arXiv subjects

P. Chen

Publications and source records attributed to P. Chen.

At least 19 recordsLinked to original sources

arXiv2026

Trigger system for the Payload for Ultrahigh Energy Observations (PUEO) balloon-borne neutrino detector

The Payload for Ultrahigh Energy Observations (PUEO) is a NASA balloon-borne instrument for the detection of ultra-high energy (UHE) neutrinos with energies above $10^{17.5}~\textrm{eV}$ via either the Askaryan effect or geomagnetic emissions from an upward-going air shower. The main instrument trigger system for PUEO is a fully digital supersample rate beamformer based on 24 Xilinx Radio Frequency System-on-a-Chip (RFSoC) digitizers sampling 192 channels operating at $3~\textrm{GSa/s}$ and a system clock frequency of $375~\textrm{MHz}$. The trigger implements frequency band conditioning, dynamic radio-frequency interference (RFI) rejection, and matched filtering, with significant emphasis on optimization to reduce both the power and resource usage while maintaining sensitivity. The system implements 48 total synthetic antenna beams with up to 8 antennas each, covering a $\sim25^\circ$ range in zenith and $\sim60^\circ$ range in azimuth. Preflight testing demonstrated a trigger performance of a minimum signal-to-noise ratio (SNR) of $\sim1.5$ using simulated signals while consuming between $5-7~\textrm{W}$ in the trigger logic.

astro-ph.IM↗

arXiv2026

The flash-ionised SN Ibn 2025kzr: H-free CSM formed during a precursor outburst 55 days prior to explosion

Type Ibn supernovae (SNe) are a class of interacting SNe characterised by narrow helium lines in their spectra. We present an extensive observational dataset of the Type Ibn SN 2025kzr at 51 Mpc, including the discovery of a precursor outburst with a peak brightness of M_r~-13.6 mag beginning ~55 days before explosion. Our photometry indicates the SN was discovered within the first day of explosion, showing fast-rising, ultraviolet-bright emission peaking at M_r=-19.26+/-0.09 mag and a peak blackbody temperature of T~29000 K, consistent with shock breakout within a region of dense and confined circumstellar material (CSM). Our high-cadence spectroscopic dataset spanning 1.9-58.5 days post-explosion shows flash-ionised emission features during the first 10 days. In our SALT spectrum at 3.8 days we observe a pronounced blueshift of the He II lines by 460 km/s compared to the He I lines at zero velocity, while a Pickering-decrement analysis reveals a CSM that is fully hydrogen-free. The timing of the disappearance of the flash features combined with the CSM velocity of 1500 km/s imply a mass-loss event ~66 days before explosion, in close agreement with the timing of the precursor observed 55 days before explosion and strongly suggestive of a physical link. We derive a CSM mass of 0.03-1.7 M_sun and a corresponding high mass-loss rate >~10^{-1} M_sun/yr. The precursor timescale and energetics suggest an extreme mass-loss event that might be explained by wave-driven mass loss during the late stages of nuclear burning, in particular the oxygen-burning phase. Overall, we favour a single massive Wolf-Rayet progenitor with M_ZAMS~30-40 M_sun to explain SN 2025kzr, although a binary origin cannot be excluded.

astro-ph.HE↗

arXiv2026

Sensitivity of the As-Built Askaryan Radio Array to Ultra-High Energy Neutrinos

The Askaryan Radio Array (ARA) is an ultra-high energy (UHE) neutrino observatory designed to detect the impulsive radio waves produced by relativistic particle cascades in the Antarctic glacial ice. Using a significantly enhanced simulation pipeline, which adds data-driven detector simulations and fully incorporates secondary particle production, we calculate the trigger-level acceptance of the entire array. We compare the resulting trigger-level sensitivity to constraints on the UHE neutrino flux from other detectors. Given its exposure from 2013 to 2023, we find that ARA achieves a world-leading sensitivity above about $10^{19}$ eV, depending on the details of the event selection used in a search. Moreover, we find that up to 13 neutrinos are predicted to have been observed in this period at trigger-level, assuming the most optimistic neutrino flux models. We show that observations of secondary particles account for up to 30\% of the total acceptance starting at $10^{19}$ eV, and we explore the potential signatures and implications of both multi-pulse (from direct and refracted pulses and/or from secondary particle interactions) and multi-station events. Finally, we comment on the implications of this study for the design of next-generation UHE neutrino experiments, in particular IceCube-Gen2 Radio.

astro-ph.HE↗

arXiv2026

The Large Array Survey Telescope-Pipeline. II. Image Subtraction and Transient Detection

Context. The Large Array Survey Telescope (LAST) is a wide-field visual-band survey designed to explore the variable and transient sky with high cadence. Its raw data stream is automatically processed in near real time at the observatory site, producing science-quality images, catalogs, and transient alerts. Transient alerts are then reported to the Transient Name Server (TNS). Aims. The LAST pipeline comprises two major components: (i) processing and calibration of single images followed by coaddition of $20\times20$s exposures, producing single-image and coadded-image catalogs; and (ii) subtraction of coadded images from calibrated reference images followed by transient detection. In this work we present a detailed description and validation of the second component of the pipeline. Methods. Transient detection is based on the algorithm for proper image subtraction (ZOGY). We combine ZOGY subtraction with the Translient statistic for sub-pixel motion discrimination, together with a sequence of deterministic filtering steps, to produce a clean stream of transient candidates without the use of machine learning. Results. Using commissioning data, the pipeline achieves a preliminary $5\sigma$ limiting magnitude of $20.3$-$20.7$mag, a single-epoch transient detection efficiency of $\sim80$%, and a purity of $\gtrsim90$% at signal-to-noise ratio of $\geq7.5\sigma$.

astro-ph.IM↗

arXiv2025

The hydrogen-free circumstellar interaction in the Type Ib supernova 2021efd: A clue to the mechanism of the helium-layer stripping

Stripped-envelope supernovae (SESNe), including Type IIb, Ib, and Ic supernovae (SNe), originate from the explosions of massive stars whose outer envelopes have been largely removed during their lifetimes. The main stripping mechanism for the hydrogen (H) envelope in the progenitors of SESNe is often considered to be interaction with a binary companion, while that for the helium (He) layer is unclear. We conducted photometric and spectroscopic observations of the Type Ib SN 2021efd, which shows signs of interaction with H-free circumstellar material (CSM). Around 30 days after the r-band light curve (LC) peak, until at least ~ 770 days, its LCs display excessive luminosity compared to regular SESNe and at least three distinct peaks. The light curve evolution is similar to that of SN 2019tsf, whose previously unpublished spectrum at 400 days is also presented here. The nebular spectrum of SN 2021efd shows narrow emission lines (~ 1000 km/s) in various species. Our observations suggest that SN 2021efd is a Type Ib SN interacting with CSM with the following parameters: The estimated CSM mass, composition, and distribution are at least a few times 0.1 M_sun, H-free, and clumpy, respectively. Based on the estimated ejecta properties, we conclude that this event is a transitional SN whose progenitor was experiencing He-layer stripping at the epoch of the explosion, and was on the way to becoming a carbon-oxygen star from a He star. The estimated CSM properties suggest that the progenitor had some episodic mass ejections with a rate of ~ 0.001-0.01 M_sun/yr for the last decade and slightly smaller before this final phase at least from ~ 200 years before the explosion, for the assumed CSM velocity of 100 km/s. For the case of ~ 1000 km/s, the necessary mass-loss rate would be increased by a factor of ten, and the timescales decreased by a factor of ten.

astro-ph.SR↗

arXiv2025

The Type IIn SN 2025cbj coincidence with the high-energy neutrino IceCube-250421A

Context. The origin of the astrophysical high-energy neutrino flux remains uncertain. Core-collapse supernovae with strong CSM interaction (Type IIn) are compelling candidates for efficient hadronic acceleration and neutrino production. Aims. We investigate the possible association between the Type IIn supernova SN2025cbj and the IceCube high-energy neutrino IceCube-250421A, and assess whether the observed properties of the SN permit an appreciable neutrino yield. Methods. We combined rapid optical follow-up with LAST and archival ZTF photometry with spectroscopy from LT/SPRAT and MMT/BINOSPEC to characterize the SN evolution and CSM interaction. We estimated the explosion and peak times from early light-curve fitting, and quantified the chance-coincidence probability with resampling simulations that scramble neutrino right ascensions while preserving declinations and error contours. Using a simple post-shock-breakout interaction model in a dense wind, we estimated the expected muon-neutrino yield for IceCube real-time Bronze stream. Results. Spectra of SN2025cbj obtained after the neutrino epoch show persistent narrow Balmer lines superposed on broad Lorentzian electron-scattering wings, consistent with sustained dense-CSM interaction. For the multi-messenger association, resampling simulations against the TNS catalog give a chance-coincidence probability for observing $k \ge 1$ events of $p \simeq 0.24$ (and $p \simeq 0.078$ against the ZTF-BTS catalog). These values are sensitive to the size of the SNe and neutrino samples. A post-breakout interaction scenario predicts an expected $N_{\nu_\mu} \sim 10^{-3}$ events in the IceCube Bronze alert stream over 76 days per this one candidate. We discuss the implications of these numbers and possible biases that may affect these results.

astro-ph.HE↗

arXiv2025

Observation of In-ice Askaryan Radiation from High-Energy Cosmic Rays

We present the first experimental evidence for in-ice Askaryan radiation -- coherent charge-excess radio emission -- from high-energy particle cascades developing in the Antarctic ice sheet. In 208 days of data recorded with the phased-array instrument of the Askaryan Radio Array, a previous analysis has incidentally identified 13 events with impulsive radiofrequency signals originating from below the ice surface. We here present a detailed reanalysis of these events. The observed event rate, radiation arrival directions, signal shape, spectral content, and electric field polarization are consistent with in-ice Askaryan radiation from cosmic ray air shower cores impacting the ice sheet. For the brightest events, the angular radiation pattern favors an extended cascade-like emitter over a pointlike source. An origin from the geomagnetic separation of charges in cosmic ray air showers is disfavored by the arrival directions and polarization. Considering the arrival angles, timing properties, and the impulsive nature of the passing events, the event rate is inconsistent with the estimation of the combined background from thermal noise events and on-surface events at the level of $5.1\,\sigma$.

astro-ph.HE↗

arXiv2025

Design Process of a Self Adaptive Smart Serious Games Ecosystem

This paper outlines the design vision and planned evolution of Blexer v3, a modular and AI-driven rehabilitation ecosystem based on serious games. Building on insights from previous versions of the system, we propose a new architecture that aims to integrate multimodal sensing, real-time reasoning, and intelligent control. The envisioned system will include distinct modules for data collection, user state inference, and gameplay adaptation. Key features such as dynamic difficulty adjustment (DDA) and procedural content generation (PCG) are also considered to support personalized interventions. We present the complete conceptual framework of Blexer v3, which defines the modular structure and data flow of the system. This serves as the foundation for the next phase: the development of a functional prototype and its integration into clinical rehabilitation scenarios.

eess.SY↗

arXiv2025

Incommensurate magnetic order drives singular angular magnetoresistance in a Weyl semimetal

We demonstrate that a multi-$\mathbf{k}$ incommensurate magnetic state in the Weyl semimetal CeAlGe gives rise to singular angular magnetoresistance (SAMR), an electrical-transport signature that detects the magnetic-field direction with exceptional precision. In contrast, the sister compound CeAlSi shows neither multi-$\mathbf{k}$ order nor SAMR. Both phenomena emerge upon $\sim57\%$ Ge substitution in CeAlSi$_{1-x}$Ge$_x$ and coincide with electronic-structure changes that soften the single-ion in-plane anisotropy and enhance Weyl-mediated magnetic interactions. These results reveal a direct connection between band topology, electronic transport, and collective magnetism in Weyl semimetals.

cond-mat.str-el↗

arXiv2025

Seeing the Outer Edge of the Infant Type Ia Supernova 2024epr in the Optical and Near Infrared

We present optical-to-near-infrared (NIR) photometry and spectroscopy of the Type Ia supernova (SN Ia) 2024epr, including NIR spectra observed within two days of first light. The early-time optical spectra show strong, high-velocity Ca and Si features near rarely-observed velocities at $\sim$0.1$c$, and the NIR spectra show a C I "knee." Despite early-time, high-velocity features, SN 2024epr evolves into a normal SN Ia, albeit with stronger peak-light Ca absorption than other SNe Ia with the same light curve shape. Although we infer a normal decline rate, $\Delta m_{15}(B)=1.09\pm0.12$ mag, from the light-curve rise, SN 2024epr is a Branch "cool" object and has red early-time colors ($g-r\approx0.15$ mag at $-10$ days). The high velocities point to a density enhancement in the outer layers of the explosion, predicted by some models, but thick-shell He-detonation models do not match the smoothly rising light curve or apparent lack of He in our early-time NIR spectra. No current models (e.g., delayed detonation or thin He shell double detonation) appear to reproduce all observed properties, particularly the unusual early-time colors. Such constraints are only possible for SN 2024epr from the earliest optical and NIR observations, highlighting their importance for constraining SN Ia models. Finally, we identify several literature SNe Ia with intermediate mass elements at $\sim$30\,000 km s$^{-1}$ within days after the explosion that evolve into otherwise normal SNe Ia at peak light, suggesting the early-time spectra of SNe Ia may hide a broad diversity of observational characteristics.

astro-ph.HE↗

arXiv2025

ZTF SN Ia DR2: High-velocity components in the Si II $\lambda$6355

The ZTF SN Ia Data Release 2 provides a perfect opportunity to perform a thorough search for, and subsequent analysis of, high-velocity components in the Si II $\lambda$6355 feature in the pre-peak regime. The source of such features remains unclear, with potential origins in circumstellar material or density/abundance enhancements intrinsic to the SN ejecta. Therefore, they may provide clues to the elusive progenitor and explosion scenarios of SNe Ia. We employ a MCMC fitting method followed by BIC testing to classify single and double Si II $\lambda$6355 components in the DR2. The detection efficiency of our classification method is investigated through the fitting of simulated features, allowing us to place cuts upon spectral quality required for reliable classification. These simulations were also used to perform an analysis of the recovered parameter uncertainties and potential biases in the measurements. Within the 329 spectra sample that we investigate, we identify 85 spectra exhibiting Si II $\lambda$6355 HVFs. We find that HVFs decrease in strength with phase relative to their photospheric counterparts - however, this decrease can occur at different phases for different objects. HVFs with larger velocity separations from the photosphere are seen to fade earlier leaving only the double components with smaller separations as we move towards maximum light. Our findings suggest that around three quarters of SN Ia spectra before -11 d show high-velocity components in the Si II $\lambda$6355 with this dropping to around one third in the six days before maximum light. We observe no difference between the populations of SNe Ia that do and do not form Si II $\lambda$6355 HVFs in terms of SALT2 light-curve parameter x1, peak magnitude, decline rate, host mass, or host colour, supporting the idea that these features are ubiquitous across the SN Ia population.

astro-ph.HE↗

arXiv2024

ZTF SN Ia DR2: The diversity and relative rates of the thermonuclear SN population

The Zwicky Transient Facility SN Ia Data Release 2 (ZTF SN Ia DR2) contains more than 3,000 Type Ia supernovae (SNe Ia), providing the largest homogeneous low-redshift sample of SNe Ia. Having at least one spectrum per event, this data collection is ideal for large-scale statistical studies of the photometric, spectroscopic and host-galaxy properties of SNe Ia, particularly of the rarer 'peculiar' sub-classes. In this paper we first present the method we developed to spectroscopically classify the SNe in the sample, and the techniques we used to model their multi-band light curves and explore their photometric properties. We then show a method to distinguish between the peculiar sub-types and the normal SNe Ia. We also explore the properties of their host galaxies and estimate their relative rates, focusing on the peculiar sub-types and their connection to the cosmologically useful SNe Ia. Finally, we discuss the implications of our study with respect to the progenitor systems of the peculiar SN Ia events.

astro-ph.HE↗

arXiv2024

Modeling the refractive index profile n(z) of polar ice for ultra-high energy neutrino experiments

We have developed an in-situ index of refraction profile n(z) for cold polar ice, using the transit times of radio signals broadcast from an englacial transmitter to 2-5 km distant radio-frequency receivers, deployed at depths up to 200 m. For propagation through a non-uniform medium, Maxwell's equations generally admit two ray propagation solutions from a given transmitter, corresponding to a direct path (D) and a refracted or reflected path (R); the measured D vs. R timing differences (dt(D,R)) are determined by the refractive index profile. We constrain n(z) near South Pole, where the Askaryan Radio Array (ARA) neutrino observatory is located, by simulating D and R ray paths via ray tracing and comparing simulations to measured dt(D,R) values. Using previous ice density data as a proxy for n(z), we demonstrate that our data strongly favors a glaciologically-motivated three-phase densification model rather than a single exponential scale height model. Effective volume simulations for a detector of ARA station antenna depths yield a 14\% increase in neutrino sensitivity over a range of $10^{17} - 10^{21}$ eV using the three-phase model compared to a single exponential.

astro-ph.IM↗

arXiv2024

A Strategy Transfer and Decision Support Approach for Epidemic Control in Experience Shortage Scenarios

Epidemic outbreaks can cause critical health concerns and severe global economic crises. For countries or regions with new infectious disease outbreaks, it is essential to generate preventive strategies by learning lessons from others with similar risk profiles. A Strategy Transfer and Decision Support Approach (STDSA) is proposed based on the profile similarity evaluation. There are four steps in this method: (1) The similarity evaluation indicators are determined from three dimensions, i.e., the Basis of National Epidemic Prevention & Control, Social Resilience, and Infection Situation. (2) The data related to the indicators are collected and preprocessed. (3) The first round of screening on the preprocessed dataset is conducted through an improved collaborative filtering algorithm to calculate the preliminary similarity result from the perspective of the infection situation. (4) Finally, the K-Means model is used for the second round of screening to obtain the final similarity values. The approach will be applied to decision-making support in the context of COVID-19. Our results demonstrate that the recommendations generated by the STDSA model are more accurate and aligned better with the actual situation than those produced by pure K-means models. This study will provide new insights into preventing and controlling epidemics in regions that lack experience.

cs.LG↗

arXiv2024

Spectroscopic observations of progenitor activity 100 days before a Type Ibn supernova

Obtaining spectroscopic observations of the progenitors of core-collapse supernovae is often unfeasible due to an inherent lack of knowledge as to which stars will go supernova and when they will explode. In this letter, we present photometric and spectroscopic observations of the progenitor activity of SN 2023fyq in the preceding 150 days before the He-rich progenitor exploded as a Type Ibn supernova. The progenitor of SN 2023fyq shows an exponential rise in flux prior to core-collapse. Complex He I emission line features are observed, with a P-Cygni like profile, as well as an evolving broad base with velocities on the order of 10,000 km/s, possibly due to electron scattering. The luminosity and evolution of SN 2023fyq are consistent with a faint Type Ibn, reaching a peak r-band magnitude of 18.1 mag, although there is some uncertainty in the distance to the host, NGC 4388, located in the Virgo cluster. We present additional evidence of asymmetric He-rich material being present prior to the explosion of SN 2023fyq, as well as after, suggesting this material has survived the ejecta-CSM interaction. Broad [O I] and the Ca II triplet lines are observed at late phases, confirming that SN 2023fyq was a genuine supernova rather than a non-terminal interacting transient. SN 2023fyq provides insight into the final moments of a massive star's life, highlighting that the progenitor is likely highly unstable before core-collapse.

astro-ph.HE↗

arXiv2023

The Large Array Survey Telescope -- Pipeline. I. Basic image reduction and visit coaddition

The Large Array Survey Telescope (LAST) is a wide-field telescope designed to explore the variable and transient sky with a high cadence and to be a test-bed for cost-effective telescope design. A LAST node is composed of 48 (32 already deployed), 28-cm f/2.2 telescopes. A single telescope has a 7.4 deg^2 field of view and reaches a 5-sigma limiting magnitude of 19.6 (21.0) in 20s (20x20s) (filter-less), while the entire system provides a 355 deg^2 field of view. The basic strategy of LAST is to obtain multiple 20-s consecutive exposures of each field (a visit). Each telescope carries a 61 Mpix camera, and the system produces, on average, about 2.2 Gbit/s. This high data rate is analyzed in near real-time at the observatory site, using limited computing resources (about 700 cores). Given this high data rate, we have developed a new, efficient data reduction and analysis pipeline. The data pipeline includes two major parts: (i) Processing and calibration of single images, followed by a coaddition of the visit's exposures. (ii) Building the reference images and performing image subtraction and transient detection. Here we describe in detail the first part of the pipeline. Among the products of this pipeline are photometrically and astrometrically calibrated single and coadded images, 32-bit mask images marking a wide variety of problems and states of each pixel, source catalogs built from individual and coadded images, Point Spread Function (PSF) photometry, merged source catalogs, proper motion and variability indicators, minor planets detection, calibrated light curves, and matching with external catalogs. The entire pipeline code is made public. Finally, we demonstrate the pipeline performance on real data taken by LAST.

astro-ph.IM↗

arXiv2023

Resolving the explosion of supernova 2023ixf in Messier 101 within its complex circumstellar environment

Observing a supernova explosion shortly after it occurs can reveal important information about the physics of stellar explosions and the nature of the progenitor stars of supernovae (SNe). When a star with a well-defined edge explodes in vacuum, the first photons to escape from its surface appear as a brief shock-breakout flare. The duration of this flare can extend to at most a few hours even for nonspherical breakouts from supergiant stars, after which the explosion ejecta should expand and cool. Alternatively, for stars exploding within a distribution of sufficiently dense optically thick circumstellar material, the first photons escape from the material beyond the stellar edge, and the duration of the initial flare can extend to several days, during which the escaping emission indicates photospheric heating. The difficulty in detecting SN explosions promptly after the event has so far limited data regarding supergiant stellar explosions mostly to serendipitous observations that, owing to the lack of ultraviolet (UV) data, were unable to determine whether the early emission is heating or cooling, and hence the nature of the early explosion event. Here, we report observations of SN 2023ixf in the nearby galaxy M101, covering the early days of the event. Using UV spectroscopy from the Hubble Space Telescope (HST) as well as a comprehensive set of additional multiwavelength observations, we trace the photometric and spectroscopic evolution of the event and are able to temporally resolve the emergence and evolution of the SN emission.

astro-ph.HE↗

arXiv2023

Calibration and Physics with ARA Station 1: A Unique Askaryan Radio Array Detector

The Askaryan Radio Array Station 1 (A1), the first among five autonomous stations deployed for the ARA experiment at the South Pole, is a unique ultra-high energy neutrino (UHEN) detector based on the Askaryan effect that uses Antarctic ice as the detector medium. Its 16 radio antennas (distributed across 4 strings, each with 2 Vertically Polarized (VPol), 2 Horizontally Polarized (HPol) receivers), and 2 strings of transmitting antennas (calibration pulsers, CPs), each with 1 VPol and 1 HPol channel, are deployed at depths less than 100 m within the shallow firn zone of the 2.8 km thick South Pole (SP) ice. We apply different methods to calibrate its Ice Ray Sampler second generation (IRS2) chip for timing offset and ADC-to-Voltage conversion factors using a known continuous wave input signal to the digitizer, and achieve a precision of sub-nanoseconds. We achieve better calibration for odd, compared to even samples, and also find that the HPols under-perform relative to the VPol channels. Our timing calibrated data is subsequently used to calibrate the ADC-to-Voltage conversion as well as precise antenna locations, as a precursor to vertex reconstruction. The calibrated data will then be analyzed for UHEN signals in the final step of data compression. The ability of A1 to scan the firn region of SP ice sheet will contribute greatly towards a 5-station analysis and will inform the design of the planned IceCube Gen-2 radio array.

astro-ph.IM↗