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

TL;DR

This paper provides the first experimental evidence of in-ice Askaryan radiation caused by high-energy cosmic ray cascades in Antarctic ice, using data from the Askaryan Radio Array over 208 days.

Contribution

It presents a detailed reanalysis confirming in-ice Askaryan radiation from cosmic rays, distinguishing it from other sources and background noise.

Findings

Detected 13 impulsive radio events consistent with Askaryan radiation

Event characteristics match expectations for cosmic ray-induced cascades in ice

Event rate significantly exceeds background noise estimates at 5.1 sigma

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 $5.1\,\sigma$.