Ultra-High-Energy Cosmic Rays and other Messengers from Radio Galaxies
Final Report Abstract
We investigated, as the main part of this project, the contribution of radio-loud active galactic nuclei (AGNs) to the observed ultra-high energy cosmic rays (UHECRs) and in addition, we successfully modelled the multimessenger data of NGC 1068 — including the potential high-energy neutrino flux. Methodologically these two sub-projects are quite different: In terms of the UHECRs from radio-loud AGNs we used the Monte-Carlo based, open-source simulation code CRPropa3 to model the propagation from these sources to Earth. With regard to the multi-messenger signal from NGC 1068, we use a semi-analytical model to describe the homogeneous, steady-state non-thermal emission from the AGN corona as well as the circumnuclear starburst ring region of that source. Since the applicant is quite familiar with both methodological approaches, we were able to achieve results that could be published in international peer-reviewed journals on relatively short timescales. Summing up, we were able to show that the inner AGN corona of NGC 1068 is a strong high-energy neutrino source, but to explain the high-energy gamma-ray data we need to account for the circumnuclear starburst ring. Moreover, we pointed out within the scope of this project that the UHECR data — in particular the flux spectrum, the mean of the observed height of the shower maximum, as well as the dipole and quadrupole anisotropy information — can be explained by using less than a handful of local sources. Hereby, it is necessary to account for the individual properties of these sources which can be inferred most reliably from its radio characteristics. It is not possible to conclude a final answer on the origin of UHECRs, due to the current lack of information on the details of the Galactic and especially extra-galactic magnetic field as well as the large uncertainties in the derived chemical composition that results from the observed characteristics of the height of the shower maximum. However, we were able to expose different possible scenarios and moreover, we could exclude large parts of the physical parameter space as well as a major contribution from certain prominent sources such as Cygnus A. Finally, we addressed the curious case of apparently hard source spectra, as inferred from the combination of the observed chemical composition and the flux spectrum. We showed that there is actually no need for these extraordinarily hard source spectra if the UHECR data is produced by a small number of local sources with a limited life-time, as the hardening is in that case a result of the so-called “magnetic horizon” suppression during the propagation. So these outcomes agree with the previous ones, even though we do not account for the anisotropy information in this analysis due to the computational effort. In terms of the main part of this project we conclude that only few, local radio-loud AGNs with individual properties in terms of their CR power, maximal rigidity and life-time are sufficient to explain the UHECR data. Hence, these few sources cannot be responsible for the observed diffuse gamma-ray and high-energy neutrino emission, and future investigations need to clarify if these additional messengers can be used to further constrain the contribution of the large scale distribution of radio-loud AGNs that are embedded in a galaxy cluster environment.
Publications
-
Explaining the UHECR spectrum, composition and large-scale anisotropies with radio galaxies. Journal of Cosmology and Astroparticle Physics, 2022(07), 006.
Eichmann, B.; Kachelrieß, M. & Oikonomou, F.
-
Solving the Multimessenger Puzzle of the AGN-starburst Composite Galaxy NGC 1068. The Astrophysical Journal, 939(1), 43.
Eichmann, Björn; Oikonomou, Foteini; Salvatore, Silvia; Dettmar, Ralf-Jürgen & Tjus, Julia Becker
-
Impact of the finite life-time of UHECR sources. Journal of Cosmology and Astroparticle Physics, 2023(02), 053.
Eichmann, B. & Kachelrieß, M.
