  ALTO is a concept/project in the exploratory phase since 2013 aiming to build
a wide-field VHE gamma-ray observatory at very high altitude in the Southern
hemisphere. The operation of such an observatory will complement the Northern
hemisphere observations performed by HAWC and will make possible the
exploration of the central region of our Galaxy and the hunt for PeVatrons, and
to search for extended Galactic objects such as the Vela Supernova Remnant and
the Fermi bubbles. The ALTO project is aiming for a substantial improvement of
the Water Cherenkov Detection Technique by increasing the altitude of the
observatory in order to lower the energy threshold, by using a layer of
scintillator below the water tank to optimize the S/B discrimination, by
minimizing the size of the tanks and having a more compact array to sample the
air-shower footprints with better precision, and by using precise electronics
which will provide time-stamped waveforms to improve the angular and energy
resolution. ALTO is designed to have as low an energy threshold as possible so
as to act as a fast trigger alert to other observatories, primarily to the
Southern part of CTA, for transient Galactic and extra-galactic phenomena. The
wide FoV resulting from the detection technique allows the survey of a large
portion of the sky continuously, thus giving the possibility to access emission
from Gamma-Ray Bursts, Active Galactic Nuclei and X-ray binary flares, and
extended emissions of both Galactic (Vela SNR, Fermi bubbles) and
extra-galactic (AGN radio lobes) origin. The ALTO observatory will be composed
of about a thousand detection units, each of which consists of a Water
Cherenkov Detector positioned above a liquid Scintillation Detector,
distributed within an area of about 200 m in diameter. The project is in the
design study phase which is soon to be followed by a prototyping phase.
