The sequence of Compton dominance in blazars based on data from WISE and Fermi-LAT
Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw, Poland
Received: 7 June 2017
Accepted: 6 August 2017
The two-component broad-band spectral energy distributions of blazars were suggested to form a sequence in which (1) the peak frequency of the low-energy (synchrotron) component νsyn is anticorrelated with the bolometric synchrotron luminosity Lsyn and (2) the luminosity ratio of the high-energy (inverse Compton) to synchrotron components q = LIC/Lsyn (Compton dominance) increases with Lsyn from the BL Lac objects (BL Lacs) to the flat spectrum radio quasars (FSRQs). The Compton dominance parameter is an important probe of plasma magnetisation in the blazar zones within relativistic jets. We investigate a sample of blazars detected by WISE in the mid-infrared (MIR) band and by Fermi-LAT in the GeV gamma-ray band, with the focus on the distribution of luminosities and photon indices. Our findings are the following: (1) the MIR photon index ΓW12 is a useful probe of the blazar sequence, with the exception of low-luminosity BL Lacs that are most likely contaminated by their host galaxies (LW1 ~ 1044erg s-1 and ΓW12< 1); (2) ΓW12 is correlated with the gamma-ray photon index Γ1 − 100GeV, with the MIR luminosity LW1, and with the Fermi/WISE Compton dominance qFW = L1GeV/LW1; (3) a clean separation between FSRQs and BL Lacs can be seen in the parameter space of ΓW12 and qFW; and (4) the observed distribution of MIR luminosity LW1 versus Compton dominance qFW for the entire sample of blazars can be modelled as a sequence of lepto-magnetic jet powers in the range log 10PeB ∈ [ 42:45 ] with the preference for sub-equipartition magnetic fields PB/Pe ∈ [ 0.05:1 ], assuming fixed bulk Lorentz factor Γj = 15, fixed jet opening angle ΓjΘj = 0.3, fixed radiative efficiency of jet electrons ϵem = 50%, and that external radiation luminosity scales like Lext ∝ PeB1.6 (parameter degeneracies are discussed).
Key words: quasars: general / BL Lacertae objects: general / galaxies: jets
© ESO, 2017