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 (IXPE) could constrain the long debated leptonic versus hadronic origin for the high energy component in the broad band spectral energy distribution (SED) of blazars. We report here the results from IXPE and SED modeling of PKS 2155−304 and 3C 454.3, a high and low synchrotron peaked blazar. For PKS 2155−304, from model-independent analysis, we found polarization angle ${\Psi }_X$ = (130 ± 2.5) deg and polarization degree ${\Pi }_X$ = (20.9 ± 1.8)% in the 2−8 keV band in agreement with spectro-polarimetric analysis. We found ${\Pi }_X$ to vary with time and indications of it to vary between energies, suggesting that the emission regions are stratified. For 3C 454.3, we did not detect X-ray polarization in the June 2023 observation, analyzed here for the first time. The detection of X-ray polarization in PKS 2155−304 and its non-detection in 3C 454.3 is in accordance with the X-ray emission from synchrotron and inverse Compton process, respectively, operating in these sources. Further, our division of the dataset into finer time bins allows a more granular view of polarization variability. Additionally, we modeled the broadband SEDs of both the sources using data acquired quasi-simultaneously with IXPE, in the optical, UV and X-rays from Swift, AstroSat and γ-rays from Fermi. In PKS 2155−304, the observed X-ray is found to lie in the high energy tail of the synchrotron component of the SED, while in 3C 454.3 the observed X-ray lies in the rising part of the inverse Compton component of the SED. Our SED modeling along with X-ray polarization observations favor a leptonic scenario for the observed X-ray emission in PKS 2155−304. The SED modeling for these specific IXPE epochs has not been presented before, allowing us to place additional constraints on the physical conditions in the jet. These results strengthen the case for a structured jet model where X-ray emission originates from a compact acceleration zone near the shock front, while lower-energy optical emission is produced in a broader, more turbulent region.