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A limit on the ultra-high-energy neutrino flux from Centaurus A
Justin Bray

For the LUNASKA collaboration
Clancy James, Ron Ekers, Chris Phillips, Paul Roberts, John Reynolds, Rebecca McFadden, Ray Protheroe, Jaime Alvarez-Mußiz, Justin Bray


Outline:
· Ultra-high-energy (UHE) cosmic rays & neutrinos · Detecting UHE neutrinos · The lunar Cherenkov technique · Observations with the ATCA · Observations with Parkes · ASKAP and the future


Cosmic rays:
· High-energy protons or nuclei · Follow curved paths in magnetic fields · Attenuated by GZK interactions at high energy · Possibly accelerated by radio galaxies like Cen A · Acceleration process may produce neutrinos


Cosmic rays:
· High-energy protons or nuclei · Follow curved paths in magnetic fields · Attenuated by GZK interactions at high energy · Possibly accelerated by radio galaxies like Cen A · Acceleration process may produce neutrinos

GZK interactions Above 61019 eV, protons interact with cosmic microwave background. Produce pions, which decay into various particles, including neutrinos. Not important for a source as close as Cen A.


Cosmic rays:
· High-energy protons or nuclei · Follow curved paths in magnetic fields · Attenuated by GZK interactions at high energy · Possibly accelerated by radio galaxies like Cen A · Acceleration process may produce neutrinos

Neutrinos:
· Weakly-interacting particles · Travel in straight lines · Low cross-section: very little attenuation · Tend to pass right through detectors Potential 'messengers' of cosmic ray acceleration.


Neutrino detectors:
· Large volumes · Look for particle showers which produce e.g. · fluorescence · Cherenkov radiation · optical · radio


Neutrino detectors:
· Large volumes · Look for particle showers which produce e.g. · fluorescence · Cherenkov radiation · optical · radio The Pierre Auger Observatory Argentina


Neutrino detectors:
· Large volumes · Look for particle showers which produce e.g. · fluorescence · Cherenkov radiation · optical · radio ANTARES Neutrino Telescope Mediterranean Sea


Neutrino detectors:
· Large volumes · Look for particle showers which produce e.g. · fluorescence · Cherenkov radiation · optical · radio IceCube Neutrino Observatory Amundsen-Scott Station South Pole


Neutrino detectors:
· Large volumes · Look for particle showers which produce e.g. · fluorescence · Cherenkov radiation · optical · radio ANITA Project Balloon flight over Antarctica


Neutrino detectors:
· Large volumes · Look for particle showers which produce e.g. · fluorescence · Cherenkov radiation · optical · radio We look for Cherenkov radiation at radio frequencies. Our detector volume...



...or at least the upper ~10 metres (the regolith).


Detection Geometry
Neutrinos are penetrating...


Detection Geometry
Neutrinos are penetrating... ...but not that penetrating.


Detection Geometry
Neutrinos are penetrating... ...but not that penetrating. Detection is more likely around the edge ('limb') of the Moon. This is where we need to point our radio telescopes.


Beam pointing


Beam pointing: ATCA (Feb `08)

Australia Telescope Compact Array Narrabri, NSW, Australia

Sees entire limb of the Moon. Most sensitive to neutrinos ~15 degrees from the Moon, in any direction.


Beam pointing: ATCA (May `08)

Centaurus A
Australia Telescope Compact Array Narrabri, NSW, Australia

Sees part of limb of the Moon. Sensitive to neutrinos from Cen A. Better sensitivity.


Signal processing:
· We're looking for a short (nanosecond-scale) pulse. · Our background is the thermal emission of the Moon. · We can sample at > 1 GHz, but we can't store that fast. So, we keep the samples in a buffer, and if a sample exceeds some threshhold, we trigger and store the buffer. Problem: pulses are dispersed in the ionosphere. Solution: de-dispersion filter. Must act in real time.
(Also, we observe at night, when the ionosphere is less active.)


Signal processing:
Buffer of pure noise (dual polarisation):


Signal processing:
Triggered; buffer stored:

This one is from a pulsing signal used for calibration.


Signal processing:
Lightning also causes triggers:

(This plot is 0.8 microseconds long. Fine time structure.)


Beam pointing: Parkes (March `09)

Parkes Radio Telescope Parkes, NSW, Australia

Centaurus A

Sensitive to neutrinos from this particular direction.


Beam pointing: Parkes (March `09)

Parkes Radio Telescope Parkes, NSW, Australia

Multibeam receiver


Signal processing:
ATCA · Using three separate antennas. · If any trigger, all buffers are stored. · Radio-frequency interference frequently does so. Parkes · Using three beams of multibeam apparatus. · Compare buffers in real time. Any trigger on more than one simultaneously is ignored. · Anticoincidence filter ­ blocks out interference. · Off-moon beam has lower system temperature ­ is more sensitive. · Remaining problem is thermal noise from Moon.


Analysis:
· Calibrate against thermal emission of Moon, or pulse calibrator. · Determine threshold for significance as E-field strength. · Many samples, so ~8-sigma required for significance. Determine effective aperture of experiment to neutrinos through Monte Carlo simulation of entire pathway: · Interaction of neutrino with Moon. · Progress of particle shower in regolith. · Propagation of Cherenkov radiation out of lunar surface.


Analysis:
ATCA · Complete ­ being published. Parkes · Preliminary results only ­ needs more work.


Analysis:
ATCA · Complete ­ being published. Parkes · Preliminary results only ­ needs more work.


Analysis:
· RICE (Radio Ice Cherenkov Experiment) flux limit is from five years of observations. · ATCA and Parkes limits are each from three nights.


Analysis:
· Cuoco & Hannestad flux prediction is based on model of cosmic ray acceleration in jets. · Kachelriess et al. model is for acceleration near core. · Both are normalised to a cosmic ray flux based on the events detected by the Pierre Auger Observatory.


Further work:
· Monitor ionosphere, and adjust dedispersion as its electron content changes. · Check sensitivity to cosmic rays. · Highly dependent on lunar surface. · Latest generation of lunar probes (eg Kaguya/SELENE) provide data for this. · Plan for capabilities of future telescopes.


Beam pointing: ASKAP

Australian Square Kilometre Array Pathfinder Murchison, Western Australia

Centaurus A Coherent tied-array beamforming in real time. Less noise; more sensitive.


Sensitivity to isotropic flux

James & Protheroe (2008) Projected neutrino flux limits from one year of observation. Flux predictions from GZK interactions & TD (topological defect) models.


Summary:
· UHE neutrinos are 'messengers' of cosmic rays. · The lunar Cherenkov technique is still being developed, but already has good directional sensitivity. · Future observations (Parkes; ASKAP) will constrain neutrino fluxes from Centaurus A and elsewhere.