Constraints on minimum electron Lorentz factor and matter content of jets for a sample of bright Fermi blazars

Abstract

We fit the (quasi-)simultaneous multi-waveband spectral energy distributions (SEDs) for a sample of low-synchrotron-peaked (LSP) blazars with a one-zone leptonic model. The seed photons that predominantly come from broad line region (BLR) and infrared (IR) molecular torus are considered respectively in external Compton process. We find that the modeling with IR seed photons is systematically better than that with BLR photons based on a $χ^2$ test, which suggest that $γ$-ray emitting region most possibly stay outside the BLR. The minimum electron Lorentz factor, $γ_{\rm min}$, is constrained from the modeling for these LSP blazars with good soft X-ray data (ranges from 5 to 160 with a median value of 55), which plays a key role in jet power estimation. Assuming one-to-one ratio of proton and electron, we find that the jet power for LSP blazars is systematically higher than that of FR II radio galaxies at given 151 MHz radio luminosity, $L_{\rm 151MHz}$, even though FR IIs are regarded as same as LSP blazars in unification scheme except the jet viewing angle. The possible reason is that there are some $e^{\pm}$ pairs in the jet of these blazars. If this is the case, we find the number density of $e^{\pm}$ pairs should be several times higher than that of $e^{-}-p$ pairs by assuming the jet power is the same for LSP blazars and FR IIs at given $L_{\rm 151MHz}$.