The gamma-ray photon spectral index view of transition blazars

ABSTRACT The Fermi Large Area Telescope (LAT) has detected a large number of $\gamma$-ray-emitting sources, among which jet-dominated active Galactic nuclei (AGNs) 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 behaviours between these two sub-classes, making their classification less straightforward. In this study, we investigate transition jet-dominated AGNs from the perspective of the $\gamma$-ray photon spectral index ($\Gamma_\gamma$) using a sample of 3934 sources from the fourth catalogue of AGNs detected by Fermi-LAT (4LAC). We employ a Gaussian Mixture Model (GMM) to classify these sources on the basis of their $\Gamma_\gamma$. We suggest that the photon indices of transition AGNs mainly fall between those of BL Lacs and flat-spectrum radio quasars, indicating clear intermediate features. We conclude that the $\Gamma_\gamma$provides a useful and physically meaningful diagnostic tool for identifying and characterizing transition jet-dominated AGNs.