Justin D. Bray

Mass entrainment and turbulence-driven acceleration of ultra-high energy cosmic rays in Centaurus A

Observations of the FR I radio galaxy Centaurus A in radio, X-ray and gamma-ray bands provide evidence for lepton acceleration up to several TeV and clues about hadron acceleration to tens of EeV. Synthesising the available observational constraints on the physical conditions and particle content in the jets, inner lobes and giant lobes of Centaurus A, we aim to evaluate its feasibility as an ultra-high-energy cosmic-ray source. We apply several methods of determining jet power and affi rm the consistency of various power estimates of∼ 1× 10 43 erg s −1 . Employing scaling relations based on previous results for 3C 31, we estimate particle number densities in the jets, encompassing available radio throug h X-ray observations. Our model is compatible with the jets ingesting ∼ 3× 10 21 g s −1 of matter via external entrainment from hot gas and∼ 7× 10 22 g s −1 via internal entrainment from jet-contained stars. This leads to an imbalance between the internal lobe pressure available from radiating particles and magnetic fie ld, and our derived external pressure. Based on knowledge of the external environments of other FR I sources, we estimate the thermal pressure in the giant lobes as 1.5× 10 −12 dyn cm −2 , from which we deduce a lower limit to the temperature of∼ 1.6× 10 8 K. Using dynamical and buoyancy arguments, we infer∼ 440− 645 Myr and∼ 560 Myr as the sound-crossing and buoyancy ages of the giant lobes respectively, inconsistent with their spectral ages. We re -investigate the feasibility of particle acceleration via stochastic processes in the lobes, placing new constraints on the energetics and on turbulent input to the lobes. The same ‘very hot’ temperature s that allow self-consistency between the entrainment calculations and the missing pressure also allow stochastic UHECR acceleration models to work.

LUNASKA experiment observational limits on UHE neutrinos from Centaurus A and the Galactic Centre

We present the first observational limits to the ultra-high-energy (UHE) neutrino flux from the Galactic Centre, and from Centaurus A which is the nearest active galactic nucleus. These results are based on our ‘Lunar UHE Neutrino Astrophysics using the Square Kilometre Array’ (LUNASKA) project experiments at the Australia Telescope Compact Array (ATCA). We also derive limits for the previous experiments and compare these limits with expectations for acceleration and superheavy dark matter models of the origin of UHE cosmic rays.

Limit on the ultrahigh-energy neutrino flux from lunar observations with the Parkes radio telescope

We report a limit on the ultra-high-energy neutrino flux based on a non-detection of radio pulses from neutrino-initiated particle cascades in the Moon, in observations with the Parkes radio telescope undertaken as part of the LUNASKA project. Due to the improved sensitivity of these observations, which had an effective duration of 127 hours and a frequency range of 1.2-1.5 GHz, this limit extends to lower neutrino energies than those from previous lunar radio experiments, with a detection threshold below 10^20 eV. The calculation of our limit allows for the possibility of lunar-origin pulses being misidentified as local radio interference, and includes the effect of small-scale lunar surface roughness. The targeting strategy of the observations also allows us to place a directional limit on the neutrino flux from the nearby radio galaxy Centaurus A.

Lunar detection of ultra-high-energy cosmic rays and neutrinos with the Square Kilometre Array

The origin of the most energetic particles in nature, the ultra-high-energy (UHE) cosmic rays, is still a mystery. Due to their extremely low flux, even the 3,000 km^2 Pierre Auger detector registers only about 30 cosmic rays per year with sufficiently high energy to be used for directional studies. A method to provide a vast increase in collecting area is to use the lunar 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 the associated flux of UHE neutrinos, which are expected from cosmic ray interactions during production and propagation, and the detection of which can also be used to identify the UHE cosmic ray source(s). An additional flux of UHE neutrinos may also 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 SKA, this requires the formation of multiple tied-array beams, with high time resolution, covering the Moon, with either SKA-LOW or SKA-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 solve the mystery of their origin.

Noise statistics in a fast digital radio receiver: the Bedlam backend for the Parkes radio telescope

The digital record of the voltage in a radio telescope receiver, after frequency conversion and sampling at a finite rate, is not a perfect representation of the original analog signal. To detect and characterise a transient event with a duration comparable to the inverse bandwidth it is necessary to compensate for these effects, altering the statistical properties of the signal and making it difficult to determine the significance of a potential detection. We present an analysis of these modified statistics and demonstrate them with experimental results from Bedlam, a new digital backend for the Parkes radio telescope.

Precision measurements of cosmic ray air showers with the SKA

Supplemented with suitable buffering techniques, the low-frequency part of the SKA can be used as an ultra-precise detector for cosmic-ray air showers at very high energies. This would enable a wealth of scientific applications: the physics of the transition from Galactic to extragalactic cosmic rays could be probed with very high precision mass measurements, hadronic interactions could be studied up to energies well beyond the reach of man-made particle accelerators, air shower tomography could be performed with very high spatial resolution exploiting the large instantaneous bandwidth and very uniform instantaneous

Ionospheric propagation effects for UHE neutrino detection with the lunar Cherenkov technique

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LUNASKA simultaneous neutrino searches with multiple telescopes

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

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

coverage of SKA1-LOW, and the physics of thunderstorms and possible connections between cosmic rays and lightning initiation could be studied in unprecedented levels of detail. In this article, we describe the potential of the SKA as an air shower radio detector from the perspective of existing radio detection efforts and discuss the associated technical requirements.