Search

The Beamforming Elevated Array for COsmic Neutrinos (BEACON) is a planned neutrino telescope designed to detect radio emission from upgoing air showers generated by ultrahigh energy tau neutrino interactions in the Earth. This detection mechanism provides a measurement of the tau flux of cosmic neutrinos. We have installed an 8-channel prototype instrument at high elevation at Barcroft Field Station, which has been running since 2018, and consists of 4 dual-polarized antennas sensitive between 30-80 MHz, whose signals are filtered, amplified, digitized, and saved to disk using a custom data acquisition system (DAQ). The BEACON prototype is at high elevation to maximize effective volume and uses a directional beamforming trigger to improve rejection of anthropogenic background noise at the trigger level. Here we discuss the design, construction, and calibration of the BEACON prototype instrument. We also discuss the radio frequency environment observed by the instrument, and categorize the types of events seen by the instrument, including a likely cosmic ray candidate event., 21 pages, 20 figures

Tau neutrinos interacting in the Earth can result in upgoing extensive air showers. These showers produce optical and radio emission that can be detected by orbital and suborbital platforms. We present results of radio emission simulations using ZHAireS for observation from low-Earth orbit as part of NASA's nuSpaceSim program to develop a comprehensive end-to-end simulation package to model these signals. Peculiar properties of the radio emission arise from the fact that these showers develop in extremely rarified portions of the Earth's atmosphere and, being observed from hundreds of kilometers distance, have distinct coherent emission features compared to ground observations.

Probing the ultra-high energy cosmic ray (UHECR) spectrum beyond the cutoff at ~40 EeV requires an observatory with large acceptance, which is challenging to implement with ground arrays. We present a concept for radio detection of UHECRs impacting the Moon's regolith from low-lunar orbit called the Zettavolt Askaryan Polarimeter (ZAP). ZAP would observe several thousands of events above the cutoff (~40 EeV) with a full-sky field of view to test whether UHECRs originate from Starburst Galaxies, Active Galactic Nuclei, or other sources associated with the matter distribution of the local universe at a distance $>$ 1 Mpc. The unprecedented sensitivity of ZAP to energies beyond 100 EeV would enable a test of source acceleration mechanisms. At higher energies, ZAP would produce the most stringent limits on super heavy dark matter (SHDM) via limits on neutrinos and gamma rays resulting from self-annihilation or decay.

At any epoch, particle physics must be open to completely unexpected discoveries, and that is reason enough to extend the reach of searches for ultra-high energy (UHE) photons. The observation of a population of photons with energies $E \gtrsim 100$ EeV would for example imply the existence of either a completely new physical phenomena, or particle acceleration mechanisms heretofore never seen or imagined. But as we outline in this Letter of Interest, there are also good arguments for super-heavy dark matter (SHDM) in a parameter range such that it could be discovered via its decays to, in particular, UHE photons. Only ultra-high energy cosmic ray observatories have capabilities to detect UHE photons. We first investigate how current and future observations can probe and constrain SHDM models in important directions, and then outline some of the scenarios that motivate such searches. We also discuss connections between constraints on SHDM and on the parameter values of cosmological models., SNOWMASS 2021 LoI. Accepted for publication in Astroparticle Physics

FARSIDE (Farside Array for Radio Science Investigations of the Dark ages and Exoplanets) is a Probe-class concept to place a low radio frequency interferometric array on the farside of the Moon. A NASA-funded design study, focused on the instrument, a deployment rover, the lander and base station, delivered an architecture broadly consistent with the requirements for a Probe mission. This notional architecture consists of 128 dual polarization antennas deployed across a 10 km area by a rover, and tethered to a base station for central processing, power and data transmission to the Lunar Gateway. FARSIDE would provide the capability to image the entire sky each minute in 1400 channels spanning frequencies from 100 kHz to 40 MHz, extending down two orders of magnitude below bands accessible to ground-based radio astronomy. The lunar farside can simultaneously provide isolation from terrestrial radio frequency interference, auroral kilometric radiation, and plasma noise from the solar wind. This would enable near-continuous monitoring of the nearest stellar systems in the search for the radio signatures of coronal mass ejections and energetic particle events, and would also detect the magnetospheres for the nearest candidate habitable exoplanets. Simultaneously, FARSIDE would be used to characterize similar activity in our own solar system, from the Sun to the outer planets, including the hypothetical Planet Nine. Through precision calibration via an orbiting beacon, and exquisite foreground characterization, FARSIDE would also measure the Dark Ages global 21-cm signal at redshifts z=50-100. The unique observational window offered by FARSIDE would enable an abundance of additional science ranging from sounding of the lunar subsurface to characterization of the interstellar medium in the solar system neighborhood.

We present a semi-analytical method for the calculation of coherent Askaryan radiation in showers induced by neutrinos of any flavor in ice. We compare our results with those of a full Monte Carlo simulation based on the ZHAireS code. This approach is able to reproduce the vector potential and hence electric field at any experimentally relevant observer position in the time domain. This work extends published results only valid for electron-induced showers. We establish the validity of the semi-analytical calculation of the radio signal produced by all types of neutrino-induced showers in ice. The method is computationally efficient and only requires as inputs the longitudinal charge excess profile of the showers and a parameterization of the vector potential in the far-field region of the shower at the Cherenkov angle that we also provide. Our methodology avoids tracking the contributions to the electric field from millions of particles every time the radio pulse has to be calculated at a given observer position. These results can be readily used in the interpretation of the data taken by experiments, and in the planning and design of future initiatives based on the radio technique in ice., 18 pages, 11 figures. Accepted in Phys. Rev. D

International audience; This paper describes four high altitude sites in southern Peru that were visited, in March 2019, by members of the SGSO Alliance (currently the SWGO Collaboration). The sites are located above 4000 m.a.s.l. with access to water resources and are located between 1 to 4 hours drive from the closest airport. Peruvian authorities, local populations and universities offer support and encourage international collaborations to consider these sites for Astroparticle Observatories.

We describe a new comprehensive, end-to-end simulation package for the modeling of the extensive air shower (EAS) signals from very-high energy (above a PeV) cosmic neutrinos applicable to sub-orbital and space-based observations. The goal is to provide an efficient and practical cosmic neutrino EAS signal generation modeling package to the community that will provide a standard to gauge the neutrino measurement performance of these experiments and aid in their development. The modeling will be performed in such a way to easily be used to provide the neutrino-induced EAS signals delivered to a specific altitude for a given instrumental field-of-view and orientation. In this paper, we discuss the modeling of tau neutrino interactions inside the Earth, propagating the tau-leptons into the atmosphere, modeling the decays, forming composite EASs, generating the air fluorescence, Cherenkov, and radio signals, and the propagation through the atmosphere. Future work on the modeling of atmospheric transmission and scattering effects will include consideration of the uncertainties and more robust modeling of atmospheric and UHECR backgrounds. Additionally, the framework will allow for the calculation of the sky coverage, the pointing requirements for target-of-opportunity follow up observations of transients, and the inclusion of different neutrino interaction cross-sections to allow for the modeling of non-standard physical processes.

The IceCube Neutrino observatory has discovered an astrophysical neutrino flux in the energy range >1 TeV with their highest energy event ~3 PeV. Although IceCube has excellent muon flavor identification, their ability to identify tau neutrinos is highly limited. We present a concept for observing the tau neutrino flux in the 1-100 PeV range, where a flux is guaranteed and its observation would give further support to the astrophysical origin of the IceCube flux. The design of the observatory requires an acceptance hA?i >400 m2 sr×( PeVE )1.5 to match the ~1 PeV IceCube all-flavor event rate in tau neutrinos provided a ?e: ?µ: ?t = 1: 1: 1 flavor ratio. The energy dependence of the acceptance is designed to match the IceCube best-fit spectral flux index of E-2.5 so the spectrum is evenly sampled in the 1-100 PeV range. We present a background-free implementation with pointing resolution < 1? to begin the era of high energy tau neutrino astronomy.

In this study we investigate the capability of the planned expansion of the Owens Valley Long Wavelength Array (OVRO-LWA) to perform radio based composition studies of the cosmic-ray flux between ~50 PeV and ~1 EeV. We use ZHAireS simulations in conjunction with a method similar to the one developed by the LOFAR experiment, but that only uses radio data between 30 and 80 MHz, to reconstruct the depth of shower maximum $X_{\mathrm{max}}$. We found that the lower antenna densities away from the central core of the array lead to a decrease in the effective area for quality $X_{\mathrm{max}}$ reconstructions of low zenith angle events, and that the asymmetry of the array layout creates the need for quality cuts that depend on arrival direction. We also investigate the dependence of $X_{\mathrm{max}}$ uncertainties on shower energy and arrival direction and show that at 500 PeV it is possible to obtain $X_{\mathrm{max}}$ uncertainties lower than 20 g/cm$^2$ for the majority of arrival directions by using a set of simple cuts. For energies below $10^{17}$ eV, alternative reconstruction methods, such as the use of hierarchical beamforming to increase SNR or the use of arrival time measurements on the ground may be needed to maintain $X_{\mathrm{max}}$ resolution at a desirable level.

Tau neutrinos are expected to comprise one third of both the astrophysical and cosmogenic neutrino flux, but currently the flavor ratio is poorly constrained and the expected flux at energies >100 PeV is low. We present a new concept for a radio detector called BEACON sensitive to tau neutrinos with energies greater than 100 PeV in which a radio interferometer searches for upgoing tau neutrinos from a high elevation mountain. Signals from several antennas are coherently summed at the trigger level, permitting not only directional masking of anthropogenic backgrounds, but also a lower trigger threshold. Simulation studies indicate that a modest array size and small number of stations can achieve competitive sensitivity, provided the receivers are at high enough elevation. As a proof of concept, an array of four 30-80 MHz dual polarized antennas was deployed at the White Mountain Research Station.

Recently, the ANITA collaboration reported on two upward-going extensive air shower events consistent with a primary particle that emerges from the surface of the ice. These events may be of $\nu_\tau$ origin, in which the neutrino interacts within the Earth to produce a $\tau$ lepton that emerges from the Earth, decays in the atmosphere, and initiates an extensive air shower. In this paper we estimate an upper bound on the ANITA acceptance to a diffuse $\nu_\tau$ flux detected via $\tau$-lepton-induced air showers within the bounds of Standard Model (SM) uncertainties. By comparing this estimate with the acceptance of Pierre Auger Observatory and IceCube and assuming SM interactions, we conclude that a $\nu_\tau$ origin of these events would imply a neutrino flux at least two orders of magnitude above current bounds., Comment: 12 pages, 7 figures

Whenever the condition of anomaly freedom is imposed within the framework of effective approaches to loop quantum cosmology, one seems to conclude that a deformation of general covariance is required. Here, starting from a general deformation we regain an effective gravitational Lagrangian including terms up to fourth order in extrinsic curvature. We subsequently constrain the form of the corrections for the homogeneous case, and then investigate the conditions for the occurrence of a big bounce and the realization of an inflationary era, in the presence of a perfect fluid or scalar field.

Tau neutrinos are expected to comprise roughly one third of both the astrophysical and cosmogenic neutrino flux, but currently the flavor ratio is poorly constrained and the expected flux at energies above 1017 eV is low. We present a detector concept aimed at measuring the diffuse flux of tau neutrinos in this energy range via a high-elevation mountaintop detector using the radio technique. The detector searches for radio signals from upgoing air showers generated by Earth-skimming tau neutrinos. Signals from several antennas in a compact array are coherently summed at the trigger level, permitting not only directional masking of anthropogenic backgrounds, but also a low trigger threshold. This design takes advantage of both the large viewing area available at high-elevation sites and the nearly full duty cycle available to radio instruments. We present trade studies that consider the station elevation, frequency band, number of antennas in the array, and the trigger threshold to develop a highly efficient station design. Such a mountaintop detector can achieve a factor of ten improvement in acceptance over existing instruments with 100 independent stations. With 1000 stations and three years of observation, it can achieve a sensitivity to an integrated −2 flux of

A successful ground array Radio Frequency (RF)-only self-trigger is demonstrated with 256 dual-polarization antennas of the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA), yielding 10 high-energy cosmic ray candidates. This RF-only capability is predicated on novel techniques for Radio Frequency Interference (RFI) identification and mitigation with an analysis efficiency of approximately 45% for shower-driven events with a Signal-to-noise ratio ≳ 5 against the galactic background noise power of individual antennas. Radio self-triggering enables more efficient detection of cosmic rays over a wider range of zenith angles than possible via triggers from in-situ particle detectors and the method presented here can be easily adapted to neutrino experiments relying on RF-only detection. This paper discusses the system design, RFI characterization and mitigation techniques, and initial results from 10 cosmic ray candidate events identified within a 40-hour observing window. A design for a future optimized commensal cosmic-ray detector for the OVRO-LWA is presented, as well as recommendations for developing a similar capability for other experiments — these designs either reduce data-rate or increase sensitivity by an order of magnitude for many configurations of radio instruments.

We report on an upward traveling, radio-detected cosmic-ray-like impulsive event with characteristics closely matching an extensive air shower. This event, observed in the third flight of the Antarctic Impulsive Transient Antenna (ANITA), a NASA-sponsored long-duration balloon payload, is consistent with a similar event reported in a previous flight. These events may be produced by the atmospheric decay of an upward-propagating $\tau$-lepton produced by a $\nu_{\tau}$ interaction, although their relatively steep arrival angles create tension with the standard model (SM) neutrino cross section. Each of the two events have $a~posteriori$ background estimates of $\lesssim 10^{-2}$ events. If these are generated by $\tau$-lepton decay, then either the charged-current $\nu_{\tau}$ cross section is suppressed at EeV energies, or the events arise at moments when the peak flux of a transient neutrino source was much larger than the typical expected cosmogenic background neutrinos., Comment: 5 pages, 4 figures. Supplemental material available from corresponding author by request

We are developing a ground-based radio interferometric phased array for radio detection of high energy neutrinos, in an effort to lower the energy threshold of radio detection experiments while increasing the effective volume at high energies. The radio detection technique looks for Askaryan emission from neutrinos interacting in large volumes of glacial ice. The principle behind the phased array technique is coherent summing of the broadband, impulsive Askaryan signal from multiple antenna channels, increasing the signal-to-noise ratio for triggering on weak signals. We first discuss simulations and validation measurements related to the phased array technique, including results from a preliminary Monte Carlo simulation, a demonstration of beamforming and measurements of thermal noise correlation in an anechoic chamber, and results from a trigger simulation. We then discuss the design and development of the first ground-based interferometric phased array trigger system, a 16-channel system that has been built and will be deployed as part of one Askaryan Radio Array (ARA) station in December 2017 at the South Pole.

Cosmic neutrinos are expected to include a significant flux of tau neutrinos due to flavor mixing over astronomical length scales. However, the tau-neutrino content of astrophysical neutrinos is poorly constrained and a significant flux of cosmogenic tau neutrinos awaits discovery. Earth-skimming tau neutrinos undergo charged-current interactions that result in a tau lepton exiting the Earth. The tau lepton decay generates anextensive air shower and geomagnetic radio emission. To target the tau neutrinos, we present a new tau neutrino detector concept that uses phased antenna arrays placed on high elevation mountains. Simulation studies indicate that a modest array size and small number of stations can achieve competitive sensitivity, provided the receivers are at highs enough elevation.

Astrophysics, National Taiwan University, Taipei, Taiwan. 7 Dept. of Physics, Univ. of Delaware, Newark, DE 19716, USA. 8 Dept. of Physics, Stanford University, Stanford, CA, 94305, USA. 9 Dept. of Physics and Astronomy, Univ. of Hawaii, Manoa, HI 96822, USA. 10SLAC National Accelerator Laboratory, Menlo Park, CA, 94025, USA. 11Karlsruher Institut fur Technologie, Institut fur Kernphysik , 76021 Karlsruhe, Germany. 12Physics Dept., College of William & Mary, Williamsburg VA 23187, USA. 13Dept. of Physics and Astronomy, University College London, London, United Kingdom. 14Karlsruher Institut fur Technologie, Institut fur Experimentelle Kernphysik, 76128 Karlsruhe

We report on four radio-detected cosmic-ray (CR) or CR-like events observed with the Antarctic Impulsive Transient Antenna (ANITA), a NASA-sponsored long-duration balloon payload. Two of the four were previously identified as stratospheric CR air showers during the ANITA-I flight. A third stratospheric CR was detected during the ANITA-II flight. Here we report on characteristics these three unusual CR events, which develop nearly horizontally, 20-30~km above the surface of the Earth. In addition, we report on a fourth steeply upward-pointing ANITA-I CR-like radio event which has characteristics consistent with a primary that emerged from the surface of the ice. This suggests a possible $\tau$-lepton decay as the origin of this event, but such an interpretation would require significant suppression of the Standard Model ${\tau}$-neutrino cross section., Comment: 5pp,4 figures, referees' comments addressed, this version accepted to Phys. Rev. Letters. Supplemental PDF material available; see source files

A general procedure for precision timing calibration of waveform digitizing systems is presented. Application specific integrated circuits (ASICs) implementing this functionality are increasingly used in high-energy physics as replacements for stand-alone time-to-digital and analog-to-digital modules. However, process variations cause such ASICs to have irregularly spaced timing intervals between samples, so careful calibration is required to improve the timing resolution of such systems. The procedure presented here exploits correlations between nearby samples of a sine wave of known frequency to obtain the time difference between them. As only the correlations are used, the procedure can be performed without knowledge of the phase of the input signal, and converges with smaller data samples than other common techniques. It also serves as a valuable diagnostic tool, allowing a fast, visual, qualitative check of gain mismatches between sampling cells and other ADC artifacts. Work is continuing to extend the procedure to fit for timing intervals in the face of such non-idealities. We present both the algorithm and example calibration results from a commercial oscilloscope and the PSEC-3 ASIC. For the latter, we have also applied the calibration to improve timing resolution in the readout of a prototype microchannel plate photomultiplier tube with a stripline anode configuration.

A flux of extra-terrestrial neutrinos at energies $\gg10^{15}$ eV has the potential to serve as a cosmological probe of the high-energy universe as well as tests of fundamental particle interactions. Cosmogenic neutrinos, produced from the interactions of ultra-high energy cosmic rays (UHECRs) with cosmic photon backgrounds, have been regarded as a guaranteed flux. However, the expected neutrino flux depends on the composition of UHECRs at the highest energies; heavier nuclei result in lower neutrino fluxes compared to lighter nuclei and protons. The objective of this study is to estimate the range of cosmogenic neutrino spectra consistent with recent cosmic-ray spectral and compositional data using a fully inferential Bayesian approach. The study assumes a range of source distributions consistent with astrophysical sources, the flux and composition of cosmic rays, and detector systematic uncertainties. The technique applied to this study is the use of an affine-invariant Markov Chain Monte Carlo, which is an effective Bayesian inference tool for characterizing multi-dimensional parameter spaces and their correlations., Comment: 28 pages, 6 figures

Using detailed Monte Carlo simulations we have characterized the features of the radio emission of inclined air showers in the Ultra-High Frequency band (300 MHz - 3 GHz). The Fourier-spectrum of the radiation is shown to have a sizable intensity well into the GHz frequency range. The emission is mainly due to transverse currents induced by the geomagnetic field and to the excess charge produced by the Askaryan effect. At these frequencies only a significantly reduced volume of the shower around the axis contributes coherently to the signal observed on the ground. The size of the coherently emitting volume depends on frequency, shower geometry and observer position, and is interpreted in terms of the relative time delays. At ground level, the maximum emission at high frequencies is concentrated in an elliptical ring-like region around the intersection of a Cherenkov cone with its vertex at shower maximum and the ground. The frequency spectrum of inclined showers when observed at positions that view shower maximum in the Cherenkov direction, is shown to be in broad agreement with the pulses detected by the Antarctic Impulsive Transient Antenna (ANITA) experiment, making the interpretation that they are due to Ultra-High Energy Cosmic Ray atmospheric showers consistent with our simulations. These results are also of great importance for experiments aiming to detect molecular bremsstrahlung radiation in the GHz range as they present an important background for its detection., 8 pages, 8 figures

The detection of electromagnetic pulses from high energy showers is used as a means to search for Ultra-High Energy cosmic ray and neutrino interactions. An approximate formula has been obtained to numerically evaluate the radio pulse emitted by a charged particle that instantaneously accelerates, moves at constant speed along a straight track and halts again instantaneously. The approximate solution is applied to the particle track after dividing it in smaller subintervals. The resulting algorithm (often referred to as the ZHS algorithm) is also the basis for most of the simulations of the electric field produced in high energy showers in dense media. In this work, the electromagnetic pulses as predicted with the ZHS algorithm are compared to those obtained with an exact solution of the electric field produced by a charged particle track. The precise conditions that must apply for the algorithm to be valid are discussed and its accuracy is addressed. This comparison is also made for electromagnetic showers in dense media. The ZHS algorithm is shown to describe Cherenkov radiation and to be valid for most situations of interest concerning detectors searching for Ultra-High Energy neutrinos. The results of this work are also relevant for the simulation of pulses emitted from air showers., Comment: 12 pages, 7 figures

An in-depth characterization of coherent radio Cherenkov pulses from particle showers in dense dielectric media, referred to as the Askaryan effect, is presented. The time-domain calculation developed in this article is based on a form factor to account for the lateral dimensions of the shower. It is computationally efficient and able to reproduce the results of detailed particle shower simulations with high fidelity in most regions of practical interest including Fresnel effects due to the longitudinal development of the shower. In addition, an intuitive interpretation of the characteristics of the Askaryan pulse is provided. We expect our approach to benefit the analysis of radio pulses in experiments exploiting the radio technique., Replaced with version published Phys. Rev. D

The Owens Valley Long Wavelength Array has made self-triggered radio detections of cosmic-ray air showers (Monroe et al. 2019 submitted, see Romero-Wolf et al. proceedings PoS(ICRC2019)405) using 256 irregularly-spaced dual-polarization dipole antennas with ~60 MHz bandwidth centered around 55 MHz and maximum antenna separations of 1.5 km. Over the next two years, an upgrade to 352 antennas up to 2.6 km apart and new signal processing infrastructure will expand the capabilities of the array, including its sensitivity to cosmic ray air showers. In addition to the benefit of a larger area, wider antenna separations will improve the sensitivity to inclined events, thus increasing the detection rate of higher-energy cosmic rays. Furthermore, an observing mode with hierarchical beam-forming (grouping subsets of dipoles in-phase) will increase the detection rate of lower-energy cosmic rays. Detecting air showers requires response to ten-nanosecond-timescale signals, coincidenced across the array, and thus requires novel FPGA firmware. This poster will focus on the preliminary design of the new digital signal processing system.

High-energy cosmic neutrinos carry unique information about the most energetic non-thermal sources in the Universe. This white paper describes the outstanding astrophysics questions that neutrino astronomy can address in the coming decade. A companion white paper discusses how the observation of cosmic neutrinos can address open questions in fundamental physics. Detailed measurements of the diffuse neutrino flux, measurements of neutrinos from point sources, and multi-messenger observations with neutrinos will enable the discovery and characterization of the most energetic sources in the Universe., Comment: White paper for the Astro2020 US decadal survey. Companion paper: "Fundamental Physics with High-Energy Cosmic Neutrinos" (1903.04333). Corresponding Authors: Markus Ahlers, Albrecht Karle, Kohta Murase, Anna Nelles, Andres Romero-Wolf, Abigail Vieregg

We present the successful radio frequency (30 - 80 MHz) self-triggered detection and identification of 10 high energy cosmic rays with Caltech's Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA). The core of the OVRO-LWA consists of 256 dual-polarization antennas that are approximately evenly spaced in a 200 m diameter disk with several outliers at longer distances. Based on this success we present future plans for an array covering a 2.5 km diameter area for measuring the flux and composition of cosmic rays in the $3\times10^{16}$ eV - $10^{18}$ eV energy band.

Two anomalous events have been reported by the ANITA collaboration that are observationally consistent with air showers from a particle emerging from the Antarctic ice. One possible interpretation of these events is that they are due to tau neutrinos interacting in the Earth, resulting in an air shower initiated by a tau lepton decay. We present a comprehensive study of the sensitivity of ANITA to tau neutrinos, using the expected Standard Model cross-sections and tau lepton energy losses, varied ice thicknesses, and radio emission simulation of upgoing tau showers with ZHAireS.

These proceedings address a recent publication by the ANITA collaboration of four upward- pointing cosmic-ray-like events observed in the first flight of ANITA. Three of these events were consistent with stratospheric cosmic-ray air showers where the axis of propagation does not inter- sect the surface of the Earth. The fourth event was consistent with a primary particle that emerges from the surface of the ice suggesting a possible {\tau}-lepton decay as the origin of this event. These proceedings follow-up on the modeling and testing of the hypothesis that this event was of {\tau} neutrino origin., Comment: 8 pages, 3 figures, presented at the International Cosmic Ray Conference 2017, Busan, South Korea