Abstract Using a large sample of blazars of the Fermi observations presented by Abdo et al., we constructed a sample of blazars including high energy peaked BL Lac objects (HBLs), low energy peaked BL Lac objects (LBLs) and flat-spectral radio quasars (FSRQs). These unique characteristics make it possible to unambiguously address the question of how HBLs, LBLs and FSRQs are related. In this paper, we investigated the relationship between X-ray and γ-ray spectral indices (αx –αγ), as well as the relationship between the broadband spectral indices (αro –αrx, αro –αox, αro –αxγ and αrx –αxγ) for this sample. The color-color diagram shows that there is a significant correlation between both quantities when all three subclasses of blazars are considered, which suggests that there is a unified scheme for blazars. On the other hand, the αx –αγ diagram reveals that three kinds of blazars have different spectral energy distributions: the trend of HBLs is different from that of FSRQs and LBLs, whereas FSRQs and LBLs have a similar trend, which hints that FSRQs and LBLs have similar spectral properties, but HBLs have distinct spectral properties. In addition, the broadband energy distributions also reveal the similar spectral properties with that of the αx –αγ diagram. The spectral properties revealed from the Fermi sample do not support the blazar sequence reported by Fossati et al. and Ghisellini et al.

Keywords galaxies: active — BL Lacertae objects: general — galaxies: fundamental parameters — quasars: general

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