Observation of Broadband In-ice Radiation from Impacting High-Energy Cosmic Rays

TL;DR

This paper provides the first experimental evidence of in-ice radiofrequency emission caused by high-energy cosmic ray impacts in Antarctic ice, supporting the detection of cosmic rays via radio signals.

Contribution

It demonstrates the first detection of in-ice radio signals from cosmic ray air showers impacting the ice, using the Askaryan Radio Array with significant statistical confidence.

Findings

13 events detected with impulsive radio pulses from below the ice surface

Event characteristics match Askaryan radiation from cosmic ray impacts

Detection rate exceeds background expectations with high statistical significance

Abstract

We present the first experimental evidence for in-ice radiofrequency emission from high-energy particle cascades developing in the Antarctic ice sheet. In 208 days of data recorded with the phased-array trigger of the Askaryan Radio Array, we detect 13 events with impulsive radiofrequency pulses originating from below the ice surface. Considering only the arrival angles and timing properties, this rate is inconsistent with an a-posteriori background expectation for thermal noise events and on-surface events at the level of 3.5$\,\sigma$, which rises to 5.1$\,\sigma$ when additionally considering impulsivity. The observed event geometry, event rate, signal shape, spectral content, and electric field polarization are consistent with Askaryan radiation from cosmic ray air shower cores impacting the ice sheet. For the brightest events, the angular radiation pattern independently favors an extended cascade-like emitter over a pointlike source.