Searching for Axion-Like Particles with VERITAS Observations of the 2017 NGC 1275 flare

Abstract: The most powerful active galactic nuclei (AGN), including blazars and radio galaxies, exhibit non-thermal radiation extending beyond 1 TeV with high luminosities and strong flux variability, indicating extreme particle acceleration in their relativistic jets. The gamma-ray spectra of these sources contain information about the energy distribution and cooling processes of high-energy particles in jets, the extragalactic background light between the source and the observer, the environment of the gamma-ray emitting region, and even exotic particle physics. These spectra have proven particularly useful in searches for the existence of axion-like particles (ALPs). ALPs are light, pseudoscalar particles that have been proposed as a beyond-the-standard-model generalization of the axion. Consequently, they are expected to couple to photons in magnetic fields to compensate for spin difference. This coupling, manifested as ALP-photon oscillations in external fields, would induce modifications to the gamma-ray spectra of AGN. In this talk, I'll discuss the theory behind ALPs, discuss their relevance to very-high-energy gamma-ray astronomy, and present preliminary results from an analysis that uses data from the VERITAS gamma-ray observatory to probe ALP-photon oscillation effects in the flaring spectrum of the galaxy cluster -embedded radio galaxy NGC 1275.