Discerning the Very-high-energy Component via the Variation of γ-Rays in PKS 1510−089

Abstract

The discovery and identification of very-high-energy (VHE) γ-ray emission of blazars is important for understanding active galactic nucleus jets. Based on Fermi-LAT observations, we find that PKS 1510−089 exhibits a softer-when-brighter trend at the low γ-ray flux level and changes to the harder-when-brighter trend at the high γ-ray flux level. Such a trend transition could be naturally explained by the presence of the VHE component, recently confirmed by the H.E.S.S. Collaboration. To reproduce the trend transition, we construct a two-component model that includes the high-energy (HE) component and the VHE component and consider two variation mechanisms including the change of the Doppler factor and the intrinsic peak frequency. The result is that such a trend transition in the Fermi-LAT range can be better explained by the shift of the peak frequency of the HE component. Combining the broadband spectral energy distribution analysis, we suggest that the HE component may originate from the jet at the broad-line region scale, and the VHE component may originate from the jet at the dust torus scale. The advantage of the spectral variation method enables us to discover the rich VHE emission across blazars.