ar X iv : a st ro - p h / 04 08 16 2 v 2 18 A ug 2 00 4 Observing the Ultrahigh Energy Universe with OWL Eyes

The goal of the Orbiting Wide-field Light-collectors (OWL) mission is to study the origin and physics of the highest energy particles known in nature, the ultrahigh energy cosmic rays (UHECRs). The OWL mission consists of telescopes with UV sensitive cameras on two satellites operating in tandem to view in stereo the development of the giant particle showers induced in the Earth’s atmosphere by UHECRs. This paper discusses the characteristics of the OWL mission. 1 The OWL Satellite Detectors The OWL (Orbiting Wide-field Light-collectors) mission is designed to obtain data on ultrahigh energy cosmic rays (UHECR) and neutrinos in order to tackle the fundamental problems associated with their origin [1]. The OWL mission is designed to provide the event statistics and extended energy range that are crucial to addressing these issues. To accomplish this, OWL makes use of the Earth’s atmosphere as a huge “calorimeter” to make stereoscopic measurements of the atmospheric UV fluorescence produced by air shower particles. This is the most accurate technique that has been developed for measuring the energy, arrival direction, and interaction characteristics of UHECR [2]. To this end, OWL will consist of a pair of satellites placed in tandem in a low inclination, medium altitude orbit. The OWL telescopes will point down at the Earth and will together point at a section of atmosphere about the size of the state of Texas (∼ 6 × 10 km), obtaining a much greater sensitivity than present ground based detectors. The ability of of OWL to detect cosmic rays, in units of km sr, is called the aperture. The instantaneous aperture at Preprint submitted to Elsevier Science 18 August 2004 the highest energies is ∼ 2×10 km sr (see Sect. 2.3). The effective aperture, reduced by the effects of the moon, man-made light, and clouds, will be conservatively ∼ 0.9× 10 km sr. For each year of operation, OWL will have 90 times the aperture of the ground based HiRes detector and 13 times the aperture of the Pierre Auger detector array (130 times its most sensitive “hybrid” mode). The OWL detectors will observe the UV flourescence light from the giant air showers produced by UHECR on the dark side of the Earth. They will thus produce a stereoscopic picture of the temporal and spatial development of the showers. OWL has been the subject of extensive technical studies, examining all aspects of the instrument and mission. This paper gives an overview of OWL as based on these studies. The technical details, as well as discussion of the science, including ultrahigh energy neutrino science with OWL, can be found at http://owl.gsfc.nasa.gov. The OWL baseline instrument and mission can be realized using current technology. The baseline OWL instrument, shown in Figure 1, is a large f/1 Schmidt camera with a 45◦ full field-of-view (FOV) and a 3.0 meter entrance aperture. The entrance aperture is filled with a Schmidt corrector. The deployable primary mirror has a 7 m diameter. The focal plane has an area of 4 m segmented into approximately 500,000 pixels distributed over 1300 multianode photomultiplier tubes. Each pixel is read out by an individual electronics chain and can resolve single photoelectrons. Taking obscuration by the focal plane and by the members supporting the focal plane and corrector plate into account, the effective aperture of the instrument is about 3.4 m. A deployable light shield, not shown in Figure 1, covers the instrument and a redundant shutter is used to close off the aperture during non-observing periods. A UV laser is located at the back of the focal plane and fires through the center of the corrector plate to a small steering mirror system. Laser light reflected by clouds is detected and measured using the OWL focal plane. OWL is normally operated in stereo mode and the instruments view a common volume of atmosphere. However, the instruments are independent and the focal plane has been designed for a time resolution of 0.1 μs so that monocular operation can be supported (with reduced performance) if one instrument fails. The instrument weight is ∼ 1800 kg and total power consumption is ∼ 600 W. The amount of data generated by the instrument is determined by calibration and atmospheric monitoring and averages 150 kbps over any 24 hour period. The satellites are launched together on a Delta rocket into a 1000 km circular orbit with a nominal inclination of 10◦. Figure 1 shows both satellites stowed for launch. Following launch, the two satellites will fly in formation with a separation of 10 to 20 km for about 3 months to search for upward going showers from ντ ’s propagating thorugh the Earth. The spacecraft will then separate to 600 km for ∼ 2.5 years to measure the high-energy end of the UHECR spectrum. Following this period, the altitude is reduced to 600