Searching for signals of photon-ALP mixing effects with gamma-ray spectra of AGNs

by Qixin Yu (UHH)

Thursday 26 Oct 2023, 15:00 → 16:30 Europe/Berlin

seminar room 3 (bldg. 1b)

High energy gamma-rays propagating in external magnetic fields may convert into axion-like particles (ALPs). This way, the gamma-ray spectra may be modified by the mixing of photons with ALPs. In a statistical study, we use energy spectra of 20 AGNs recorded by Fermi-LAT in 10-year observations to search for possible modulations on the spectra induced by photon-ALP mixing effect. Methods of likelihood and chi2-minimization are both used in the analysis for estimating relevant parameters of magnetic fields. Hypotheses tests based on likelihood ratio and Delta-chi2 statistics are used to estimate the significance of one model (with ALPs) over the other model (without ALPs). 18 out of 20 sources are found a favorable fit with the introduction of ALPs. A combined significance of all sampled sources is estimated to be 5.3sigma using a bootstrapping method. The estimated B-field parameters fall into ranges that are expected for large-scale magnetic fields present in relevant astrophysical environments.