Identification of cosmic ray sources amongst jetted galaxies

Cosmic Rays (CRs) are extremely energetic and mostly charged particles of cosmic origin. The identity of the CR sources are to date not known. A dedicated method is developed in this project with the goal to contribute to the identification of the sources of the ultra-high energy (UHE, = 10 EeV) cosmic rays. Recent results from UHECR measurements favor conventional scenarios of UHECR production at astrophysical sites. Prominent jet outflows in powerful active galactic nuclei (AGN) provide suitable conditions and energetics for charged particle acceleration up to the highest energies, and are therefore considered prime contributors to the observed UHECR flux. During acceleration of the CRs hadronic interactions occur where photons and neutrinos are produced. As neutral messengers they provide information about CR source direction on the sky, and the physical conditions at the CR production site, while the charged CRs suffer from loss of directional information by deflecting at cosmic magnetic fields. To gain insights into the identity of the UHECR sources and the dominating acceleration and emission mechanisms at source we therefore employ a multi-messenger, multi-wavelength approach including Pierre Auger Observatory measured UHECR events, high- (HE, = 100 MeV) and very-high energy (VHE, = 30 GeV) gamma-ray emission collected by the Fermi-LAT detector and ground-based Cherenkov telescopes, and VHE neutrinos observed with the IceCube experiment. In the proposed multistep procedure first all nearby AGN are classified according to their capability to accelerate particles to UHEs. For this purpose we use multi-frequency measurements of them complemented with energetics arguments to build a list of suitable nearby UHECR AGN candidates. Auger data point to heavy mass nuclei composition at extreme energies. Therefore current state-of-the art AGN emission models will be extended by the Austrian team members to allow the treatment of heavy nuclei. Subsequent simulations of possible UHECR arrival directions, given this candidate list, for various cosmic magnetic field configurations using a propagation code (developed by the Slovenian project partners) will predict the UHECR patterns observed at Earth, to be cross-correlated with Auger data. In a complementary approach the expected signatures of the produced photon and neutrino emission are analyzed in conjunction with LAT- and neutrino data. The results of this study are finally fed into predictions of characteristic features that signal UHECRs in AGN jets and that is verifiable by the near- future Cherenkov Telescope Array (CTA). Combining the expertise of the proposing project teams in this project will therefore lead to valuable results towards identifying the source origin of the UHECRs and also optimize the CTA observation strategies for this science case.

The origin of the cosmic-ray particles impinging on Earth, in particular those of extreme energies, is largely not identified so far. Jets of Active Galactic Nuclei (AGN) are considered prime candidate sources that may contribute to the observed flux of cosmic rays (CRs) reaching into the ultra-high energy (UHE, >10EeV) domain. By a multi-messenger multi-wavelength approach this joint Austrian-Slovenian project contributed to assess low-luminosity jetted AGN as possible UHECR sources, with a particular focus on the recently discovered Fanaroff-Riley 0 (FR 0) galaxies which constitute the most abundant persistent jet source population in the local Universe. For this purpose we developed state-of-the-art simulation frameworks that model cosmic-ray particle and photon propagation and multi-messenger production within jets and jet-accretion flow systems of AGN, and that propagate UHE cosmic-ray nuclei and secondary cosmogenic photons and neutrinos from FR0 galaxies to Earth through intergalactic environments. Both simulation chains were used to assess the survival, workings and multi-messenger signatures of UHECRs in those environments, and confront the resulting energy spectra and composition on Earth with the current observational situation. Using these tools and a combined analysis of broadband photon and Pierre Auger Observatory (PAO) CR data we appraised FR0 galaxies as viable and possibly significant contributors to the CR-flux up to energies of several EeV and particle multi-messenger emitters. Our modelling implies a proton-dominated composition of CRs that have escaped the source, while inside the source charged particles survive and could gain energy up to tens of EeV favourably through the gradual shear acceleration mechanism with bulk flows within structured jets of mildly-relativistic speeds being sufficient. If CR protons are accelerated in jets of FR0 galaxies up to EeV energies, our multi-messenger modelling further predicts FR0s as weak >VHE-neutrino emitters, an extremely faint photon flux from this source class in the energy range accessible to the Cherenkov Telescope Array (CTA), and potentially observable, distinct photon signatures at MeV energies from their core region that includes the accretion flow. The high predictive power of our modeling results allows for near-future observational tests of the UHECR-hypothesis of FR0s, and motivates hadronic non-thermal AGN emission model developments that take into account jet structures.