Very-High-Energy gamma-ray astronomy with the ALTO observatory

TL;DR

ALTO is a proposed high-altitude gamma-ray observatory in the Southern hemisphere designed to improve detection of very high-energy gamma rays, enabling continuous sky surveys and rapid transient alerts.

Contribution

It introduces a novel detection technique combining water Cherenkov and scintillation detectors at high altitude for enhanced gamma-ray detection.

Findings

Conceptual design completed, upcoming prototyping phase

Enhanced sensitivity to low-energy gamma rays

Wide field of view for continuous sky monitoring

Abstract

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.