A parametric study of plasma instability cooling and its impact on intergalactic magnetic field constraints in GeV cascades

TL;DR

This study models plasma instability effects on electromagnetic cascades from TeV gamma rays in the intergalactic medium, constraining the intergalactic magnetic field strength using observational data.

Contribution

It introduces a parameterized plasma instability model into cascade simulations and derives new constraints on the intergalactic magnetic field from blazar observations.

Findings

Plasma instabilities with ~100 kpc scale can explain observed spectra.

The IGMF strength is constrained to around 10^{-17} G.

The model fits observational data across different viewing angles.

Abstract

Electromagnetic cascades are initiated by TeV gamma rays propagating through the intergalactic medium (IGM), and they can be used to constrain the weak intergalactic magnetic field (IGMF) in cosmic voids. Primary TeV photons produce electrons and positrons through electromagnetic pair production, which can be deflected out of the line-of-sight to the observer by IGMF. In addition, electron-positron pairs can perturb the IGM, triggering plasma instabilities that can cool down the pairs before they upscatter cosmic background photons to GeV energies via inverse Compton (IC) scattering. We investigate the influence of plasma instabilities on the cascade spectrum by introducing a parameterized model for the instability using a publicly available Monte Carlo framework CRPropa. We use extended-emission observations within the field of view of the observer to constrain the IGMF in the presence of plasma instability cooling. Based on spectral observations of the blazar 1ES 0229+200 from Fermi-LAT, we find the best-fit photon spectrum including the plasma instability and IGMF parameters that reproduces the observational data for different observer field-of-view angles and obtain the IGMF constraint in cosmic voids. We find that plasma instabilities with a characteristic length scale of order $10^{2}~\text{kpc}$ reproduce the observed photon spectrum and imply an IGMF strength of order $10^{-17}~\text{G}$.