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

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 Letter, 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 X = 21.9% +/- 1.9% (MDP99 similar to 6.0%). An even higher polarization is observed in the 3-4 keV band, reaching pi X = 28.6% +/- 2.7% (MDP99 similar to 8.1%) with a confidence level of 10.8 sigma. Furthermore, no polarization is detected above the 5 keV energy band. For 1ES 1959+650, the highest detected polarization degree in the 2-8 keV band is pi X = 12.4% +/- 0.7% (MDP99 similar to 2.2%), with an electric vector position angle (EVPA) of psi X = 19.degrees 7 +/- 1.degrees 6. The X-ray polarization of 1ES 1959+650 exhibits evident variability, accompanied by the variations of psi 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.