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Register a new userLunar detection of ultra-high-energy cosmic rays and neutrinos with the Square Kilometre Array
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The origin of the most energetic particles in nature, the ultra-high-energy (UHE) cosmic rays, is still a mystery. Only the most energetic of these have sufficiently small angular deflections to be used for directional studies, and their flux is so low that even the 3,000 km^2 Pierre Auger detector registers only about 30 cosmic rays per year of these energies. A method to provide an even larger aperture is to use the lunar Askaryan technique, in which ground-based radio telescopes search for the nanosecond radio flashes produced when a cosmic ray interacts with the Moons surface. The technique is also sensitive to UHE neutrinos, which may be produced in the decays of topological defects from the early universe. Observations with existing radio telescopes have shown that this technique is technically feasible, and established the required procedure: the radio signal should be searched for pulses in real time, compensating for ionospheric dispersion and filtering out local radio interference, and candidate events stored for later analysis. For the Square Kilometre Array (SKA), this requires the formation of multiple tied-array beams, with high time resolution, covering the Moon, with either SKA1-LOW or SKA1-MID. With its large collecting area and broad bandwidth, the SKA will be able to detect the known flux of UHE cosmic rays using the visible lunar surface - millions of square km - as the detector, providing sufficient detections of these extremely rare particles to address the mystery of their origin.
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Radio waves, perhaps because they are uniquely transparent in our terrestrial atmosphere, as well as the cosmos beyond, or perhaps because they are macroscopic, so the basic instruments of detection (antennas) are easily constructable, arguably occupy a privileged position within the electromagnetic spectrum, and, correspondingly, receive disproportionate attention experimentally. Detection of radio-frequency radiation, at macroscopic wavelengths, has blossomed within the last decade as a competitive method for measurement of cosmic particles, particularly charged cosmic rays and neutrinos. Cosmic-ray detection via radio emission from extensive air showers has been demonstrated to be a reliable technique that has reached a reconstruction quality of the cosmic-ray parameters competitive with more traditional approaches. Radio detection of neutrinos in dense media seems to be the most promising technique to achieve the gigantic detection volumes required to measure neutrinos at energies beyond the PeV-scale flux established by IceCube. In this article, we review radio detection both of cosmic rays in the atmosphere, as well as neutrinos in dense media.
Various experiments have been conducted to search for the radio emission from ultra-high-energy particles interacting in the lunar regolith. Although they have not yielded any detections, they have been successful in establishing upper limits on the flux of these particles. I present a review of these experiments in which I re-evaluate their sensitivity to radio pulses, accounting for effects which were neglected in the original reports, and compare them with prospective near-future experiments. In several cases, I find that past experiments were substantially less sensitive than previously believed. I apply existing analytic models to determine the resulting limits on the fluxes of ultra-high-energy neutrinos and cosmic rays. In the latter case, I amend the model to accurately reflect the fraction of the primary particle energy which manifests in the resulting particle cascade, resulting in a substantial improvement in the estimated sensitivity to cosmic rays. Although these models are in need of further refinement, in particular to incorporate the effects of small-scale lunar surface roughness, their application here indicates that a proposed experiment with the LOFAR telescope would test predictions of the neutrino flux from exotic-physics models, and an experiment with a phased-array feed on a large single-dish telescope such as the Parkes radio telescope would allow the first detection of cosmic rays with this technique, with an expected rate of one detection per 140 hours.
One of the five key science projects for the Square Kilometre Array (SKA) is The Origin and Evolution of Cosmic Magnetism, in which radio polarimetry will be used to reveal what cosmic magnets look like and what role they have played in the evolving Universe. Many of the SKA prototypes now being built are also targeting magnetic fields and polarimetry as key science areas. Here I review the prospects for innovative new polarimetry and Faraday rotation experiments with forthcoming facilities such as ASKAP, LOFAR, the ATA, the EVLA, and ultimately the SKA. Sensitive wide-field polarisation surveys with these telescopes will provide a dramatic new view of magnetic fields in the Milky Way, in nearby galaxies and clusters, and in the high-redshift Universe.
We review the current status of the Square Kilometre Array (SKA) by outlining the science drivers for its Phase-1 (SKA1) and setting out the timeline for the key decisions and milestones on the way to the planned start of its construction in 2016. We explain how Phase-2 SKA (SKA2) will transform the research scope of the SKA infrastructure, placing it amongst the great astronomical observatories and survey instruments of the future, and opening up new areas of discovery, many beyond the confines of conventional astronomy.
We present a strong hint of a connection between high energy $gamma$-ray emitting blazars, very high energy neutrinos, and ultra high energy cosmic rays. We first identify potential hadronic sources by filtering $gamma$-ray emitters %from existing catalogs that are in spatial coincidence with the high energy neutrinos detected by IceCube. The neutrino filtered $gamma$-ray emitters are then correlated with the ultra high energy cosmic rays from the Pierre Auger Observatory and the Telescope Array by scanning in $gamma$-ray flux ($F_{gamma}$) and angular separation ($theta$) between sources and cosmic rays. A maximal excess of 80 cosmic rays (42.5 expected) is found at $thetaleq10^{circ}$ from the neutrino filtered $gamma$-ray emitters selected from the second hard {it Fermi}-LAT catalogue (2FHL) and for $F_gammaleft(>50:mathrm{GeV}right)geq1.8times10^{-11}:mathrm{ph},mathrm{cm}^{-2},mathrm{s}^{-1}$. The probability for this to happen is $2.4 times 10^{-5}$, which translates to $sim 2.4 times 10^{-3}$ after compensation for all the considered trials. No excess of cosmic rays is instead observed for the complement sample of $gamma$-ray emitters (i.e. not in spatial connection with IceCube neutrinos). A likelihood ratio test comparing the connection between the neutrino filtered and the complement source samples with the cosmic rays favours a connection between neutrino filtered emitters and cosmic rays with a probability of $sim1.8times10^{-3}$ ($2.9sigma)$ after compensation for all the considered trials. The neutrino filtered $gamma$-ray sources that make up the cosmic rays excess are blazars of the high synchrotron peak type. More statistics is needed to further investigate these sources as candidate cosmic ray and neutrino emitters.
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