Abstract
This paper discusses the spectral energy distribution (SED) of all blazars with redshift detected by the Fermi satellite and listed in the 3LAC catalog. The so called “blazar sequence” from the phenomenological point of view will be updated, with no theory or modelling. It will be shown that: (i) pure data show that jet and accretion power are related; (ii) the updated blazar sequence maintains the properties of the old version, albeit with a less pronounced dominance of the γ-ray emission; (iii) at low bolometric luminosities, two different types of objects have the same high energy power: low black hole mass flat spectrum radio quasars and high mass BL Lacs. Therefore, at low luminosities, there is a very large dispersion of SED shapes; (iv) in low power BL Lacs, the contribution of the host galaxy is important. Remarkably, the luminosity distribution of the host galaxies of BL Lacs are spread in a very narrow range; (v) a simple sum of two smoothly joining power laws can describe the blazar SEDs very well.
Highlights
About 10% of Active Galactic Nuclei have relativistic jets whose emission is strongly boosted.When pointing at us, these jetted sources are called blazars
Blazars come in two varieties: BL Lacs, with weak or absent broad emission lines; and Flat Spectrum Radio Quasars, with strong broad emission lines
The original phenomenological blazar sequence was a function of only one parameter: the observed bolometric luminosity
Summary
About 10% of Active Galactic Nuclei have relativistic jets whose emission is strongly boosted. Compton (IC) process can use these seed photons (produced externally to the jet) to produce a very powerful high energy luminosity This implies strong radiative cooling, which inhibits the emitting electrons from reaching very high energies. The radiative cooling rate is weaker, allowing the emitting electrons to reach high energies, producing a “blue” spectrum This scenario predicted that low luminosity BL Lacs should be the bluest blazars, and relatively strong TeV emitters. The original phenomenological blazar sequence was a function of only one parameter: the observed bolometric luminosity (which correlates with the radio one) This is likely the result of considering only a small sample of blazars that is inevitably biased towards the most luminous sources. The information on the flux at other wavelengths provided especially by the SDSS survey and the Planck, WISE, and Swift satellites
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