arXiv subjects

Stijn Buitink

Publications and source records attributed to Stijn Buitink.

At least 19 recordsLinked to original sources

arXiv2026

Reconstruction of anomalous air showers with SKA-Low

Double-bump showers are a surprising class of extensive air showers (EAS) predicted by Monte Carlo simulations, which, so far, no experiment has been able to directly detect. They occur when a high-energy secondary particle, the leading particle, travels significantly farther than the rest, creating a distinct double-peaked longitudinal profile. The unique radio footprint of double-bump showers, characterized by multiple pulses in the signals and interference patterns in the frequency spectra, enables reconstruction of longitudinal profiles from radio observations. With its dense antenna array and broad frequency range, SKA-Low will be the first observatory capable of detecting these features, offering a new opportunity to probe hadronic interactions and use the distinctive signatures of elements to provide new mass composition measurements. The goal of this analysis is to take the first steps toward using these radio signatures to reconstruct the relevant parameters of the longitudinal profile of a double-bump shower. We will start by explaining the radio signal of double-bump}showers compared to that of average showers. Then we will create a simple 2-point emission model to explain the interference patterns in the frequency spectra, which can be inverted to obtain rudimentary estimates of atmospheric depth of both peaks. Lastly, we implement a brute-force approach to reconstruct multiple parameters of the double bump.

astro-ph.HE↗

arXiv2026

Beyond $X_\mathrm{max}$ : Reconstructing Air Shower Profiles with Information Field Theory with SKA-Low

While radio measurements of extensive air showers have shown to achieve a high precision of $X_\mathrm{max}$ sensitivity, it has been shown that parameters beyond $X_\mathrm{max}$ can also be reconstructed. These shape parameters contain additional sensitivity to the hadronic physics in the shower as well as its mass composition. In this work, we showcase a reconstruction framework to recover the full longitudinal profile from realistic radio measurements. The framework is based on Information Field Theory that infers the full profile with a forward-based model, which uses a Gaisser-Hillas profile with weakly informative shower priors, SMIET with a template library to synthesise pulses at any event geometry, and a realistic antenna response and noise level emulating that of SKA-Low. We verify the self-consistency of our framework with $\sim 900$ events generated with SMIET with antennas placed on the $\vec{v} \times (\vec{v} \times \vec{B})$ axis. The framework recovers the full profile within uncertainty and capture correlations between shower parameters. We yield an $X_\mathrm{max}$ resolution of $< 9$ g cm$^{-2}$ as well as resolutions of the width and asymmetry with minimal bias. The profile is also recovered with a bias of $< 4$% at all atmospheric depths $< 1200$ g cm$^{-2}$. We aim to apply this framework with pulses simulated from CoREAS with measured noise, ultimately extending the framework to realistic antenna layouts such as from LOFAR or SKA-Low.

astro-ph.IM↗

arXiv2026

Unveiling the Mysteries of Lightning: Exploring its fundamental Physical Processes with SKA-LOW

Lightning is a surprisingly poorly understood phenomena. It consists of a wide variety of complex processes such as initiation, propagation, connection to ground, even emission of high-energy radiation. However, due to the extreme challenges in observing lightning at fast time scales, small spatial scales, and behind obscuring clouds, these processes are not well understood. In the past, interferometers such as the LOFAR radio telescope have provided unique insight and discoveries into the physics of lightning. The new SKA-LOW being built in western Australia will provide unrivaled spectral bandwidth and sensitivity, which will be combined with high resolution resulting from large antenna baselines. We will use SKA-LOW to observe lightning in order to explore its fundamental plasma physics, such as how it initiates and propagates. SKA's high bandwidth will allow us to test how lightning emits VHF radiation, giving tremendous insight into precisely how the plasma behaves. SKA's sensitivity will allow us to explore extremely faint lightning processes, such as the very first radio emission from a lightning flash. Here, we detail the lightning physics that can be explored with SKA, as well as the observation strategy needed explore such physics.

astro-ph.EP↗

arXiv2026

Anomalous Air Showers and What They Reveal About Hadronic Interactions and Cosmic-ray Masses

The identification of the sources and acceleration mechanisms of cosmic rays require precise measurements of their mass composition. Currently, the most reliable method is to measure the atmospheric depth at which cosmic ray air showers in our atmosphere reach their maximum (\Xmax). However, the hadronic interaction properties that govern the longitudinal development of air showers are not precisely known, which is a major source of systematic uncertainty on the mass composition. SKA-Low will observe cosmic rays in the 10$^{16}$ - 10$^{18}$ eV energy range with unprecedented resolution and bandwidth. This allows for a much more detailed reconstruction of the longitudinal shower evolution, which can be used to gain better understanding of the hadronic interactions, as well as the primary mass composition. After the first interaction of the cosmic ray with an atom in an air molecule, the secondary particles still carry a significant fraction of the total energy. When one of these particle travels very far before interacting again, it produces a sub-shower that can be recognized as a secondary bump in the longitudinal profile. Simulations have demonstrated that SKA-Low can resolve such double bump profiles by virtue of its high antenna density and broad bandwidth. In this chapter, we demonstrate how double-bump showers and other anomalous longitudinal developments can be used to constrain hadronic interaction properties, and to determine the mass composition of cosmic rays in the Galactic-to-extragalactic transition region.

astro-ph.HE↗

arXiv2026

Measuring High-Energy Cosmic Particles with the SKA

The origin of high-energy cosmic rays remain one of astrophysics' greatest unsolved mysteries. SKA-Low will be able to measure air showers initiated by cosmic rays with unprecedented precision in the PeV - EeV energy range, covering the critical transition region between Galactic and extragalactic sources. SKA-Low's densely instrumented core and broad bandwidth will allow for measurements of individual air showers with a level of detail unmatched by any existing or planned detector. The depth of shower maximum, the primary mass-sensitive observable, will be reconstructed with a resolution of better than 8~g/cm$^2$, a significant improvement over existing methods. Additionally, new reconstruction methods are expected to enable full air shower reconstruction across a wide energy range, down to PeV levels. At these energies, efficient photon/hadron separation may offer an opportunity to measure PeV gamma-ray air showers. Furthermore, SKA-Low opens a window into studying high-energy hadronic interactions, including via the unique channel of anomalous air showers. This combination of measurements provides a unique opportunity to investigate the origins and physics of high-energy cosmic rays. A dedicated particle detector array will provide triggered readout of raw antenna-level voltage buffers, enabling fully commensal cosmic-ray observations alongside regular operations. We outline our science case and discuss the observational strategy, signal properties and detector design underpinning these measurements. We also summarize the accompanying book chapters, which address composition measurements in the Galactic-to-extragalactic transition region, next-generation interferometric reconstruction techniques, hadronic interaction physics through anomalous air showers, the prospects for detecting PeV gamma-rays from Galactic sources, and the related project of imaging lightning using SKA-Low.

astro-ph.HE↗

arXiv2026

Interferometric Analysis of Air-shower Radio Emission in the Near Field with an Information Field Theory Approach

Current reconstruction techniques for air-shower radio emission generated by cosmic rays have shown great success, having been applied to several radio detectors over the last decade. Nevertheless, they are limited by their high computational cost, simplified approximations, and signal information used for reconstruction. As such, advanced analyses are required to not only be able to perform a holistic reconstruction of all parameters, but also to conduct near-field interferometry of the air shower. This can be achieved through Information Field Theory (IFT), an imaging reconstruction framework based on Bayesian inference that can extract all available information within the signal to infer distributions of field-like quantities. In this chapter, we highlight current novel approaches that use IFT for air shower reconstruction, and the potential of their applicability towards SKA-Low.

astro-ph.IM↗

arXiv2026

Using SKA-Low to Detect PeV Gamma-rays from Galactic Sources

Detecting so called PeVatrons is considered one of the prime goals of $\gamma$-ray astronomy. PeVatrons are astrophysical objects in the Galaxy that are sources of cosmic rays exceeding PeV ($10^{15}$ eV) energies, the highest in our Galaxy. Their nature is unknown as of now, with some candidates reaching barely above PeV energies just having been identified. Serendipitously, the energy threshold of air shower detection using radio emission, has been proven at 50 PeV. There is a case to be made that SKA-Low with its unprecedented number of antennas, can reach lower in energy, while the size of the core is sufficiently large provide a significant effective area to measure PeV fluxes. While this promises a novel angle towards understanding the cosmic ray accelerators in our Galaxy, it also would be the first detection of $\gamma$-ray air showers using radio emission.

astro-ph.HE↗

arXiv2026

A Bayesian Method for Air-Shower Reconstruction using Information Field Theory

The radio detection of extensive air showers provides a powerful method for studying the origin of high-energy cosmic rays. The Low-Frequency Array (LOFAR) offers unprecedentedly detailed measurements of the radio emission footprint. However, fully exploiting this information requires advanced reconstruction techniques. In this paper, we introduce a novel framework for air shower reconstruction based on Bayesian inference and Information Field Theory (IFT). Our method is built on a fully differentiable forward model of the radio signal, which incorporates a physical emission parameterization and a precise wavefront model. Additionally, we augment this physical model with Gaussian processes to account for systematic uncertainties in both the signal fluence and arrival timing. By leveraging gradient information, our approach enables efficient (three orders of magnitude acceleration w.r.t.\ the legacy method) and robust inference of the underlying physical shower parameters, such as primary energy and the depth of shower maximum, $X_\text{max}$. This work provides not only point estimates but also a rigorous quantification of uncertainties. We achieve a resolution in $X_\text{max}$ of $25\,\mathrm{g/cm^2}$ and a radiation energy resolution of $12\%$ on simulations for LOFAR.

astro-ph.HE↗

arXiv2025

Measuring the locations and properties of VHF sources emitted from an aircraft flying through high clouds

We show that it is possible to locate the few places on the body of an airplane, while it is flying through high clouds, from which broad-band, pulsed, radiation is emitted at Very High Frequency (VHF) radio frequencies. This serendipitous discovery was made whilst imaging a lightning flash using the Low-Frequency Array (LOFAR). This observation provides insights into the way the airplane sheds the electrical charge it acquires when flying through clouds. Furthermore, this observation allowed us to test and improve the precision and accuracy for our lightning observation techniques. Our new results indicate that with the improved procedure the location precision for strong pulses is better than 50~cm, with the orientation of linear polarization being accurate to within 25$^\circ$. For the present case of a Boeing 777-300ER, VHF emissions were observed exclusively associated with the two engines, as well as a specific spot on the tail. Despite the aircraft flying through clouds at an altitude of 8~km, we did not detect any emissions from electrostatic wicks.

physics.plasm-ph↗

arXiv2025

SMIET: Fast and accurate synthesis of radio pulses from extensive air shower using simulated templates

Interpreting the data from radio detectors for extensive air showers typically relies on Monte-Carlo based simulation codes, which, despite their accuracy are computationally expensive and present bottlenecks for analyses. To address this issue we developed a novel method called template synthesis, which synthesises the radio emission from cosmic ray air showers in seconds. This hybrid approach uses a microscopically simulated, sliced shower (the origin) as an input. It rescales the emission from each slice individually to synthesise the emission from a shower with different properties (the target). In order to be able to change the arrival direction during synthesis, we adjust the phases based on the expected geometrical delays. We benchmark the method by comparing synthesised traces to CoREAS simulations over a wide frequency range of [30, 500] MHz . The synthesis quality is primarily influenced by the difference in $X_{max}$ between the origin and target shower. When $\Delta X_{max} \leq 100 g/cm^2$ , the scatter on the maximum amplitudes of the geomagnetic traces is at most 4%. For the traces from the charge-excess component this scatter is smaller than 6%. We also observe a bias with $\Delta X_{max}$ up to 5% for both components, which appears to depend on the antenna position. Since the bias is symmetrical around $\Delta X_{max} = 0 g/cm^2$, we can use an interpolation approach to correct for it. We have implemented the template synthesis algorithm in a Python package called \texttt{SMIET}, which includes all the necessary parameters. This package has been successfully tested with air showers with zenith angles up to $50^{\circ}$ and can be used with any atmosphere, observation level and magnetic field. We demonstrate that the synthesis quality remains comparable to our main benchmarks across various scenarios and discuss use cases, including machine-learning-based analyses.

astro-ph.HE↗

arXiv2024

Applying template synthesis to the radio emission from air showers with generic geometries

Studying high-energy cosmic-ray air showers through the radio emission produced by their secondary particles is a well-established technique. However, due to the increasing size and density of the radio arrays, analyses are running into computational limits, as these rely on Monte Carlo simulations to model the emission. To address this, we have been developing template synthesis. With this method, we use semi-analytical expressions to describe how the radio emission from an air shower depends on the shower age and the position of the antenna with respect to the shower. These expressions are extracted from a set of microscopic simulations, thus benefiting from their accuracy. Once obtained, we can use these relations to synthesise the emission from an air shower with any longitudinal profile, by using a single Monte Carlo simulation as an input. Previously we have demonstrated that this hybrid approach can synthesise the radio emission from air showers and agrees with results from microscopic simulations within 10%. The method was however limited to a specific geometry. Here we present our first step towards generalising template synthesis across geometries. We found a set of scaling relations which correct for the shower geometry as well as the viewing angle under which the radiation is observed. This allows us to reformulate the semi-analytical relations in a way that does not longer depend on the geometry, significantly reducing the number of parameters that need to be fitted. We apply these scaling relations to a simulation library of CORSIKA showers with a zenith angle of 50 degrees. We then extract the semi-analytical expressions required for template synthesis, and use them to synthesise the emission from air showers with lower zenith angles. We investigate the accuracy by comparing both to microscopic simulations as well as the single geometry version of template synthesis.

astro-ph.HE↗

arXiv2024

Template synthesis approach for radio emission from extensive air showers

We present a novel way to synthesise the radio emission from extensive air showers. It is a hybrid approach which uses a single microscopic Monte-Carlo simulation to generate the radio emission from a shower with a different longitudinal evolution, primary particle type and energy. The method employs semi-analytical relations which only depend on the shower parameters to transform the radio signal in the simulated antennas. We apply this method to vertical air showers with energies ranging from $10^{17}$ eV to $10^{19}$ eV and compare the results with CoREAS using two different metrics. In order to gauge the performance over our simulation set, we subsequently use every shower in the set as a template to synthesise the emission from the other showers. Depending on the scoring metric, template synthesis reconstructs the radio emission with an accuracy of 5 to 10%.

astro-ph.HE↗

arXiv2024

Generalising template synthesis of EAS radio emission to other geometries

Over the last few decades, radio detection has become one of the standard techniques to study high-energy cosmic-ray air showers. For the purpose of analysing the data, we heavily rely on Monte Carlo simulations. Upcoming dense radio array experiments such as LOFAR2.0 and SKA will, however, reach the limit of what is computationally feasible with these. Other techniques are available, based on macroscopic quantities, but their accuracy has thus far not been adequate to use them in precision analyses. In this contribution we present the latest update on the template synthesis approach, a hybrid model using both micro- and macroscopic inputs to synthesise the radio emission for an air shower with an arbitrary longitudinal profile. The method starts from the emission of a given shower and employs semi-analytical relations which only depend on the atmospheric depth at shower maximum and antenna position in order to transform it. Core to the template synthesis approach is the slicing of the atmosphere. By considering the radio emission from each slice separately, we only need to explicitly account for shower age effects. In previous work it was shown this could be done over a wide range of primary energy and across primary types for vertical air showers, with an accuracy of 10%. Here, we generalise the method to other zenith angles. We investigate the potential to synthesise between different geometries using a data set consisting of several hundreds of CORSIKA showers with primary energies ranging from $10^{17}$ eV to $10^{19}$ eV.

astro-ph.HE↗

arXiv2024

Validation of straight-line signal propagation for radio signal of very inclined cosmic ray air showers

An ongoing challenge for radio-based detectors of high-energy cosmic particles is the accurate description of radio signal propagation in natural nonuniform media. For radio signals originating from extensive air showers, the current state of the art simulations often implicitly assume straight-line signal propagation. The refraction due to a nonuniform atmosphere is however expected to have an effect on the received signal and associated reconstruction. This effect is currently not completely understood for the most inclined geometries. Here, we present a study regarding the validity of straight-line signal propagation when simulating radio emission associated with very inclined air shower geometries. To this end, the calculation of the electric field based on the end point formalism used in CoREAS was improved by use of tabulated ray tracing data. We find a difference of 2% in radiation energy and a difference of O(0.01{\deg}) on direction reconstruction when working at frequencies below 600 MHz. We thus find that, for frequencies up to 600 MHz and zenith angles up to 88{\deg}, the current straight-line based simulation approaches are accurate.

astro-ph.HE↗

arXiv2024

Simulation of radio signals from cosmic-ray cascades in air and ice as observed by in-ice Askaryan radio detectors

A new generation of neutrino observatories will search for PeV-EeV neutrinos interacting in the ice by detecting radio pulses. Extended air showers propagating into the ice will form an important background and could be a valuable calibration signal. We present results from a Monte-Carlo simulation framework developed to fully simulate radio emission from cosmic-ray particle cascades as observed by in-ice radio detectors in the polar regions. The framework involves a modified version of CoREAS (a module of CORSIKA 7) to simulate in-air radio emission and a GEANT4-based framework for simulating in-ice radio emission from cosmic-ray showers as observed by in-ice antennas. The particles that reach the surface of the polar ice sheet at the end of the CORSIKA 7 simulation are injected into the GEANT4-based shower simulation code that takes the particles and propagates them further into the ice sheet, using an exponential density profile for the ice. The framework takes into account curved ray paths caused by the exponential refractive index profiles of air and ice. We present the framework and discuss some key features of the radio signal and radio shower footprint for in-ice observers.

astro-ph.HE↗

arXiv2024

Proof of principle for template synthesis approach for the radio emission from vertical extensive air showers

The radio detection technique of cosmic ray air showers has gained renewed interest in the last two decades. While the radio experiments are very cost-effective to deploy, the Monte-Carlo simulations required to analyse the data are computationally expensive. Here we present a proof of concept for a novel way to synthesise the radio emission from extensive air showers in simulations. It is a hybrid approach which uses a single microscopic Monte-Carlo simulation, called the origin shower, to generate the radio emission from a target shower with a different longitudinal evolution, primary particle type and energy. The method employs semi-analytical relations which only depend on the shower parameters to transform the radio signals in the simulated antennas. We apply this method to vertical air showers with energies ranging from $10^{17}$ eV to $10^{19}$ eV and compare the results with CoREAS simulations in two frequency bands, namely the broad [20, 500] MHz band and a more narrow one at [30, 80] MHz. We gauge the synthesis quality using the maximal amplitude and energy fluence contained in the signal. We observe that the quality depends primarily on the difference in $X_{\text{max}}$ between the origin and target shower. After applying a linear bias correction, we find that for a shift in $X_{\text{max}}$ of less than 150 $\text{g}/\text{cm}^2$ , template synthesis has a bias of less than 2% and a scatter up to 6%, both in amplitude, on the broad frequency range. On the restricted [30, 80] MHz range the bias is similar, but the spread on amplitude drops down to 3%. These fluctuations are on the same level as the intrinsic scatter we observe in Monte-Carlo ensembles. We therefore surmise the observed scatter in amplitude to originate from intrinsic shower fluctuations we do not explicitly account for in template synthesis.

astro-ph.HE↗

arXiv2023

Detection of Ultra High Energy Cosmic Rays and Neutrinos with Lunar Orbital Radio Telescope

Particle cascades induced by ultra-high-energy (UHE) cosmic rays and neutrinos impacting on the lunar regolith usually radiate Cherenkov radio emissions due to the presence of excess negative charge, which is known as Askaryan effect. Several experiments have been carried out to detect the Cherenkov radio emissions in the lunar regolith. To prepare for future lunar Ultra-Long Wavelength (ULW, frequencies below 30 MHz) radio astronomy missions, we study the detection of the Cherenkov radio emissions with the ULW radio telescope that are operating at the lunar orbit. We have carried out instrument modelling and analytic calculations for the analysis of aperture, flux and event rate, and the analyses show the detectability of the Cherenkov radiation. Based on the properties of the Cherenkov radiation, we have demonstrated that the cosmic ray and neutrino events could be reconstructed with the three ULW vector antennas onboard the lunar satellites via measurements of the Askaryan radio pulse intensity, polarizations, etc. The results obtained by this study would be useful for future lunar radio explorer mission, where the detections of UHE cosmic rays and neutrinos could be successfully attempted.

astro-ph.IM↗

arXiv2021

Interferometric imaging of Intensely Radiating Negative Leaders

The common phenomenon of lightning still harbors many secrets and only recently a new propagation mode was observed for negative leaders. While propagating in this `Intensely Radiating Negative Leader' (IRNL) mode a negative leader emits 100 times more very-high frequency (VHF) and broadband radiation than a more normal negative leader. We have reported that this mode occurs soon after initiation of all lightning flashes we have mapped as well as sometimes long thereafter. Because of the profuse emission of VHF the leader structure is very difficult to image. In this work we report on measurements made with the LOFAR radio telescope, an instrument primarily built for radio-astronomy observations. For this reason, as part of the present work, we have refined our time resolved interferometric 3-Dimensional (TRI-D) imaging to take into account the antenna function. The images from the TRI-D imager show that during an IRNL there is an ionization front with a diameter in excess of 500~m where strong corona bursts occur. This is very different from what is seen for a normal negative leader where the corona bursts happen at the tip, an area of typically 10~m in diameter. The observed massive ionization wave supports the idea that this mode is indicative of a dense charge pocket.

physics.ao-ph↗