2024ApJ...963L..41H

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 Π_X = 21.9% ± 1.9% (MDP₉₉ ∼ 6.0%). An even higher polarization is observed in the 3-4 keV band, reaching Π_X = 28.6% ± 2.7% (MDP₉₉ ∼ 8.1%) with a confidence level of 10.8σ. 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 Π_X = 12.4% ± 0.7% (MDP₉₉ ∼ 2.2%), with an electric vector position angle (EVPA) of ψ_X = 19.°7 ± 1.°6. The X-ray polarization of 1ES 1959+650 exhibits evident variability, accompanied by the variations of ψ_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.