The Fermi blazar sequence

• Published in: Monthly notices of the Royal Astronomical Society 2017-07, Vol.469 (1), p.255-266
• Main Authors: Ghisellini, G., Righi, C., Costamante, L., Tavecchio, F.
• Format: Article
• Language: English
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stx806

Abstract We revisit the blazar sequence exploiting the complete, flux-limited sample of blazars with known redshift detected by the Fermi satellite after 4 yr of operations (the 3LAC sample). We divide the sources into γ-ray luminosity bins, collect all the archival data for all blazars, and construct their spectral energy distribution (SED). We describe the average SED of blazars in the same luminosity bin through a simple phenomenological function consisting of two broken power laws connecting with a power law describing the radio emission. We do that separately for BL Lacs and for flat spectrum radio quasars (FSRQs) and also for all blazars together. The main results are: (i) FSRQs display approximately the same SED as the luminosity increases, but the relative importance of the high-energy peak increases; (ii) as a consequence, the X-ray spectra of FSRQs become harder for larger luminosities; (iii) BL Lacs indeed form a sequence: they become redder (i.e. smaller peak frequencies) with increasing luminosities, with a softer γ-ray slope and a larger dominance of the high-energy peak; (iv) for all blazars (BL Lacs+FSRQs), these properties become more prominent, as the highest luminosity bin is populated mostly by FSRQs and the lowest luminosity bin mostly by BL Lacs. This agrees with the original blazar sequence, although BL Lacs never have an average γ-ray slope as hard as found in the original sequence. (v) At high luminosities, a large fraction of FSRQs show signs of thermal emission from the accretion disc, contributing to the optical–UV (ultraviolet).