Hunting Very High-Energy ($>$100 GeV) Emitting High-Synchrotron Peaked Blazars

TL;DR

This work targets the identification of VHE-emitting blazars by leveraging ~16 years of Fermi-LAT data to search for VHE activity among high-synchrotron-peaked BL Lacs. By cross-matching the 4FGL-DR4 and 3HSP catalogs and incorporating unassociated Fermi sources, the authors assemble a comprehensive, LAT-based candidate pool and perform a likelihood analysis in the VHE band, correcting for EBL effects where appropriate. The study reports 92 new VHE-emitting HSP blazars (≥5σ) and recovers 52 previously known VHE blazars, with detections spanning quiescent to elevated activity states, and confirms the robustness of most detections against stricter event selections. These results significantly expand the VHE blazar census, offering valuable targets for current and next-generation Cherenkov telescopes and providing a platform to probe particle acceleration, multi-messenger connections, and EBL models in relativistic jets.

Abstract

Very-high energy (VHE; $>$100 GeV) $γ$-ray emission originates via some of the most extreme particle acceleration processes in the universe. Considering beamed active galactic nuclei, i.e., blazars, only a small fraction, mainly high synchrotron peak BL Lacs, have been detected in the VHE band with the ground-based Cherenkov telescopes. We utilized $\sim$16 years of Fermi-Large Area Telescope (LAT) observations in the 0.1$-$2 TeV energy range to systematically search for potential VHE emitters in a sample of high synchrotron peaked ($ν^{\rm peak}_{\rm syn}>10^{15}$ Hz) BL Lac sources. We identified, for the first time, 92 VHE emitting blazars at $\geq 5σ$ confidence level. A significant VHE emission was also detected from 52 sources previously reported as VHE blazars. Comparing with the general blazar population, these VHE emitting blazars are found to be located at low redshifts (mean $z=0.2 \pm 0.1$) and exhibit bright synchrotron emission ($\log F^{\rm peak}_{\rm syn}=-11.2 \pm 0.4$, in erg cm$^{-2}$ s$^{-1}$). We also investigated the coincidence of VHE photon arrivals with the source activity states and found that Fermi-LAT has detected VHE photons during both quiescent and elevated activity epochs. These VHE emitting blazars represent promising targets for current and next-generation ground-based Cherenkov telescopes, and provide powerful laboratories for probing particle acceleration in relativistic jets, testing multi-messenger connections, and constraining extragalactic background light models.