The gamma-ray photon spectral index view of transition blazars

Abstract The Fermi Large Area Telescope (LAT) has detected a large number of γ-ray-emitting sources, among which jet-dominated active galactic nuclei (AGN) are typically classified as BL Lac objects or flat-spectrum radio quasars based on their spectral properties. However, recent observations have revealed a growing number of jet-dominated AGNs exhibiting transition behaviors between these two subclasses, making their classification less straightforward. In this study, we investigate transition jet-dominated AGN from the perspective of the γ-ray photon spectral index (Γγ) using a sample of 3934 sources from the fourth catalog of AGNs detected by Fermi-LAT (4LAC). We employ a Gaussian Mixture Model (GMM) to classify these sources on the basis of their Γγ. We suggest that the photon indices of transition AGNs mainly fall between those of BL Lacs and FSRQs, indicaing clear intermediate features. We conclude that the Γγ provides a useful and physically meaningful diagnostic tool for identifying and characterizing transition jet-dominated AGNs.