Observation of In-ice Askaryan Radiation from High-Energy Cosmic Rays

TL;DR

This paper provides the first in-situ observation of Askaryan radiation from high-energy cosmic-ray cascades within Antarctic ice, using the ARA phased array to identify 13 impulsive events consistent with in-ice Askaryan emission. By combining detailed background modeling, polarization and arrival-direction analyses, and both parameterized and microscopic simulations, the study demonstrates that the observed signals arise from shower cores impacting the ice and not from geomagnetic or surface processes. A two-band spectral discriminant further supports a shower-like, diffractive emission pattern, while a rigorous significance calculation yields a discovery significance of 5.1σ when impulsivity information is included. The results validate the in-ice Askaryan detection concept and provide crucial calibration data and methods for future ultrahigh-energy neutrino searches with radio detectors, with the expectation of many more events in the full dataset.

Abstract

We present the first experimental evidence for in-ice Askaryan radiation -- coherent charge-excess radio emission -- from high-energy particle cascades developing in the Antarctic ice sheet. In 208 days of data recorded with the phased-array instrument of the Askaryan Radio Array, a previous analysis has incidentally identified 13 events with impulsive radiofrequency signals originating from below the ice surface. We here present a detailed reanalysis of these events. The observed event rate, radiation arrival directions, signal shape, spectral content, and electric field polarization are consistent with in-ice Askaryan radiation from cosmic ray air shower cores impacting the ice sheet. For the brightest events, the angular radiation pattern favors an extended cascade-like emitter over a pointlike source. An origin from the geomagnetic separation of charges in cosmic ray air showers is disfavored by the arrival directions and polarization. Considering the arrival angles, timing properties, and the impulsive nature of the passing events, the event rate is inconsistent with the estimation of the combined background from thermal noise events and on-surface events at the level of $5.1\,σ$.

Figures (25)

• Figure 1: The dense core of a CR air shower (black) emits Askaryan radiation in the near-surface ice, showing a characteristic radial electric field polarization with respect to the shower axis (bottom left). The radiation is beamed around the Cherenkov angle, represented by the yellow cone. The VPol (red) and HPol (blue) receivers of A5 are also shown.
• Figure 2: Top, center: Impulse response of the instrument (black) and VPol waveforms of the passing events (blue), with the instrumental phase response removed and normalized by the peak-to-peak amplitude. Bottom: Relative power density spectra normalized by the maximum power density. Simulated signals are shown in red. The envelope indicates a range of $\approx$$3^\circ$ around the approximate in-ice Cherenkov angle, representative of events passing the selection.
• Figure 3: Zenith angles measured at the phased array for the passing events (vertical lines), with statistical uncertainties indicated by the blue distributions. The simulated distribution for impacting shower cores, normalized to the observed yield, is shown in red.
• Figure 4: Reconstructed polarization angles for the passing events (markers, vertically offset for clarity) with statistical uncertainties (error bars) and a kernel density estimate (solid blue). The red histogram shows the simulation prediction for in-ice Askaryan radiation from shower cores, normalized to the observed event yield. The grey histogram shows the expectation for radiation with identical zenith angle distribution, but geomagnetic polarization. The dashed histograms show variations of the HPol response detailed in the text.
• Figure 5: Top: Noise-normalized VPol signal intensities in the LF and HF bands at multiple receivers (markers), for a high-SNR passing event (left) and a calibration pulser event (right). Model predictions are overlaid for impacting shower cores (dashed) and a pointlike source (dotted). Error bars show statistical uncertainties, and colored envelopes indicate the systematic effects on relative signal arrival direction and propagation distance described in the text. The respective $\Delta \log L$ values are also shown. Bottom: Distributions of $\Delta \log L$ for the passing events (vertical lines, with SNR at the phased array shown by color) and a sample of calibration pulser events.