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Unusual Near-Horizon Cosmic-Ray-like Events Observed by ANITA-IV

P. W. Gorham et al.
Phys. Rev. Lett. 126, 071103 – Published 19 February 2021
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  • ACKNOWLEDGMENTS
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    Abstract

    ANITA’s fourth long-duration balloon flight in 2016 detected 29 cosmic-ray (CR)-like events on a background of 0.37−0.17+0.27 anthropogenic events. CRs are mainly seen in reflection off the Antarctic ice sheets, creating a phase-inverted waveform polarity. However, four of the below-horizon CR-like events show anomalous noninverted polarity, a p=5.3×10−4 chance if due to background. All anomalous events are from locations near the horizon; ANITA-IV observed no steeply upcoming anomalous events similar to the two such events seen in prior flights.

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    • Received 19 August 2020
    • Revised 20 October 2020
    • Accepted 12 November 2020

    DOI:https://doi.org/10.1103/PhysRevLett.126.071103

    © 2021 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Cosmic rays & astroparticlesExtrasolar neutrino astronomy
    1. Physical Systems
    Neutrinos
    1. Techniques
    Cherenkov detectorsCosmic ray & astroparticle detectorsNeutrino detectionTelescopes
    Gravitation, Cosmology & AstrophysicsParticles & Fields

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    Vol. 126, Iss. 7 — 19 February 2021

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    Images

    • Figure 1
      Figure 1

      Diagram of ANITA-IV signal channels, in the near-vertical Antarctic geomagnetic field. Geomagnetic signals appear from CRs either directly or in reflection and include a small Askaryan radiation component. Askaryan signals dominate the under-ice neutrino channel and can arise from multiple showers due to both primary and secondary interactions. Geomagnetic signals can also arise from EAS initiated by τ-lepton decay after ντ charged-current (CC) interactions within the ice.

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    • Figure 2
      Figure 2

      Top: ANITA-IV flight path and location of payload and apparent event source location and ice surface elevation above sea level for each of the 29 events in the final CR sample. Bottom: Geomagnetic correlation of 29 candidate events revealed in our CR analysis.

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    • Figure 3
      Figure 3

      Normalized overlays of the 21 normal reflected CR in our sample, in each of the four deconvolution methods used, as noted by the title for each pane. clean [23] is a time-domain method based on iterative correlation and subtraction of the impulse response [24, 25]. The all-pass method applies simple Fourier-division deconvolution to the phases only. Wiener deconvolution uses a noise-optimized Fourier deconvolution method for both phases and amplitudes [26]. The wavelet method [27, 28] uses wavelet, rather than Fourier, basis functions for the deconvolution. The bottom plot gives the normalized average waveform of the four methods.

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    • Figure 4
      Figure 4

      The incident field strength vs time for the six near-horizon events, all of which have the same (noninverted) polarity: two above-horizon (pale blue background) and four below-horizon (white background) with anomalous polarity. These plots use the clean deconvolution method as the waveform estimate.

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