The IXPE and multifrequency polarimetric view of the extreme blazars 1ES 1101-232 and RGB J0710+591

TL;DR

This study leverages IXPE X-ray polarimetry together with Swift, NuSTAR, and optical/mm/radio data to probe the magnetic-field structure and particle acceleration in two extreme HSP BL Lac objects, 1ES 1101-232 and RGB J0710+591. By applying and refining a stratified shock model, the authors demonstrate that the observed polarization behavior—high X-ray polarization with modest optical polarization in 1ES 1101-232, and an X-ray polarization upper limit near the optical level in RGB J0710+591—can be reconciled within a single framework sensitive to viewing angle and magnetic-field stratification. The results challenge the simple monotonic increase of polarization with frequency and underscore the need for multiwavelength polarization data to constrain jet physics in EHBLs. The improved model highlights how the interplay of a self-generated, decaying perpendicular field and downstream cooling can reproduce diverse polarization trends, with implications for the geometry and magnetization of relativistic jets in EHBLs.

Abstract

Multiwavelength polarimetry is a powerful tool to probe magnetic field and flow geometries in the relativistic jets of blazars. In this respect, particularly interesting are the sources whose synchrotron emission covers a broad range of frequencies, from radio to X-rays, such as the BL Lac objects of the HSP type. Previous measurements including radio, optical and X-ray data show a clear trend, with the degree of polarization increasing with frequency. Here we report radio, optical and X-ray observations ($Swift$, $Nustar$ and $IXPE$) of 1ES 1101-232 and RGB J0710+591, two blazars belonging to the puzzling subclass of extreme BL Lacs (EHBL). For 1ES 1101-232 we found a strong frequency-dependency of the degree of polarization (with a ratio $Π_X/Π_O\simeq 5.2$). For RGB J0710+591, IXPE derived a 1$σ$ upper limit $Π_X<11.6\%$, comparable with the measured optical degree of polarization (average $Π_O\sim 12\%$). We discuss the results in the framework of current interpretations and, in particular, we report an improved version of the stratified shock model that is able to reproduce the observed data of both sources.

Figures (8)

• Figure 1: Detection significance of X-ray polarization for 1ES 1101-232 (left) and RGB J0710+591 (right), measured from the spectropolarimetric analysis. The tangential direction indicates the polarization angle, while the radial direction corresponds to the polarization degree. The colored dot at the center of each contour denotes the measured X-ray polarization. Each contour plot represents the detection confidence levels at 68%, 90%, and 99%.
• Figure 2: Light curve of the IXPE observation of RGB J0710+591. The green and orange shaded areas indicate periods 1 and 2, respectively, which were split based on the observation gap in the middle. The central green dashed line represents the average count rate of RGB J0710+591 during the IXPE observation.
• Figure 3: A joint spectropolarimetric analysis of 1ES 1101-232. The left panel presents the energy flux spectra, expressed as photon flux multiplied by energy squared, for the Swift (red), IXPE I (yellow), and NuSTAR (blue) data together with the corresponding data-model residuals. The black line indicates the best-fit model. The right panel displays the IXPE Q and U spectra, shown in green and orange, respectively, together with their deviations from the best fit model.
• Figure 4: A joint spectropolarimetric analysis of RGB J0710+591. The panels follow the same format as Figure \ref{['fig:xspec_joint_1es']}.
• Figure 5: Optical polarization observations of RGB J0710+591. The top panel shows the brightness in different optical bands, the middle panel the polarization degree in the R-band, and the bottom panel the polarization angle. The grey shaded area marks the duration of the IXPE observation, whereas the horizontal green band marks the direction of jet as projected on the sky.