Science

Reports of the highest energy cosmic rays, beyond 1020 eV, presents us with the challenge of understanding their origin. They are unlikely to be galactic and, in light of the serious energy loss processes (p, gamma) of the Greisen-Zatsepin-Kuzmin (GZK) effect, protons and nuclei cannot reach us from far-distant extra-galactic sources. Identifying acceleration mechanisms that can get to such extremely high energies is a longstanding unsolved mystery in astrophysics. The OWL observations will explore the frontiers of cosmic ray and gamma ray astrophysics, fundamental particle physics, and early universe cosmology, and, depending on the results, could have important impact on them all. The decades of data obtained with imaginative pioneering experiments have raised puzzling fundamental questions, which can only be answered by instruments capable of detecting hundreds of events per year. OWL will measure the highest energy cosmic ray air showers by observing atmospheric fluorescence light from orbit. By using the atmosphere as a target, we can get an effective detection area solid angle efficiency product of 2.3 x 105 km2 sr. At each instant, a spacecraft's field of view can encompass the atmosphere above a very large area of the Earth. The resulting geometry factor exceeds by a factor of 30 that of any proposed Earth-based cosmic ray detector.

The cosmic ray observations above 10¹⁷ eV reported by the Fly's Eye, Haverah Park, AGASA, and HiRes show a break in the spectrum at ~5 x 10¹⁸ eV. The composition changes from being predominantly heavy nuclei below the break to light nuclei above the break. If these particles were produced in the galaxy, they should be anisotropic because their gyroradii in the few microgauss galactic magnetic fields are bigger than galactic scales. Thus the spectral change, composition, and the observed arrival directions are strongly suggestive that the cosmic rays above 10¹⁹ eV are predominantly extra-galactic particles.

If the particles observed are initiated by protons of extra-galactic origin, and if their sources are correlated with luminous matter, then the inhomogeneity of the large scale galaxy distribution, on scales < 100 h^-1 Mpc, should be imprinted on their arrival directions. The expected anisotropy associated with the large scale structure should be apparent once the number of events detected above 10¹⁹ eV is increased by an order of magnitude.

In any case, the arrival direction distribution will be an important clue in determining their origin. OWL will provide a statistically significant study of the arrival directions of particles beyond the GZK cutoff.

Physics objectives of the OWL include measuring the anisotropy, energy spectrum, and composition of the highest energy cosmic rays. Composition analysis should include the identification of gamma rays or neutrinos if they are present.

This file was last modified May 4, 2004