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

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 Kilometre Array” (LUNASKA) project is to develop the lunar Cherenkov technique of using terrestrial radio telescope arrays for ultrahigh energy (UHE) cosmic ray (CR) and neutrino detection, and, in particular, to prepare for using the Square Kilometre Array (SKA) and its path-finders such as the Australian SKA Pathfinder (ASKAP) and the Low Frequency Array (LOFAR) for lunar Cherenkov experiments.

Figure 1

A photograph of three of the six ATCA antennas. For the observations described here, the distance between the antennas was greater.

Figure 2

Diagram of the signal path at each antenna. Abbreviations are: “LNA”: low noise amplifier, “ADC”: analogue–digital converter.

Figure 3

Trigger rates for the three antennas (three lower curves) and efficiency (upper curve) from Eq. (1) for the night of May 18th, 2008. The increased trigger rate at UT 15:00–17:00 results from a “Type 1” noise feature caused by an unknown source—see Sec. 5 and the top-center of Fig. 10.

Figure 4

Printed circuit board layout of analog dedispersion filter. Note the variation in the width of the waveguide along its length. The physical size is approximately $25\times 25\mathrm{cm}$.

Figure 5

(a) Predictions of an observed impulse of different origins. (b) A typical noise-calibration pulse—in this case, from CA01 A on Feb 26th. The small prepulse at $-35\mathrm{ns}$ is caused by a spurious reflection from the filter connection point.

Figure 6

(color online) Maximum sampled value as a fraction of the true pulse height as a function of the arbitrary phase offset for sampling rate of $2.048\mathrm{Gs}/\mathrm{s}$ and two extreme lunar Cherenkov pulse types.

Figure 7

Greatest detected amplitude of the time-domain signal relative to the maximum undispersed amplitude, for two extreme lunar Cherenkov pulse types and four sampling rates, as a function of the difference between the line-of-sight total electron content, in TECU, and the constant value used in the dedispersion.

Figure 8

Ionospheric influence over our May 2008 observations. Dashed curves: measured VTEC (TECU) from Ref. 40. Dotted curves: STEC (slant TEC along the line-of-sight). Solid curves: mean fractional loss for coherent (lower curve) and incoherent (upper curve) lunar Cherenkov pulses due to ionospheric effects and triggering inefficiency due to our noninfinite sampling rate (see Fig. 6): the shading gives the range. Vertical lines indicate the times of the observations, horizontal lines give the fractional loss averaged over each observation and over the two extreme cases for the expected spectrum.

Figure 9

Consistency check of correlation times in the February data: alignment relative to the first calibration of the February period, in ($x$-axis) chronological order.

Figure 10

For each twofold coincident event the angle, $\theta$, between the East direction and the direction of propagation of a plane wave fitting the arrival times at the two relevant antennas is calculated, and $\cos \theta$ is plotted (dots) against the time of the occurrence of each event for February (top), and May (bottom). Red dots represent coincidences between antennas CA01 and CA03, green dots for CA01 and CA05, and blue dots for CA03 and CA05. In dot-dense regions, only blue points show, since they are plotted last. The lunar direction (grey dotted line) is also plotted, and positions of candidate events (crosses) are marked.

Figure 11

Raw (top) and cleaned (bottom) spectra for the off-Moon pointing of May 18th on CA05.

Figure 12

$k(\nu )$ as measured for February 28th for CA01 A based on lunar center/off-Moon, piecewise fits to $k(\nu )$. Dashed line shows the mean $\overline{k}(\nu )$.

Figure 13

A narrow time structure event from February 27th. Note that the signal has been displaced vertically by $\pm 30$ as indicated for display purposes.

Figure 14

Comparison of best far field-fitted alignment (top) with the best unrestricted fit (bottom) for polarization B on one of the 16 narrow-time-structure candidates on February 27th, for the values of the offsets ($\Delta t_{31}$, $\Delta t_{51}$, $\Delta t_{53}$) shown. The “wave front curvature” is given by subtracting the measured $t_3$ from the expected value $t_3^{\prime }$ given by Eq. (23).

Figure 15

The range of effective apertures (see text) for the LUNASKA February 2008 center-pointing (C) and May 2008 limb-pointing (L) ATCA observations, compared to that from previous experiments at Parkes (“Pks”), Kalyazin (‘Kal.’), and Goldstone (“GLUE”), in both limb- (“L”), half-limb- (“HL”), and center- (“C”) pointing modes (see 30), assuming the existence of a subregolith layer of comparable dielectric properties to the regolith itself. We also show the effect on the aperture for the (center-pointing) Feb. 2008 observations including our adjusted toy model of small-scale surface roughness (C, SSR incl.) (see Appendix ), which is the reason for the abrupt (and artificial) change in aperture near $2\times {10}^{21}\mathrm{eV}$.

Figure 16

Model-independent 90% confidence limits, i.e. $2.3/[t_{\mathrm{eff}}{A}_{\mathrm{eff}}(E_{\nu })]$ for our nominal 33.5 hr observations (effective on-time $t_{\mathrm{eff}}=26.15\mathrm{hr}$), on a total flux of UHE neutrinos (adjusted for all neutrino flavours) from our 2008 ATCA observations assuming a subregolith layer, both using the standard modeling (“ATCA no SSR”) and using our adjusted toy model of small-scale surface roughness (“ATCA SSR incl.”). In the “SSR” case, the abrupt transition near $2\times {10}^{12}\mathrm{GeV}$ is a model artefact (see Appendix ). Also, from previous experiments: GLUE [11]; IceCube [55]; RICE [48]; ANITA [49]; FORTE [43]; NuMoon [24]; revised estimates by James and Protheroe [30] for Parkes, GLUE, and Kalyazin are shown by hatched bands (upper boundary—limit for 10 m regolith; lower boundary—10 m regolith plus 2 km subregolith); Auger surface detectors [56]. The range on the band labeled ATCA reflects experimental uncertainties, while for previous experiments (where applicable) reflects the inclusion or otherwise of a subregolith layer.

Figure 17

Effective apertures of a fictitious experiment, calculated using large-scale roughness only ($A_{\mathrm{eff}}^S$), our small-scale roughness approximation ($A_{\mathrm{eff}}^R$), and an adjusted aperture which is a linear combination of them both ($A_{\mathrm{eff}}^A$—see Sec. ).

Figure 18

The ratio $A_{\mathrm{eff}}^R(N_S{E}_{\nu })/A_{\mathrm{eff}}^S(E_{\nu })$. The error bars reflect both uncertainties in the apertures themselves from the Monte Carlo simulation (inner error bars), and also including the error from $N_S=13\pm 1$. The fit is to the range $E_{\nu }\ge {10}^{21}\mathrm{eV}$ only.