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

Many of the blazars observed by Fermi actually have the peak of their time-averaged gamma-ray emission outside the similar to GeV Fermi energy range, at similar to MeV energies. The detailed shape of the emission spectrum around the similar to 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 approximate 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 (gamma less than or similar to 10(2)) 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 similar to 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.