GeV-TeV $\gamma$-ray energy spectral break of BL Lac objects

Abstract

In this paper, we compile the very-high-energy and high-energy spectral indices of 43 BL Lac objects from the literature. Based on a simple math model, $\Delta {\Gamma }_{obs}=\alpha \mathrm{z}+\beta$, we present evidence for the origin of an observed spectral break that is denoted by the difference between the observed very-high-energy and high-energy spectral indices, $\Delta {\Gamma }_{obs}$. We find by linear regression analysis that $\alpha \ne 0$ and $\beta \ne 0$. These results suggest that the extragalactic background light attenuation and the intrinsic curvature dominate on the GeV-TeV $\gamma$-ray energy spectral break of BL Lac objects. We argue that the extragalactic background light attenuation is an exclusive explanation for the redshift evolution of the observed spectral break.

8 pages, 5 figures, accepted for publication in APSS