X-Ray Polarization Variability of High Spectral Peak BL Lacertaes: Cases of 1ES 1959+650 and PKS 2155-304

Abstract

The high-energy-peaked BL Lacertae objects (HBLs) are the main targets of the Imaging X-ray Polarimetry Explorer (IXPE) for investigating the mechanisms of radiation and particle acceleration in jets. In this paper, we report the first IXPE observations of two HBLs, 1ES 1959+650 and PKS 2155--304. Both sources exhibit X-ray polarization with a confidence level exceeding 99\%, as well as significant variability in polarization across different time intervals and energy ranges. Notably, PKS 2155--304 demonstrates the highest X-ray polarization among all blazars detected by IXPE within its entire energy band (2--8 keV), with a polarization degree of $\Pi_{\rm X}=21.9\%\pm1.9\%$ (MDP$_{99}\sim$6.0\%). An even higher polarization is observed in the 3--4 keV band, reaching $\Pi_{\rm X}=28.6\%\pm2.7\%$ (MDP$_{99}\sim$8.1\%) with a confidence level of 10.8$\sigma$. Furthermore, no polarization is detected above 5 keV energy band. For 1ES 1959+650, the highest detected polarization degree in the 2--8 keV band is $\Pi_{\rm X}=12.4\%\pm0.7\%$ (MDP$_{99}\sim$2.2\%), with an electric vector position angle (EVPA) of $\psi_{\rm X}=19.7^{\circ}\pm1.6^{\circ}$. The X-ray polarization of 1ES 1959+650 exhibits evident variability, accompanied by the variations of $\psi_{\rm X}$, flux, spectrum, and energy bin. We discuss possible implications of these observational findings, including the variability in polarization, rotation of EVPA, and transition between synchrotron and synchrotron-self-Compton. We speculate that the X-rays observed during different IXPE observations originate from distinct regions in the jet and may involve diverse mechanisms for particle acceleration.