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

TL;DR

This paper proposes a new class of blazars called ultra extreme high-energy-peaked BL Lacs (UEHBLs) with synchrotron peaks in the MeV range, explores their detectability, and discusses their significance for understanding particle acceleration.

Contribution

It introduces the concept of UEHBLs, models their spectral energy distribution, and identifies potential observational signatures, expanding the blazar sequence beyond current limits.

Findings

UEHBLs have synchrotron peaks between 0.2 and 2 MeV.

They are undetectable by current GeV and TeV facilities due to Klein-Nishina suppression.

Potential observational hints of UEHBLs found in Swift-BAT catalog.

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 apply 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 multiwavelenght 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 identify a sample of hard X-ray sources from the Swift-BAT catalogs that exhibit the expected spectral properties of UEHBLs, potentially representing the first observational hints 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.