2025ApJS..278...20D

We present a comprehensive study of the X-ray spectral variability observed in 13 TeV photon-emitting high-energy-peaked BL Lacertae objects (HBLs). These data come from 54 XMM-Newton EPIC-pn pointed observations made during its operational period from 2001 June through 2023 July. We performed spectral studies in the energy range of 0.6-10 keV by fitting X-ray spectra of the pointed observations with power-law and log-parabolic (PL and LP) models. We found at a 99% confidence level that 31 of these X-ray spectra were best fitted with a range of LP models with local photon indices (at 1.0 keV), α ≃ 1.75-2.66, and convex curvature parameter β ≃ 0.02-0.25. PL models with photon index Γ ≃ 1.78-2.68 best described the spectra of 14-pointed observations. Nine PN spectra resulted in negative curvature parameters in fitting an LP model, and eight among them were significant (β ≥ 2β_err). We fitted broken power-law models to these eight X-ray spectra and found spectral hardening in the range of ΔΓ ≃ 0.06-0.54 for these observations. EPIC-MOS spectra were also studied for those eight observations to search for similar trends, and we were able to find them in only two, one observation each of PKS 0548-322 and Mrk 501. This indicates the possibility of the coexistence of an inverse Compton component along with the dominant synchrotron component for these two observations. We also performed correlation studies between various LP spectral parameters and briefly discuss their possible implications.