Magnetic fields are omnipresent in the universe on all scales from magnetic moments of elementary particles to large-scale fields in the intergalactic space of clusters of galaxies. Astrophysical magnetic fields in galaxies and galaxy clusters are ubiquitously observed and are believed to have been seeded by an intergalactic magnetic field (IGMF), which should still be present at its seed field strength in cosmic voids. Such a field should have a small field strength, making it extremely difficult to observe directly.
The overarching goal of the project is to probe the IGMF through observations of so-called blazars. These active galaxies produce jets of relativistic plasma outflows which point towards the observer. These blazars can produce highly energetic gamma ray emission, roughly a million times more energetic than X-rays. The gamma rays can interact with background radiation fields and produce electron-positron pairs. In turn, these pairs can scatter photons of the cosmic microwave background (CMB) up to gamma-ray energies, thereby initiating an electromagnetic cascade. Deflections of these pairs in the IGMF cause extended halos around otherwise pointlike blazars. These halos can be searched for using observations of the Fermi Large Area Telescope (LAT) and imaging air Cherenkov Telescopes (IACTs) such as the High Energy Stereoscopic System (H.E.S.S.).
Detecting or significantly constraining the IGMF would be revealing about processes in the early Universe; it would be possible to determine whether the IGMF was created during inflation or phase transitions in the early Universe and to probe contributions of beyond-the-standard-model physics during these epochs. Since it acts as a seed for fields in galaxies and galaxy clusters, its determination would be crucial for understanding the formation of such structures.
In order to reach the goal to either detect or constrain the IGMF through a search of the halo with gamma-ray observations, several objectives have been formulated. In order to achieve the highest sensitivity, the gamma-ray data from different instruments should be analyzed jointly in a uniform fashion. To this end, a joint analysis of Fermi-LAT data and IACT data has to be developed building on the already available gammapy software. The analysis results obtained with gammapy need to be cross check with instrument specific analysis tools. The halo emission needs to be modeled through Monte Carlo simulations in order to fully capture its shape and spectrum. Therefore, the another objective is to build a library of cascade templates that can then be confronted with Fermi-LAT and H.E.S.S. data. Lastly, the objective was also to assess the sensitivity of the upcoming Cherenkov Telescope Array (CTA) to the detection of the cascade signal.
In conclusion, most of these objectives were fulfilled during the project period. Mainly due to an early termination of the project (due to the beginning of an ERC starting grant) not all final results are published in scientific journals yet. However, all the software machinery is in place, the gammapy analysis is validated, and the cascade templates have been generated.
