Numoon: Status of ultra-high energy cosmic ray detection with LOFAR and improved limits with the WSRT.

LOFAR (Low Frequency Array) is a new distributed digital radio telescope built in the Netherlands and surrounding countries. It can be used to detect radio emission induced by cosmic rays as well as other transient signals, due to its design of stations of simple antennas. We will present LOFAR and how the NuMoon project plans to use the telescope to detect ultra-high-energy cosmic rays (> 10²¹eV). The flux at these energies is very low, therefore, the Moon is chosen as target because of its large surface area of 10⁷ km². When a cosmic ray hits theMoon surface it will produce a cascade of secondary particles with an excess of electrons. This causes radio emission, a process known as the Askaryan effect. Until recently, an unsolved problem was the possibility of formation-zone suppression of near-surface cascades, as produced by cosmic rays, which could prevent this radiation from being visible from Earth. We will show an analytic calculation that solves this problem. With this result we are able to set a limit on the flux of cosmic rays at the highest energies with data from the Westerbork Synthesis Radio Telescope and provide the expected sensitivity for LOFAR and the Square Kilometre Array.