Detection of very-high-energy gamma-ray transients with monitoring facilities

Abstract

The study of the sky in Very High Energy gamma rays (VHE, E > 100 GeV) has led to the identification of a wealth of processes that are responsible for the acceleration of particles at the highest observed energies within the Milky Way and beyond. Observations with VHE facilities, like the Cherenkov Telescope Array (CTA), will be fundamental to investigate the characteristics of these processes. Still, the transient and unpredictable nature of the most powerful sources requires that effective monitoring strategies should be adopted to track them. With this study, we focus on the type of VHE transients that can be effectively detected with monitoring facilities. Using the data collected by Fermi-LAT during its observing campaign, we investigate the frequency, luminosity and timescales of different VHE transients, focusing on blazar flares and Gamma-Ray Bursts. We compare their properties with the performance of existing and future instruments. We show that pursuing an enhanced spectral coverage in the sub-TeV range with a large field-of-view instrument, operating in the Southern hemisphere, will effectively contribute to the investigation of different types of transients, both by providing prompt alerts to activate follow-up observations of the most energetic events, as well as by collecting critical information on their temporal and spectral evolution.