Extensive air shower (EAS) arrays directly sample the shower particles that reach the observation altitude. They are wide field of view (FoV) detectors able to view the whole sky simultaneously and continuously. In fact, EAS arrays have an effective FoV of about 2 sr and operate with a duty cycle of $\sim$100\%. This capability makes them well suited to study extended sources, such as the Galactic diffuse emission and measure the spectra of Galactic sources at the highest energies (near or beyond 100 TeV). Their sensitivity in the sub-TeV/TeV energy domain cannot compete with that of Cherenkov telescopes, but the wide FoV is ideal to perform unbiased sky surveys, discover transients or explosive events (GRBs) and monitor variable or flaring sources such as Active Galactic Nuclei (AGN). An EAS array is able to detect at the same time events induced by photons and charged cosmic rays, thus studying the connection between these two messengers of the non-thermal Universe. Therefore, these detectors are, by definition, multi-messenger instruments.
Wide FoV telescopes are crucial for a multi-messenger study of the Gravitational Wave events due to their capability to survey simultaneously all the large sky regions identified by LIGO and VIRGO, looking for a possible correlated $\gamma$-ray emission.
In this contribution we summarize the scientific motivations which push the construction of new wide FoV air shower detectors and introduce the future instruments currently under installation. Finally, we emphasize the need of an EAS array in the Southern hemisphere to monitor the Inner Galaxy and face a number of important open problems.
Wed, 14 Feb 18
Comments: 19 pages, Invited talk, XII Multifrequency Behaviour of High Energy Cosmic Sources Workshop, 12-17 June, 2017, Palermo, Italy