An Empirical "High-confidence" Candidate Zone for Fermi BL Lacertae Objects

In the third catalog of active galactic nuclei detected by the Fermi Large Area Telescope Clean (3LAC) sample, there are 402 blazar candidates of uncertain type (BCU). The proposed analysis will help to evaluate the potential optical classification flat spectrum radio quasars (FSRQs) versus BL Lacertae (BL Lac) objects of BCUs, which can help to understand which is the most elusive class of blazar hidden in the Fermi sample. By studying the 3LAC sample, we found some critical values of γ-ray photon spectral index (Γ_ph), variability index (VI), and radio flux ($F_R$) of the sources separate known FSRQs and BL Lac objects. We further utilize those values to defined an empirical "high-confidence" candidate zone that can be classified as BCUs. Within such a zone (Γ_ph < 2.187, log F _R < 2.258, and log VI < 1.702), we found that 120 BCUs can be classified as BL Lac object candidates with a higher degree of confidence (with a misjudged rate <1%). Our results suggest that an empirical "high-confidence" diagnosis is possible to distinguish the BL Lac objects from the Fermi observations based on only the direct observational data of Γ_ph, VI, and F _R.