The sequence of Compton dominance in blazars based on data from WISE and Fermi/LAT

Abstract

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 nu_syn is anticorrelated with the synchrotron luminosity L_syn, and (2) the luminosity ratio of the high-energy (inverse Compton) to synchrotron components q = L_IC / L_syn (Compton dominance) increases with L_syn 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 Gamma_W12 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 (L_W1 ~ 10^44 erg/s and Gamma_W12 < 1); (2) Gamma_W12 is correlated with the gamma-ray photon index Gamma_1-100GeV, with the MIR luminosity L_W1, and with the Fermi/WISE Compton dominance q_FW = L_1GeV/L_W1; (3) a clean separation between FSRQs and BL Lacs can be seen in the parameter space of Gamma_W12 and q_FW; (4) the observed distribution of MIR luminosity L_W1 vs. Compton dominance q_FW for the entire sample of blazars can be modeled as a sequence of lepto-magnetic jet powers in the range (log_10 P_eB) in [42:45] with the preference for sub-equipartition magnetic fields P_B/P_e in [0.05:1], assuming fixed bulk Lorentz factor Gamma_j = 15, fixed jet opening angle Gamma_j Theta_j = 0.3, fixed radiative efficiency of jet electrons epsilon_em = 50%, and that external radiation luminosity scales like L_ext propto (P_eB)^(1.6) (parameter degeneracies are discussed).