The Study of a Cosmic Ray Candidate Detected by the Askaryan Radio Array

TL;DR

This work analyzes a cosmic-ray candidate detected by the ARA Station 2 array, focusing on a downward-going shower with a characteristic double-pulse signature from geomagnetic in air and Askaryan in ice emissions. By combining the FAERIE CR shower framework with the AraSim detector simulation, the authors reconstruct the two emission sources, compare arrival-time delays, and show that the data are consistent with a CR-induced air shower geometry within a few nanoseconds and a polarization pattern consistent with the two emission mechanisms. The study provides a concrete CR topology for ARA events, demonstrates good agreement between simulated and observed vertex directions (residuals < $2^\circ$) and time delays (residuals < $5$ ns), and estimates a geomagnetic-to-Askaryan power ratio around 1.7, with ongoing work to constrain the primary energy. The results highlight the ability of radio techniques to both calibrate detectors and characterize CR backgrounds for UHE neutrino searches in Antarctic ice.

Abstract

Experiments designed to detect ultra-high energy (UHE) neutrinos using radio techniques are also capable of detecting the radio signals from cosmic-ray (CR) induced air showers. These CR signals are important both as a background and as a tool for calibrating the detector. The Askaryan Radio Array (ARA), a radio detector array, is designed to detect UHE neutrinos. The array currently comprises five independent stations, each instrumented with antennas deployed at depths of up to 200 meters within the ice at the South Pole. In this study, we focus on a candidate event recorded by ARA Station 2 (A2) that shows features consistent with a downward-going CR-induced air shower. This includes distinctive double-pulse signals in multiple channels, interpreted as geomagnetic and Askaryan radio emissions arriving at the antennas in sequence. To investigate this event, we use detailed simulations that combine a modern ice-impacting CR shower simulation framework, FAERIE, with a realistic detector simulation package, AraSim. We will present results for an optimization of the event topology, identified through simulated CR showers, comparing the vertex reconstruction of both the geomagnetic and Askaryan signals of the event, as well as the observed time delays between the two signals in each antenna.

Figures (5)

• Figure 1: Waveforms(voltage traces in the time domain) of the CR candidate event. Double pulses with variable time delays can be seen in the figure. The top two rows correspond to the VPol channels, while the bottom two rows show the waveforms in the HPol channels.
• Figure 2: The predicted event geometry for the CR candidate event, where the in-ice antennas at the A2 center detect both geomagnetic emission produced in air and Askaryan emission generated within the ice.
• Figure 3: The four strings (S1 to S4) at the A2 station and their distances from the point where the shower core hits the ice (impact distance) are provided in a magnetic coordinate system. The simulated signals will be detected by antennas located along these strings at the specified x-y coordinates.
• Figure 4: Vertex reconstruction directions for the first and second pulses are compared, respectively, with geomagnetic and Askaryan emissions recorded by the A2 station. Calculations are performed in the ASC coordinate system, with the origin at the center of the A2 station. The green box indicates the direction of the IceCube Lab (ICL), a known anthropogenic source at the South Pole. The errors are estimated from the ten simulated events.
• Figure 5: Channel-to-channel time delays, defined as the difference in arrival times between the two pulses, where the arrival time is determined by the peak of the pulse in the Hilbert envelope of the waveform. Errors are based on ten simulated events.