Probing the strength of the intergalactic magnetic field through high-energy gamma ray observations and modelling of hard and non-variable blazar sources

Abstract

Galaxies and galaxy clusters are separated by large distances of nearly empty regions called the intergalactic space. In these regions a weak magnetic field is thought to be present that is predicted to be of a primordial (early universe) origin. This is called the intergalactic magnetic field (IGMF) and knowledge about its strength, coherence length, origin etc. is limited. Understanding the origin of the IGMF is crucial because of the impact it may have had on early star and galaxy formations. It has been proposed that the IGMF can be indirectly constrained through gamma-ray observations and SED analysis in the GeV-TeV energy range of high redshift sources. TeV photons will undergo γγ absorption with the extra-galactic background light (EBL) and could initiate a cascade producing more emission at lower energies. However the IGMF could deflect the electrons from the initial path and, therefore, the GeV-TeV energy emission may indirectly measure the magnetic field strength. In this paper, seven hard and non-variable BL Lacs were selected and reanalysed with the improved Pass 8 analysis pipeline. Using previous Imaging Atmospheric Cherenkov Telescopes (IACTs) observational results, the secondary cascade component was modelled using the ELMAG code of Kachelrieß (2012) and the Kneiske (2004) EBL model. From this, the total spectrum was determined by the sum of the cascade and primary intrinsic spectra, and consequently was compared to the Fermi-LAT spectrum, allowing constraints to be placed on the strength of the IGMF. We assumed that there is indeed a contributing cascade spectrum superimposed on the primary intrinsic spectrum and consequently used three different scenarios to constrain the IGMF strength namely: cascade dominant, primary intrinsic dominant and an in-between scenario. All three scenarios were modelled with the cascade spectrum. In addition, the primary intrinsic dominant scenario was also modelled with a power law (PL) model and the in-between scenario was modelled with broken power law (BPL) model. The modelling allowed us to preliminary determines a lower limit (cascade dominant), the upper limit (primary intrinsic dominant), and a best-fit (in-between) value for the IGMF strength. Consequently, the IGMF strength was preliminary constrained with lower limits between $4 \times 10^{-17} < B_{IGMF} (G) < 1 \times 10^{-15}$ and for a best-fit the IGMF strength was preliminary found to be $B_{IGMF} = (6 \pm 1) \times 10^{-17}$ G.