Depth of shower maximum and mass composition of cosmic rays from 50 PeV to 2 EeV measured with the LOFAR radio telescope

We present an updated cosmic-ray mass composition analysis in the energy range $10^{16.8}$ to $10^{18.3}$ eV from 334 air showers measured with the LOFAR radio telescope, and selected for minimal bias. In this energy range, the origin of cosmic rays is expected to shift from galactic to extragalactic sources. The analysis is based on an improved method to infer the depth of maximum $X_{\rm max}$ of extensive air showers from radio measurements and air shower simulations. We show results of the average and standard deviation of $X_{\rm max}$ versus primary energy, and analyze the $X_{\rm max}$-dataset at distribution level to estimate the cosmic ray mass composition. Our approach uses an unbinned maximum likelihood analysis, making use of existing parametrizations of $X_{\rm max}$-distributions per element. The analysis has been repeated for three main models of hadronic interactions. Results are consistent with a significant light-mass fraction, at best fit $23$ to $39$ $\%$ protons plus helium, depending on the choice of hadronic interaction model. The fraction of intermediate-mass nuclei dominates. This confirms earlier results from LOFAR, with systematic uncertainties on $X_{\rm max}$ now lowered to 7 to $9$ $\mathrm{g/cm^2}$. We find agreement in mass composition compared to results from Pierre Auger Observatory, within statistical and systematic uncertainties. However, in line with earlier LOFAR results, we find a slightly lower average $X_{\rm max}$. The values are in tension with those found at Pierre Auger Observatory, but agree with results from other cosmic ray observatories based in the Northern hemisphere.Comment: 24 pages, 14 figures. Accepted for publication in Phys. Rev.

The Relationship of Lightning Radio Pulse Amplitudes and Source Altitudes as Observed by LOFAR

When a lightning flash is propagating in the atmosphere it is known that especially the negative leaders emit a large number of very high frequency (VHF) radio pulses. It is thought that this is due to streamer activity at the tip of the growing negative leader. In this work, we have investigated the dependence of the strength of this VHF emission on the altitude of such emission for two lightning flashes as observed by the Low Frequency ARray (LOFAR) radio telescope. We find for these two flashes that the extracted amplitude distributions are consistent with a power-law, and that the amplitude of the radio emissions decreases very strongly with source altitude, by more than a factor of 2 from 1 km altitude up to 5 km altitude. In addition, we do not find any dependence on the extracted power-law with altitude, and that the extracted power-law slope has an average around 3, for both flashes

Extension of the LOFAR Radboud Air Shower Array

The LOFAR Radboud Air Shower Array (LORA) is an array of scintillators situated at the core of the LOFAR radio telescope. LORA detects particles from extensive air showers and acts as a trigger for the readout of the LOFAR antennas, which are densely spaced and routinely measure radio emission from air showers around 1017 eV. LORA originally consisted of 20 scintillators. An extension is underway that doubles the number of scintillators and increases the effective area of the array. This will result in a 45% increase in the number of triggers from higher energy cosmic rays, which are more likely to produce a strong radio signal. In addition, it will reduce the composition bias inherent in detecting low energy showers. In this contribution we discuss the status of the LORA extension and prospects for the science that can be done with the expanded triggering capabilities and improved calibration of the detector

Reconstructing air shower parameters with LOFAR using event specific GDAS atmospheres

The limited knowledge of atmospheric parameters like humidity, pressure, temperature, and the index of refraction has been one of the important systematic uncertainties in reconstructing the depth of the shower maximum from the radio emission of air showers. Current air shower Monte Carlo simulation codes like CORSIKA and the radio plug-in CoREAS use various averaged parameterized atmospheres. However, time-dependent and location-specific atmospheric models are needed for the cosmic ray analysis method used for LOFAR data. There, dedicated simulation sets are used for each detected cosmic ray, to take into account the actual atmospheric conditions at the time of the measurement. Using the Global Data Assimilation System (GDAS), a global atmospheric model, we have implemented time-dependent, realistic atmospheric profiles in CORSIKA and CoREAS. We have produced realistic event-specific atmospheres for all air showers measured with LOFAR, an event set spanning several years and many different weather conditions. A complete re-analysis of our data set shows that for the majority of data, our previous correction factor performed rather well; we found only a small systematic shift of 2 g/cm$^2$ in the reconstructed $X_{\rm max}$. However, under extreme weather conditions, for example, very low air pressure, the shift can be up to 15 g/cm$^2$. We provide a correction formula to determine the shift in $X_{\rm max}$ resulting from a comparison of simulations done using the US-Std atmosphere and the GDAS-based atmosphere.Comment: Accepted for publication in Astroparticle Physics. arXiv admin note: text overlap with arXiv:1911.0285

Constraining the cosmic-ray mass composition by measuring the shower length with SKA

The current generation of air shower radio arrays has demonstrated that the atmospheric depth of the shower maximum Xmax can be reconstructed with high accuracy. These experiments are now contributing to mass composition studies in the energy range where a transition from galactic to extragalactic cosmic-ray sources is expected. However, we are still far away from an unambiguous interpretation of the data. Here we propose to use radio measurements to derive a new type of constraint on the mass composition, by reconstructing the shower length L. The low-frequency part of the Square Kilometer Array will have an extremely high antenna density of roughly 60.000 antennas within one square kilometer, and is the perfect site for high-resolution studies of air showers. In this contribution, we discuss the impact of being able to reconstruct L, and the unique contribution that SKA can make to cosmic-ray science.Comment: Proceedings 9th International Workshop on Acoustic and Radio EeV Neutrino Detection Activities - ARENA2022, 7-10 June 2022, Santiago de Compostela, Spain (8 pages

Using pulse-shape information for reconstructing cosmic-ray air showers and validating antenna responses with LOFAR and SKA

The Low Frequency Array (LOFAR) is capable of measuring extensive air showers through their radio emission in the frequency range of 30–80 MHz, while the Square Kilometer Array (SKA) will be able to expand this range to 50–350 MHz. A very important characteristic of cosmic rays is the mass of the primary particle, which is associated with the atmospheric depth of the shower maximum (max). The standard max reconstruction procedure with LOFAR involves the use of a library of CORSIKA/CoREAS simulations for a specific measured event and uses the energy deposited to the ground in terms of radio fluence. In this study, to extract information about shower development, not only the energy fluence is considered but the possibility of using information from the pulse shape is investigated in both frequency ranges (30–80 MHz and 50–350 MHz). The study of the pulse shape through the pulse agreement of measured data and simulations also provides a way to diagnose the proper functioning of individual LOFAR dipoles

On the cosmic-ray energy scale of the LOFAR radio telescope

Cosmic rays are routinely measured at LOFAR, both with a dense array of antennas and with the LOFAR Radboud air shower Array (LORA) which is an array of plastic scintillators. In this paper, we present two results relating to the cosmic-ray energy scale of LOFAR. First, we present the reconstruction of cosmic-ray energy using radio and particle techniques along with a discussion of the event-by-event and absolute scale uncertainties. The resulting energies reconstructed with each method are shown to be in good agreement, and because the radio-based reconstructed energy has smaller uncertainty on an event-to-event basis, LOFAR analyses will use that technique in the future. Second, we present the radiation energy of air showers measured at LOFAR and demonstrate how radiation energy can be used to compare the energy scales of different experiments. The radiation energy scales quadratically with the electromagnetic energy in an air shower, which can in turn be related to the energy of the primary particle. Once the local magnetic field is accounted for, the radiation energy allows for a direct comparison between the LORA particle-based energy scale and that of the Pierre Auger Observatory. They are shown to agree to within (6$\pm$20)% for a radiation energy of 1 MeV, where the uncertainty on the comparison is dominated by the antenna calibrations of each experiment. This study motivates the development of a portable radio array which will be used to cross-calibrate the energy scales of different experiments using radiation energy and the same antennas, thereby significantly reducing the uncertainty on the comparison