On invisible plasma content in radio-loud AGNs: The case of TeV blazar Markarian 421

Abstract

Invisible plasma content in blazar jets such as protons and/or thermal electron-positron (e±) pairs is explored through combined arguments of dynamical and radiative processes. By comparing physical quantities required by the internal shock model with those obtained through the observed broad-band spectra for Mrk 421, we obtain that the ratio of the Lorentz factors of a pair of cold shells resides in about 2 ∼ 20, which implies that the shocks are at most mildly relativistic. Using the obtained Lorentz factors, the total mass density ρ in the shocked shells is investigated. The upper limit of ρ is obtained from the condition that thermal bremsstrahlung emission should not exceed the observed γ-ray luminosity, whilst the lower limit is constrained from the condition that the energy density of non-thermal electrons is smaller than that of the total plasma. Then, we find ρ is 10 2-103 times heavier than that of non-thermal electrons for pure e± pairs, while 102-106 times heavier for pure electron-proton (e/p) content, implying the existence of a large amount of invisible plasma. The origin of the continuous blazar sequence is briefly discussed and we speculate that the total mass density and/or the blending ratio of e± pairs and e/p plasma could be new key quantities for the origin of the sequence. © 2007 The Authors. Journal compilation © 2007 RAS.