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

Multiwavelength polarimetry offers a powerful tool to probe magnetic field and flow geometries in the relativistic jets of blazars. Sources with synchrotron emission that spans a broad frequency range, from radio to X-rays, such as High Synchrotron Peak (HSP) type BL Lac objects, are particularly interesting. 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 find a strong frequency dependence of the degree of polarization, with a ratio Π_X/Π_O ≃ 5.2. For RGB J0710+591, IXPE derives a 1σ upper limit Π_X < 11.6%, comparable to the measured optical degree of polarization (average Π_O ∼ 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 reproduces the observed data of both sources.