LUNASKA experiments using the Australia Telescope Compact Array to search for ultrahigh energy neutrinos and develop technology for the lunar Cherenkov technique

Abstract

We describe the design, performance, sensitivity and results of our recent experiments using the Australia Telescope Compact Array (ATCA) for lunar Cherenkov observations with a very wide (600 MHz) bandwidth and nanosecond timing, including a limit on an isotropic neutrino flux. We also make a first estimate of the effects of small-scale surface roughness on the effective experimental aperture, finding that contrary to expectations, such roughness will act to increase the detectability of near-surface events over the neutrino energy-range at which our experiment is most sensitive (though distortions to the time-domain pulse profile may make identification more difficult). The aim of our "Lunar UHE Neutrino Astrophysics using the Square Kilometer Array" (LUNASKA) project is to develop the lunar Cherenkov technique of using terrestrial radio telescope arrays for ultra-high energy (UHE) cosmic ray (CR) and neutrino detection, and in particular to prepare for using the Square Kilometer Array (SKA) and its path-finders such as the Australian SKA Pathfinder (ASKAP) and the Low Frequency Array (LOFAR) for lunar Cherenkov experiments.

A lunar radio experiment with the Parkes radio telescope for the LUNASKA project

“…Note that s peak may also be represented as s peak = n σ s rms (11) in terms of the significance n σ of the pulse relative to the RMS noise. The value of this threshold significance is found through analysis of the observational data in Sec.…”

“…James et al [11] note that, for a finite sampling rate, there will be a random offset between the peak of a pulse and the closest sampling time, which reduces the sensitivity of an experiment of this type, as the sampled digital data will not record the full magnitude of the pulse. They calculate the loss of sensitivity this effect causes for their experiment, finding it to be ∼ 0-30%, depending on the actual offset between the peak of the pulse and the closest sampling time, and on the coherency of the pulse; other similar experiments have neglected this effect entirely.…”

“…As the electric field is proportional to the square root of the power, the uncertainty in this value will be half the uncertainty quoted above. To relate the electric field strength to the recorded signal s(t), we use its relation [11] to the flux density for a limited interval ∆t:…”

“…Buitink et al [13], whose experiment was the most severely affected by dispersion due to its lower frequency, recorded voltage data continuously over the course of their observations, and dedispersed it afterwards to restore the original pulse. James et al [11] used an analog dedispersion filter developed by Roberts [27] which allowed the same correction to be performed in real time, but it was limited to a fixed dispersion characteristic, whereas the effect of the ionosphere varied over the course of the observations. For this experiment, we developed a digital dedispersion filter (see Sec.…”

“…The first lunar particle detection experiment was performed in 1995 with the Parkes radio telescope [8]; subsequent experiments have been carried out with the Goldstone Deep Space Communications Complex (GDSCC) [9,GLUE], the Kalyazin radio telescope [10], the Australia Telescope Compact Array (ATCA) [11,LUNASKA], the Lovell telescope [12,La Luna], the Westerbork Synthesis Radio Telescope (WSRT) [13,NuMoon] and the Expanded Very Large Array (EVLA) [14,RESUN]. A key feature of these experiments is that they make use of existing radio telescopes, whereas other experiments searching for UHE cosmic rays or neutrinos require the development of expensive dedicated instruments.…”