Livio Scarsi

AIR WATCH: a space mission to observe the UV fluorescence induced in the Earth atmosphere by extreme-energy cosmic radiation

One of the most challenging tissues in Astroparticle Physics is represented today by the observation of the energy spectrum of the Extreme Energy Cosmic Radiation. The very existence of particles with energy above 1020 eV and of neutrinos of comparable energy raises fundamental scientific questions in connection with their origin and propagation in the interstellar/intergalactic space. These particles can be detected through the gain showers produced in the Earth Atmosphere. The shower development is accompanied by emission of fluorescence in the atmosphere, in particular that induced in Nitrogen with characteristics spectral lines in the UV. Following a first suggestion by J. Linsley in the early 1980s, taken over by Y/ Takahashi, the fluorescence observation can be advantageously carried out by space. By using wide angel optics with large collecting surface, we can monitor a target area of atmosphere of the order of millions square kilometers x sr and corresponding mass above 1013 tons, allowing the detection of the very small flux values typical of the EECR and making possible the search of the elusive high energy neutrinos. AIRWATCH follows this approach. We describe the main scientific goals for the investigation of the EECR, High Energy neutrinos and of the Gamma Ray Bursts, together with the relevant connections to the problem of their origin. The experimental framework is outlined and a description is given of the space mission and of the observational strategy.

AIRWATCH: the fast detector

The discovery of the extreme energy cosmic rays (EERC) with energy greater than 1020 eV has opened a new research branch of astrophysics on both observational and interpretative point of views. Together with the EECR one has also to consider the neutrino component which, independently on its primary or secondary origin, can reach comparable energies. These particles can be detected through the giant showers (EAS) produced in the Earth atmosphere and the induced fluorescent molecular nitrogen emission. Observing the EECR signals is very difficult; we need forefront technology or new developments. The main reason is that their flux is very weak, typically of the order of a few events/year/1000 km2 per EECR of E approximately equals 1020 eV. The proposed Airwatch mission, base don a single orbiting telescope which can measure both intensity and direction of the EAS, impose new concepts for the detectors; single photon sensitivity, fast response of the order of few microseconds with sampling times of tenths of nanoseconds, low noise and good S/N ratio, large area, adaptability to a curved surface. Fortunately the spatial resolution requirements are somehow relaxed. The peculiar characteristics of this application are such that no available detectors satisfies completely the requirements. Therefore the final detector has to be the result of a R and D program dedicated to the specific problem. In this paper we survey a number of possible detectors and identify their characteristics versus the Airwatch mission requirements.

Maximum-energy Auger-shower satellite (MASS/AIRWATCH)

A concept for observation from space of the highest energy cosmic rays above 1020 eV with a satellite-borne observatory has been considered. A maximum-energy auger (air)-shower satellite (MASS) would use segmented lenses (and/or mirrors) and an array of imaging devices (about 106 pixels) to detect and record fluorescent light profiles of cosmic ray cascades in the atmosphere. The field-of-view of MASS could be extended to about (1000 km)2 so that more than 103 events per year could be observed above 1020 eV. From far above the atmosphere, MASS would be capable of observing events at all angles including near horizontal tracks, and would have considerable aperture for high energy photon and neutrino observation. With a large aperture and the spatial and temporal resolution, MASS could determine the energy spectrum, the mass composition, and arrival anisotropy of cosmic rays from 1020 eV to 1022 eV, a region hitherto not explored by ground-based detectors such as the flys eye and air-shower arrays. MASSs ability to identify comic neutrinos and gamma rays may help providing evidence for the theory which attributes the above cut-off cosmic ray flux to the decay of topological defects.