Ultra-extreme high-frequency-peaked BL Lacs: A potential population of MeV synchrotron blazars

Abstract

We investigate the potential existence of a new population of BL Lacs, called ultra-extreme high-energy-peaked BL Lacs (UEHBLs), whose synchrotron emission component peaks in the MeV band, extending the blazar sequence beyond its current limit. To model the spectral energy distribution of these new sources, we applied the hybrid shock-turbulence acceleration framework previously developed for extreme high-frequency-peaked BL Lacs. We present three representative realizations that produce synchrotron peaks between $0.2$ and $2$ MeV and evaluate their multiwavelength signatures. Our results show that UEHBLs would be undetectable with current GeV (Fermi) and future TeV (CTA) facilities due to severe Klein-Nishina suppression of inverse-Compton scattering, but are ideal targets for proposed MeV missions such as COSI, AMEGO-X, and e-ASTROGAM. We further identified a sample of hard X-ray sources from the Swift-BAT catalogs that exhibit the spectral properties expected of UEHBLs, representing promising follow-up targets of this population. If confirmed, UEHBLs would provide unique insight into particle acceleration in relativistic jets, imposing strong constraints on the maximum achievable electron energies. We also discuss the expected polarization and variability signatures, including the possibility of synchrotron-driven thermal instabilities leading to MeV flares. These findings underscore the critical importance of the MeV band for discovering and characterizing the most extreme accelerators among blazars.