A Filter for Selection of Inclined Cosmic-Ray Air Showers at IceTop

Muon Lateral Distribution in Cosmic Ray Air Showers with VERITAS

VERITAS is an array of Imaging Atmospheric Cherenkov Telescopes (IACTs) which observes the light from gamma-ray and cosmic-ray (CR) air showers. Muons produced in CR air showers create distinct ring images. These are typically used for calibration of optical throughput, since the relation between the ring radius and brightness is well-known. If an array of IACTs can detect both a muon ring and the light from the rest of the shower, the muon distance to the shower core can be inferred. This allows for the reconstruction of the muon lateral distribtuion function (LDF), or muon density vs distance from shower core. The muon LDF is sensitive to the elemental composition of the primary CR. This works aims to understand if VERITAS observations of muon rings in CR air showers can be used to describe the average composition of CRs seen by VERITAS.

Search for TeV emission from SNR W49B with HAWC

Studying the Cas A supernova remnant through VHE gamma-rays with VERITAS

A model-Independent Search for Time Variation in the TeV Cosmic Ray Anisotropy

There is an observed anisotropy in the arrival direction distribution of cosmic rays in the TeV-PeV regime with variations on the scale of one part in a thousand. While the origin of this anisotropy is an open question, a possible factor is cosmic-ray interactions with interstellar and heliospheric magnetic fields. These magnetic fields may change over time - for example, due to changes in solar activity throughout its 11-year solar cycle. The cosmic-ray anisotropy can reflect these time-dependent magnetic fields. I will present a preliminary, model-independent search for time variation in this anisotropy undertaken by the IceCube Neutrino Observatory.

Two Years, Two Telescopes: SST-1M Stereo Observations of Markarian 421 at Very High Energies

Markarian 421 (Mrk 421) is the closest and one of the brightest high-frequency–peaked blazars, located at a redshift of z=0.031. It is a prominent and highly variable gamma-ray source whose broadband emission has been extensively investigated through multi-wavelength observations. Despite these efforts, the physical mechanisms driving blazar emission remain incompletely understood, largely due to limited simultaneous multi-wavelength coverage and the moderate sensitivity of earlier gamma-ray measurements.  To extend existing observations into the very-high-energy (VHE) gamma-ray regime, Mrk 421 was targeted during dedicated observation campaigns in 2024 and 2025 using the SST-1M telescopes at the Ondřejov Observatory in the Czech Republic. The SST-1M system consists of two single-mirror, small-size imaging atmospheric Cherenkov telescopes operated in both mono and stereoscopic modes, enabling the detection of VHE gamma rays in the energy range from 1 to 300 TeV. In this contribution, we present the SST-1M stereo observations from the 2024–2025 campaigns, describe the data analysis procedures, and report preliminary results from physical modeling aimed at constraining the emission mechanisms of Mrk 421.

Time-dependent search for neutrino emission correlated with Fermi AGN flares 

High-energy neutrinos provide a unique probe of hadronic particle acceleration in astrophysical sources, but their signals require precise time-dependent and multi-wavelength analyses to identify their origins. I will present a time-dependent search for neutrino emission associated with gamma-ray flares from active galactic nuclei (AGN), using Fermi-LAT observations to construct physically motivated temporal probability density functions for IceCube likelihood analyses. Motivated by the TXS 0506+056 case, where a neutrino alert coincided with a gamma-ray flare, this approach enhances sensitivity to transient neutrino emission that would be diluted in time-integrated searches.

Using the Fermi 4FGL-DR4 data, I identify bright and variable AGN based on energy flux and variability index, and rank them using a figure of merit that accounts for IceCube sensitivity to select the most promising neutrino-emitting candidates. This analysis framework enables a population-level search for transient neutrino emission from flaring AGN, testing hadronic particle acceleration in relativistic jets by combining Fermi gamma-ray variability with IceCube neutrino data.

X-ray Data Analysis of a Flaring High-Redshift Gamma-Ray Blazar

High-redshift blazars detected at gamma-ray energies are extremely rare, with only a handful of sources known to date. The blazar 4FGL J1428.9+5406 (5BZQJ1429+5406) is one of only a few such systems and is the third high-redshift blazar caught flaring at gamma-ray energies in real time, enabling the collection of a simultaneous multiwavelength data set. Due to their distance and limited photon statistics, detailed multiwavelength studies of these sources remain scarce, making X-ray observations a critical probe of their high-energy emission.

In this talk, I will present my ongoing data analysis project focused on the X-ray emission of this source during a flaring episode, using observations from Swift/XRT and NuSTAR. I perform spectral and variability analyses using the Interactive Spectral Interpretation System (ISIS), including joint spectral modeling across soft and hard X-ray bands. A key goal of this work is to investigate whether the X-ray spectrum is best described by intrinsic spectral curvature, a spectral break, or absorption-related features, all of which have important implications for the physical origin of the emission.

I will show preliminary results from the X-ray analysis and discuss how they constrain the dominant emission processes in this extreme system. These results form the foundation for a forthcoming multiwavelength study that will incorporate Fermi-LAT data, and they highlight the essential role of X-ray observations in interpreting gamma-ray–detected high-redshift blazars.

The curious case of EP241021a

Fast X-ray Transients (FXTs) are rare and poorly understood, with various potential progenitors. The launch of the Einstein Probe (EP) mission has greatly increased the rate of real-time FXT discoveries and enabled timely multi-wavelength follow-up observations. We have recently begun to do follow-up observations of FXTs with the Three Channel Imager (3KK) on the Fraunhofer Telescope at Wendelstein Observatory (FTW), located in the German Alps and operated by the Ludwig-Maximilians-Universität München.

EP241021a was the first FXT that we observed in this new campaign. The optical and NIR (OIR) counterpart shows a very peculiar panchromatic behavior in its light curve, rebrightening by almost 1.5 mag within ~2 days 5.5 days after the initial trigger. This event stands out due to its extreme brightness, peaking at an absolute magnitude of approximately -22 mag in the r band after rebrightening, and its consistently red color of $J - r \approx 1\,\mathrm{mag}$. While its overall light curve can be described by two decaying power laws $\sim t^{-1}$ with a steep rising phase $\sim t^4$, its brightness and timescale distinguish it from other FXTs, including EP240414a, which exhibited similar behavior but evolved more rapidly and at lower luminosity. These characteristics, along with the absence of a gamma-ray detection, constrain possible origins for this event.

In this talk, I will discuss possible explanations for the extreme rebrightening. Traditional supernova models struggle to account for both the high luminosity and rapid evolution of EP241021a, but alternative scenarios related to the collapse of a massive star may offer a more promising explanation.

Jordan Stanley

Dissecting the Radiative Puzzle of VHE GRBs: Insights from Multi-Wavelength Modeling

The recent detection of very high-energy (VHE, $>$ 100 GeV) gamma-ray emission from gamma-ray bursts (GRBs) has provided new insights into afterglow physics. Understanding the temporal and spectral evolution of VHE GRBs requires detailed modeling and simultaneous multi-wavelength observations spanning radio to VHE including \textit{Fermi} GBM and LAT data. The afterglow emission primarily consists of synchrotron radiation from forward and reverse shocks, synchrotron self-Compton (SSC) emission, and external Compton (EC) emission. We conducted a detailed multi-wavelength modeling of six long-duration VHE GRBs - GRB~180720B, GRB~190114C, GRB~190829A, GRB~201216C, GRB~210619B, and GRB~221009A; using the NAIMA code, which employs radiative models and Markov chain Monte Carlo (MCMC) techniques. Our analysis constrains key parameters governing the emission and surrounding environments of these GRBs. The results indicate that synchrotron self-Compton (SSC) is the dominant VHE emission mechanism, with negligible contribution from external Compton (EC). Most VHE GRBs are well described by a forward shock model in a spherical jet configuration, except for GRB 221009A, which requires additional considerations. Additionally, we find that VHE GRBs tend to occur in environments with lower magnetic fields and higher ambient medium densities. Interestingly, VHE GRBs lie outside the 3-$\sigma$ region of the $E_{k,\rm iso}$ - $\epsilon_B$ correlation observed in other high-energy GRBs. These findings provide crucial constraints on VHE GRB emission mechanisms and serve as a benchmark for future observations and theoretical studies in the era of CTA and next-generation gamma-ray observatories.

A Search for GeV Emission from Magnetar Giant Flare Candidates with Fermi-LAT

Around 11.4 million years ago, a magnetar in the galaxy NGC 253 emitted a giant flare, which was detected on April 15, 2020, by multiple gamma-ray telescopes, including the Fermi Gamma-ray Burst Monitor and the Large Area Telescope. This event, designated GRB 200415A, has been interpreted as an extragalactic magnetar giant flare, contributing to the small but growing number of such confirmed detections. Recent events and archival analyses have more than doubled the number of known giant flares. Here, we present our search for GeV counterparts to these candidates using Fermi-LAT data. Specifically, we search for spatial and temporal coincidences — within an expected delay window — between GeV photons and the GBM triggers, as well as a cumulative signal obtained by stacking the LAT data for the candidate events. These efforts aim to shed light on the high-energy behavior of magnetar flares and contribute to a more complete understanding of their emission mechanisms.

Linda Lattanzi

Grace Nelson

Search for High Energy Flares in afterglow of GRB221009A Using HAWC Data

Predicting Galactic Diffuse Emission in Gamma Rays using Machine Learning

Angus B. Jameson

Beyond the Label Bottleneck-Supervised and Unsupervised Deep Learning for the Automated Discovery of AGNs in the JWST Era

The unprecedented data volume from JWST wide-field programs, such as the COSMOS-Web survey, has created a critical "label bottleneck," rendering traditional human-driven AGN classification untenable. To address this, this thesis systematically compares two deep learning paradigms for automated AGN identification using multi-band NIRCam imagery: a supervised ConvNeXt architecture and an unsupervised Variational Autoencoder (VAE) coupled with Gaussian Mixture Model (GMM) clustering. The supervised network achieved 84.00% completeness at a 56% purity threshold, proving highly effective for precision cataloging. Meanwhile, the unsupervised pipeline autonomously isolated 48.9% of the AGN population at a comparable purity level without human annotations. To resolve the physical degeneracy between compact star-forming galaxies and active nuclei in the latent space, a novel composite ratio was introduced to detect the infrared signatures of dust-obscured accretion disks. Our results demonstrate that while supervised networks excel at targeted extraction, unsupervised representation learning provides a highly scalable, unbiased engine for discovery, paving the way for semi-supervised pipelines in the JWST era.

Systematic Study of Gamma-ray Pulsars above 10 GeV

The First Fermi Large Area Telescope (LAT) catalog of  >10 GeV sources (1FHL), released in 2013, was carried out using three years of data and contained 27 sources associated with known pulsars. The 1FHL Catalog presented a detailed study of these pulsars, including their possible high-energy emission, as determined through detailed pulsation searches. In the years since the publication of 1FHL, the Fermi LAT collaboration has released additional high-energy catalogs, but none of them contained a similar detailed analysis of pulsars.  Using the latest available Fermi LAT data, I will conduct a systematic study of pulsations at the highest Fermi-LAT energies, and use the results to test and constrain competing emission scenarios, and provide a standardized reference for prioritizing and interpreting follow-up observations with current and next-generation very-high-energy facilities (e.g., HAWC, MAGIC, LHAASO, CTA). We will utilize the latest  Fermi-LAT pulsar catalog (3PC), analyzing only events above 10 GeV over the mission lifetime, extending earlier exploratory studies presented in 1FHL, which were based on a much shorter data set (3 vs > 17 years), and an older instrument response (Pass 7 vs Pass 8).