Exploring cosmic magnetism with gamma-ray burst afterglow emission

Abstract

The nature and origin of magnetic fields on cosmological scales are still unclear. Magnetic fields detected in galaxies and galaxy clusters are typically interpreted as the result of the amplification of weak seed fields, but their nature remains largely unknown with two scenarios considered: the cosmological and the astrophysical origin. Signatures of magnetization in cosmic voids from observations of very high energy (VHE, E > 100 GeV) photons from extragalactic sources can provide crucial results. Indeed, if a non-negligible intergalactic magnetic field (IGMF) is present in the voids, a time-delayed emission known as pair-echo is expected. The timing and intensity of this signal encode information on the IGMF strength B and properties. Given the recent detection of gamma-ray burst (GRB) afterglows at TeV energies, for this study we performed a detectability study of pair-echo signatures from GRBs. We simulated afterglow emission for different values of the jet kinetic energy (E$_{k,iso}$ = 10$^{49}$ - 10$^{55}$ erg), redshift (z = 0.03 - 1), and lightcurve break times, and estimated the expected pair-echo radiation for IGMF strengths in the range B = 10$^{-19}$ - 10$^{-16}$ G. We investigated the capability of CTAO to detect the resulting emission at tens of GeV. We find that a subsample of GRBs in the z - E$_{k,iso}$ parameter space can produce a detectable pair-echo component for CTAO for all the tested IGMF strengths. A steepening of the GRB afterglow light curve, caused e.g. by an early (0.1 - 1 days) jet break, is a key factor to increase the chances of detection. CTAO observations starting from 10 - 12 h after the GRB trigger and extending up to a few days can provide valuable information on the IGMF.