Probing the low-energy particle content of blazar jets through MeV observations

TL;DR

This paper investigates how MeV observations can reveal the low-energy particle populations in blazar jets, especially the thermal-like electron component, which impacts the emitted radiation spectrum.

Contribution

It introduces the potential for MeV spectral observations to detect thermal electron populations in blazar jets, highlighting their influence on emission spectra and the capabilities of upcoming MeV missions.

Findings

Spectral signatures of thermal electron populations are expected around 1 MeV in FSRQs.

Upcoming MeV missions like COSI can constrain the presence of these thermal components.

Thermal electron populations can produce observable excesses over power-law extrapolations in blazar spectra.

Abstract

Many of the blazars observed by Fermi actually have the peak of their time-averaged gamma-ray emission outside the $\sim$ GeV Fermi energy range, at $\sim$ MeV energies. The detailed shape of the emission spectrum around the $\sim$ MeV peak places important constraints on acceleration and radiation mechanisms in the blazar jet and may not be the simple broken power law obtained by extrapolating from the observed X-ray and GeV gamma-ray spectra. In particular, state-of-the-art simulations of particle acceleration by shocks show that a significant fraction (possibly up to $\approx 90\%$) of the available energy may go into bulk, quasi-thermal heating of the plasma crossing the shock rather than producing a non-thermal power law tail. Other ``gentler" but possibly more pervasive acceleration mechanisms such as shear acceleration at the jet boundary may result in a further build-up of the low-energy ($\gamma \lesssim 10^{2}$) electron/positron population in the jet. As already discussed for the case of gamma-ray bursts, the presence of a low-energy, Maxwellian-like ``bump'' in the jet particle energy distribution can strongly affect the spectrum of the emitted radiation, e.g., producing an excess over the emission expected from a power-law extrapolation of a blazar's GeV-TeV spectrum. We explore the potential detectability of the spectral component ascribable to a hot, quasi-thermal population of electrons in the high-energy emission of flat-spectrum radio quasars (FSRQ). We show that for typical FSRQ physical parameters, the expected spectral signature is located at $\sim$ MeV energies. For the brightest Fermi FSRQ sources, the presence of such a component will be constrained by the upcoming MeV Compton Spectrometer and Imager (COSI) satellite.