Photohadronic emission of VHE gamma rays in blazars and the CTA neutrino target of opportunity program

Abstract

Blazars are an extremely luminous and highly variable type of AGN, which possess a relativistic jet with a small viewing angle towards the observer. Recent results, such as the 3-sigma spatial and temporal coincidence of TXS~0506+056 with the IceCube neutrino alert IC-170922A, have motivated an ongoing discussion of how these astrophysical sources can produce high-energy neutrinos during a flaring state and which scenario can successfully describe the observed gamma-ray behaviour.

In this thesis, the possibility of a hadronic contribution to the very high energy (VHE) gamma-ray emission of blazars, as well as the possible detection of neutrino events, are explored by considering photohadronic interactions in a lepto-hadronic scenario. The model is applied to fit the flaring period of Markarian 421 in 2010, for which a dedicated analysis of Fermi-LAT data from the source was performed in the MeV range (100 MeV - 1 GeV). The fit results were compared with two leptonic models using the Akaike Information Criterion (AIC) test. In all cases, the photohadronic model was favoured as a better fit description in comparison to the one-zone leptonic model.

The photohadronic model was also applied to the blazar 4FGL J0658.6+0636, which was found within the 90% localisation region of the IceCube neutrino alert IC-201114A. By analysing 12.3-years of Fermi-LAT data, the periods in which the blazar was detected significantly were identified and studied. For one of these periods, it was found that the photohadronic flaring model results are consistent with the observed gamma-ray behaviour of 4FGL J0658.6+0636 and the IC-201114A is discussed under the photohadronic scenario. These results show the potential of a photohadronic contribution to a lepto-hadronic origin of gamma-ray flux of blazars.

The final part of this thesis describes the neutrino and gamma-ray simulations performed for the Neutrino Target of Opportunity (NToO) program for the Cherenkov Telescope Array (CTA). The detection probability with CTA of the gamma-ray flux associated with the simulated IceCube hot-spots for steady neutrino source populations is investigated, and the performance of the CTA Omega configuration array is analysed to predict the potential science reach of the NToO program.