamma$, respectively. The red and blue points signify the results derived via the PCUBE analysis and SpecPol fits, respectively. In the $\Pi_{\rm X}$ panels, the violet shaded histograms donate the values of MDP$_{99}$ for different time bins, and the blue points with downward arrows mark the 1-D upper limits of $\Pi_{\rm X}$ at the 99% CL, estimated using the error task within Xspec when the best-fit values are lower than the associated values of MDP$_{99}$. Sub-figures (a), (b) and (c) correspond to the cases that the IXPE observation is divided into two, three and ten time bins, respectively.