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Probing the origin of intergalactic magnetic fields and cosmic rays with gamma-ray cascades

Periodic Reporting for period 1 - GammaRayCascades (Probing the origin of intergalactic magnetic fields and cosmic rays with gamma-ray cascades)

Reporting period: 2019-10-01 to 2021-09-30

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.
Extensive work has been performed in terms of cross checking data analysis results obtained with gammapy compared to the instrument specific software chains. This verification project led to interesting physics results in itself such as a state-of-the-art description of the high energy emission of the Crab Nebula using a newly developed 3D model of the emission (which is publicly available). The results of this work will be published in an upcoming paper.

Together with an international research team within the H.E.S.S. collaboration, we identified blazars observed with H.E.S.S. that are best suited for the search for a cascade component. The final data sample not only included archival data but also so-far unpublished observations. Thanks to the gammapy analysis I performed of these data, two new sources were firmly detected and their spectra are published in a conference proceeding. I also performed extensive Monte-Carlo simulations to generate templates for the cascade emission for different IGMF strengths. The analysis and simulation code is made publicly available through the GitHub repository. These templates were then compared against the data. The results still need to finalized but they already suggest that no evidence for the cascade is present in the data which in turn provides evidence for high values of the IGMF strength. The cascade templates were also used in a sensitivity study to assess the prospects of the future Cherenkov Telescope Array (CTA) to detect the IGMF. Together with an international research team, which I co-led, we determined that CTA will be able to probe so-far un-tested regions of the IGMF parameter space.

Additionally, I had the opportunity to investigate one mechanism that might be responsible for the particle acceleration in the blazar jet and thereby the production of gamma-ray emission in the first place, namely so-called magnetic reconnection. Together with an international research team, we used theoretical predictions for magnetic reconnection events as input for simulations of Fermi-LAT data. We then tested whether specific features of the magnetic reconnection events could be recovered in actual simulations.

Beyond the publication in peer-reviewed journals, conference proceedings, and public code repositories, the research has been disseminated at in total 5 international conferences (three talks were invited) that were all held online due to the COVID-19 pandemic. I also presented the research at 4 seminars at Universities and at a public lecture at the Nuremberg observatory.
The project has led to significant progress beyond the state of the art. The feasibility of a joint LAT and H.E.S.S. analysis was successfully demonstrated which led to an unprecedented description of the high energy emission of Crab Nebula over 5 orders of magnitude in energy. Furthermore, the project led to the detection of two blazars with H.E.S.S. which show promising spectral properties in terms of searches for the halo. Preliminary results on the halo search suggest no evidence for such a component. This rules out a weak IGMF strength and improves previous constraints. This finding might suggest that other mechanism for the energy loss of the electron-positron pairs are at work such as plasma instabilities. The final results will also provide important input for large scale structure simulations and the propagation of charged particles (cosmic rays) over cosmological distances. Lastly, for the first time, magnetic reconnection observations of the LAT were simulated. The results show that magnetic reconnection can lead to fast variability on time scale of minutes in blazar jets.
Cartoon to illustrate the formation of the electromagnetic cascade together with a halo simulation.