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

TL;DR

This paper investigates the invisible plasma content in blazar jets, particularly in Mrk 421, by analyzing dynamical and radiative processes to infer the presence of significant unseen plasma components.

Contribution

It introduces a method to constrain plasma density and composition in blazar jets using broadband spectra and shock physics, revealing substantial invisible plasma presence.

Findings

The shocks in Mrk 421 are mildly relativistic with Lorentz factors between 2 and 20.

The plasma density in shocked shells exceeds non-thermal electron density by 10^2 to 10^3 for pairs.

The plasma density exceeds non-thermal electron density by 10^2 to 10^6 for electron-proton content.

Abstract

Invisible plasma content in blazar jets such as protons and/or thermal electron-positron ($e^{\pm}$) 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 broadband spectra for Mrk 421, we obtain that the ratio of the Lorentz factors of a pair of cold shells resides in about $2\sim 20$, which implies that the shocks are at most mildly relativistic. Using the obtained Lorentz factors, the total mass density $\rho$ in the shocked shells is investigated. The upper limit of $\rho$ is obtained from the condition that thermal bremsstrahlung emission should not exceed the observed $\gamma$-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 $\rho$ is $10^2$-$10^3$ times heavier than that of non-thermal electrons for pure $e^{\pm}$ pairs, while $10^2$-$10^6$ 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 shortly discussed and we speculate that the total mass density and/or the blending ratio of $e^{\pm}$ pairs and $e/p$ plasma could be new key quantities for the origin of the sequence.