Letter to the Editor

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

¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate, Italy
² Dipartimento di Fisica, Universita‘ degli Studi di Genova, Via Dodecaneso 33, I-16146 Genova, Italy
³ Department of Astronomy, Yale University, PO Box 208101, New Haven, CT 06520-8101, USA

^⋆ Corresponding author; fabrizio.tavecchio@inaf.it

Received: 12 December 2024
Accepted: 13 January 2025

Abstract

Many of the blazars observed by Fermi actually have the peak of their time-averaged gamma-ray emission outside the ∼GeV Fermi energy range, at ∼MeV energies. The detailed shape of the emission spectrum around the ∼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 ≈90%) of the available energy may go into bulk quasi-thermal heating of the plasma crossing the shock rather than producing a nonthermal 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 (γ ≲ 10²) particle 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, for example 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 (FSRQs). We show that for the typical physical parameters of FSRQs, the expected spectral signature is located at ∼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.