Clues on the X-ray emission mechanism of blazars PKS 2155$-$304 and 3C 454.3 through polarization studies

Abstract

X-ray polarization measurable with the imaging X-ray Polarimetry Explorer (\textit{IXPE}) could constrain the long-debated leptonic versus hadronic origin of the high-energy component in the broadband spectral energy distribution (SED) of blazars. We report \textit{IXPE} results and SED modeling of PKS 2155$-$304 and 3C 454.3, a high- and low-synchrotron-peaked blazar. For PKS 2155$-$304, model-independent analysis gives polarization angle $\Psi_X$ = (130$\pm$2.5) deg and polarization degree $\Pi_X$ = (20.9$\pm$1.8)\% in the 2$-$8 keV band, in agreement with spectro-polarimetric analysis. We found $\Pi_X$ varies with time and shows indications of energy dependence, suggesting stratified emission regions. For 3C 454.3, no X-ray polarization is detected in the June 2023 observation, analyzed here for the first time. The detection in PKS 2155$-$304 and non-detection in 3C 454.3 are consistent with X-ray emission from synchrotron and inverse Compton processes, respectively. Dividing the dataset into finer time bins allows a more granular view of polarization variability. We modeled the broadband SEDs using quasi-simultaneous optical, UV, and X-ray data from {\it Swift}, {\it AstroSat}, and $\gamma$-rays from {\it Fermi}. In PKS 2155$-$304, X-rays lie in the high-energy tail of the synchrotron component, while in 3C 454.3 they lie in the rising part of the inverse Compton component. Our SED modeling with X-ray polarization favors a leptonic scenario for PKS 2155$-$304. These results support a structured jet model where X-ray emission originates from a compact acceleration zone near the shock front, while lower-energy optical emission comes from a broader turbulent region.