Effective spectral index properties for Fermi blazars

Abstract

Blazars are a special subclass of active galactic nuclei with extreme observation properties. This subclass can be divided into two further subclasses of flat spectrum radio quasars (FSRQs) and BL Lacertae objects (BL Lacs) according to their emission line features. To compare the spectral properties of FSRQs and BL Lacs, the 1.4 GHz radio, optical R-band, 1 keV X-ray, and 1 GeV γ-ray flux densities for 1108 Fermi blazars are calculated to discuss the properties of the six effective spectral indices of radio to optical (α_RO), radio to X-ray (α_RX), radio to γ ray (α_Rγ), optical to X-ray (α_OX), optical to γ ray (α_Oγ), and X-ray to γ ray (α_Xγ). The main results are as follows: For the averaged effective spectral indices, $\overline {{\alpha _{OX}}} > \overline {{\alpha _{O\gamma }}} > \overline {{\alpha _{X\gamma }}} > \overline {{\alpha _{R\gamma }}} > \overline {{\alpha _{RX}}} > \overline {{\alpha _{RO}}}$ for samples of whole blazars and BL Lacs; $\overline {{\alpha _{X\gamma }}} \approx \overline {{\alpha _{R\gamma }}} \approx \overline {{\alpha _{RX}}}$ for FSRQs and low-frequency-peaked BL Lacs (LBLs); and $\overline {{\alpha _{OX}}} \approx \overline {{\alpha _{O\gamma }}} \approx \overline {{\alpha _{X\gamma }}}$ for high-synchrotron-frequency-peaked BL Lacs (HBLs). The distributions of the effective spectral indices involving optical emission (α_RO, α_OX, and α_Oγ) for LBLs are different from those for FSRQs, but if the effective spectral index does not involve optical emission (α_RX, α_Rγ, and α_Xγ), the distributions for LBLs and FSRQs almost come from the same parent population. X-ray emissions from blazars include both synchrotron and inverse Compton (IC) components; the IC component for FSRQs and LBLs accounts for a larger proportion than that for HBLs; and the radiation mechanism for LBLs is similar to that for FSRQs, but the radiation mechanism for HBLs is different from that for both FSRQs and LBLs in X-ray bands. The tendency of α_Rγ decreasing from LBLs to HBLs suggests that the synchrotron self-Compton model explains the main process for highly energetic γ rays in BL Lacs.