Energy and flux measurements of ultra-high energy cosmic rays observed during the first ANITA flight

Abstract

The first flight of the Antarctic Impulsive Transient Antenna (ANITA) experiment recorded 16 radio signals that were emitted by cosmic-ray induced air showers. The dominant contribution to the radiation comes from the deflection of positrons and electrons in the geomagnetic field, which is beamed in the direction of motion of the air shower. For 14 of these events, this radiation is reflected from the ice and subsequently detected by the ANITA experiment at a flight altitude of ∼36 km. In this paper, we estimate the energy of the 14 individual events and find that the mean energy of the cosmic-ray sample is 2.9 × 10 18 eV, which is significantly lower than the previous estimate. By simulating the ANITA flight, we calculate its exposure for ultra-high energy cosmic rays. We estimate for the first time the cosmic-ray flux derived only from radio observations and find agreement with measurements performed at other observatories. In addition, we find that the ANITA data set is consistent with Monte-Carlo simulations for the total number of observed events and with the properties of those events.

Characteristics of Four Upward-Pointing Cosmic-Ray-like Events Observed with ANITA

“…In Table III, we provide estimates of the energy of each of the air showers considered here, based on the assumption of scaling from simulations of down-going CR [10]. For event D, we consider only the upcoming hypothesis.…”

“…In the highest event, which develops above 30 km, the shower was near its maximum development when it passed by ANITA. Geometric estimates of ANITA's expected rate of CRs at these angles, using the acceptance determined by the reflected CRs [10], indicates that the number of detected events is consistent with the known CR spectrum at EeV energies.…”

Measurement of the cosmic ray flux with the ANITA experiment

“…The detailed analysis can be found in [8]. When treating the data for the cosmic ray events that ANITA found, the exponential fall-off of the spectrum with frequency was observed.…”

Upward-Pointing Cosmic-Ray-like Events Observed with ANITA

“…However, the smallest peak electric field value in the ANITA extensive air shower event ensemble was 446 µV/m corresponding to a peak voltage of 145 µV (or 18σ above thermal noise) [12]. This is due to the the multiple band coincidence design of the triggering system, which was not optimal for the extensive air shower pulse spectrum (see [11]). With the objective of setting an upper bound, we set trigger the threshold at half this value requiring that V pk > 72 µV.…”

“…Such simulations have been applied to neutrino [4,5] and cosmic ray [2,11] analyses. In this study, we take a simplified approach with the objective of setting an upper bound to the sensitivity of ANITA.…”