Lei Hu

The explosion of a star is first marked by the shock wave breaking out of the stellar surface, producing a burst of ultraviolet and X-ray radiation. These events are observationally rare, despite likely accompanying the majority of supernovae (SNe). Here, we report on our multiwavelength observing campaign of the closest Einstein Probe fast X-ray transient (FXT) EP260321a at z = 0.0344. The thermal (kT = 130 eV) X-ray emission with peak luminosity of 1.0 × 10⁴⁵ erg s⁻¹ points to a shock breakout origin. We demonstrate that EP260321a is accompanied by a broad-lined Type Ic supernova, SN 2026gzf. The supernova (SN) properties, including its spectral evolution, lightcurve evolution, and expansion velocities, are all typical of the energetic stripped-envelope SNe associated with gamma-ray bursts (GRBs). However, deep X-ray upper limits obtained with the Chandra X-ray Observatory do not detect an X-ray afterglow, and instead exclude the afterglow of known GRBs or FXTs. If the stellar explosion launched a successful relativistic jet, we require that it had both a low Lorentz factor of Γ₀ < 30 and a kinetic energy of E_kin < 10⁴⁹ erg for a stellar wind density of A_* ≳ 1. We propose that EP260321a originated from a mildly relativistic, weak outflow that was choked by the progenitor star. This scenario is capable of naturally explaining its low X-ray luminosity and lack of prompt gamma-ray emission. EP260321a bridges the gap between SN 2008D and low-luminosity GRBs, suggesting a greater diversity in the physical parameters of stripped stars as they undergo terminal collapse.

We present a GPU-accelerated transient detection pipeline developed for time-domain surveys with the Dark Energy Camera (DECam). It enables real-time-capable image processing, incorporating science-driven candidate filtering to support rapid transient identification in time-critical observing programs. The pipeline serves as the core transient discovery engine for multiple long-term DECam programs, including the GW-MMADS gravitational-wave follow-up campaign and the DESIRT survey for intermediate-redshift transients with DESI synergy. The pipeline ingests calibrated imaging products from the DECam Community Pipeline and performs image differencing using SFFT, a GPU-accelerated Fourier-domain image-subtraction method, coupled with convolutional neural network-based real–bogus classification, to produce science-ready transient alerts and light curves that are delivered to community brokers. We validate the pipeline using archival DECam data from the DESIRT survey. Our AI-based real–bogus classifier achieves a completeness of ∼99% of real transients while rejecting ∼96% of subtraction artifacts, and the workflow typically reduces the candidate load to a manageable level for survey operations. With GPU acceleration, the typical processing time per DECam exposure is ∼50 s from calibrated image processing to alert generation using a modest allocation of computing resources.

We present the first systematic spectroscopic observations of extragalactic transients from the Dark Energy Spectroscopic Instrument (DESI), as part of the DESI Transients Survey program. With 5000 fibers and an ∼8 deg ² field of view, we exploit DESI as a machine for the discovery and classification of transients. We present transient classifications from archival DESI data in Data Releases 1 and 2, relying on a combination of a secondary target program and serendipitous observations. We also present observations from the first 6 months of the DESI spare fiber program dedicated to transients. The program is run in coordination with a dedicated DECam time-domain survey, serving as a pathfinder for what we will be able to achieve in conjunction with the Rubin Observatory Legacy Survey of Space and Time (LSST). We classify over 250 transients, of which the majority were previously unclassified. The sample comprises thermonuclear and core-collapse supernovae and tidal disruption events (TDEs), including a TDE observed before its discovery in imaging. We demonstrate DESI’s ability to classify a population of faint transients down to r ∼ 22.5 mag during main survey operations, with negligible impacts on DESI’s main observations. With the start of Rubin LSST operations, we expect to classify O ( 1000 ) transients per year.

Type Ia supernovae (SNe Ia) are key tools for addressing major cosmological questions like the Hubble tension and dark energy. Modern surveys are predominantly photometry based, making photometric classification of SNe Ia essential for precision cosmology. We evaluate whether functional principal component analysis (FPCA) scores derived from light curves, combined with ensemble learning, can reliably distinguish SNe Ia from other transients using a subset of PLAsTiCC sources with well-sampled light curves in a fully photometric approach. FPCA offers a data-driven, flexible characterization without strict model-based assumptions. Light curves are fitted by minimizing residuals with penalty terms from clean samples, ensuring robustness to outliers and poor band sampling. The first two FPCA scores and peak magnitudes in five Legacy Survey of Space and Time (LSST) bands serve as classification features. We implement two binary classifiers: an ensemble boosting model (CatBoost) and a statistical probabilistic method based on Euclidean distances. CatBoost slightly outperforms the statistical method, achieving 96.5% accuracy and 96.6% precision. Both accuracy and precision remain satisfactory (>90%) for photometric redshift uncertainties up to σ = 0.05. On the spectroscopic DES Y5 sample, both methods maintain ∼90% accuracy and 95% precision, demonstrating excellent cross-survey generalization. Applied to DECam DDF and DESIRT transients, predictions strongly agree, and their intersection yields a high-confidence SN Ia sample for cosmological analyses. This FPCA-based framework provides a powerful, flexible tool for transient classification in upcoming surveys such as LSST and Roman.

We present comprehensive photometric and spectroscopic observations of a transitional Type Ia supernova (SN Ia), SN 2024aedt, discovered by the 2.5 m Wide Field Survey Telescope within 1 day of the explosion. Its light curve is characterized by a peak absolute magnitude of M _B = −18.49 ± 0.03 mag and a decline rate of Δ m ₁₅ ( B ) = 1.53 ± 0.36 mag, placing the object on the Δ m ₁₅ ( B )– M _B diagram in the transition region between normal and subluminous SNe Ia. Furthermore, the early color evolution and host galaxy environment of SN 2024aedt underscore its transitional nature, sharing properties with both normal and SN 1991bg–like SNe Ia. Light-curve modeling with MOSFiT yields a synthesized ⁵⁶ Ni mass of 0.414 ± 0.042 M _⊙ and a total ejecta mass of 0.548 ± 0.108 M _⊙ . A comparison with theoretical models suggests that the evolutionary trend can be broadly explained by both delayed-detonation and double-detonation (DDet) scenarios, while possible early-excess emissions predicted by DDet cannot be identified given the limited detections soon after the SN explosion. Although the overall spectral evolution of SN 2024aedt is similar to that of other transitional SNe Ia, the spectroscopic comparison reveals diversity in the early-phase blue-end features, which becomes more homogeneous at later phases. This result indicates the importance of early-time observations in understanding the origin of SN Ia diversity.

On 2025 August 18, the LIGO–Virgo–KAGRA collaboration reported a subthreshold gravitational-wave candidate detection consistent with a subsolar-mass neutron star merger, denoted S250818k. An optical transient, AT2025ulz, was discovered within the localization region. AT2025ulz initially appeared to meet the expected behavior of kilonova emission, the telltale signature of a binary neutron star merger. The transient subsequently rebrightened after ∼5 days and was classified as a Type IIb supernova. In this work, we analyze the observations of its host galaxy obtained by the Dark Energy Spectroscopic Instrument (DESI). From the DESI spectrum, we obtain a secure redshift of z = 0.084840 ± 0.000006. If S250818k has an astrophysical origin, this places the transient within 2 σ of the gravitational-wave distance and results in an integral overlap between the gravitational-wave alert and the transient location of log 10 I ≈ 3.9 − 4.2 . Our analysis of the host galaxy’s spectral energy distribution reveals a star-forming, dusty galaxy with stellar mass ∼10 ¹⁰ M _⊙ , broadly consistent with the population of both short gamma-ray bursts and core-collapse supernova host galaxies. We also present our follow-up of DESI-selected candidate host galaxies using the Fraunhofer Telescope at the Wendelstein Observatory, and show the promise of DESI for associating or rejecting candidate electromagnetic counterparts to gravitational-wave alerts. These results emphasize the value of DESI’s extensive spectroscopic dataset in rapidly characterizing host galaxies, enabling spectroscopic host subtraction, and guiding targeted follow-up.

Using Spitzer and Wide-field Infrared Survey Explorer images, we have discovered 42 mid-infrared (mid-IR) luminous dusty supernovae (SNe) with local integral field spectroscopy data. The observed mid-IR emission indicates the presence of newly formed dust or preexisting dust heated by radiation from SN or circumstellar medium (CSM) interactions. We carry out a systematic analysis of the SN host environments and their dust properties, to understand the dust-veiled exploding stars and whether such an intense dust production process is associated with their local environments. We find that dusty SNe prefer the locations with higher EW(H α ), lower metallicity, and heavier host extinctions compared to typical SN types, and they show the same increasing sequence in the values of EW(H α ) and oxygen abundance, from hydrogen-rich to type IIn to hydrogen-poor dusty SNe. These differences in the environmental properties of different dusty SN types indicate the diversity of their progenitors. We also find that one marginal correlation is a negative correlation between the SN dust mass and star formation rate. This means that SNe would be more mid-IR-luminous and more dust-rich in regions with lower star formation rates. However, the SN dust mass shows no correlation with the metallicity and the host extinction, which were thought to be key factors affecting the mass-loss history of progenitors and the CSM environment of SNe. Therefore, the dust formation process in SNe might be insensitive to metallicity and the dust conditions of their host environments.

The gravitational wave (GW) event S250206dm, as the first well-localized neutron star (NS) merger candidate potentially located in the mass gap, presented a unique opportunity to probe the electromagnetic signatures from such a system. Here we report a deep, multiband search with the new 2.5 m Wide Field Survey Telescope (WFST), covering ∼64% of the localization region up to a 5 σ limiting magnitude of 23 mag. In total, 12 potential candidates have been identified, but none of them are likely related to S250206dm. This nondetection provides the most stringent constraint to date on any associated kilonova. Crucially, an AT 2017gfo-like event at 269 Mpc can be excluded only by WFST observations. Based on ejecta mass limits, a NS-black hole with a large mass ratio ( Q ≳ 3.2) is disfavored. This optically derived constraint on the mass ratio reaches, for the first time, a precision comparable to that inferred from the GW signal. This work presents the best observation of this type of event until now, and demonstrates the power of rapid, deep follow-up observations to constrain the properties of compact binary progenitors, offering key insights into the constituents of the mass gap.

We present the searches conducted with the Zwicky Transient Facility (ZTF) in response to S250206dm, a bona fide event with an online false alarm rate of one in 25 yr, detected by the International Gravitational Wave Network. Although the event is significant, the nature of the compact objects involved remains unclear, with at least one likely neutron star. ZTF covered 68% of the last refined Bilby localization region, though we did not identify any likely optical counterpart. We describe the ZTF strategy, potential candidates, and the observations that helped rule out candidates, including sources circulated by other collaborations. Similar to Ahumada et al., we perform a frequentist analysis, using simsurvey , as well as Bayesian analysis, using nimbus , to quantify the efficiency of our searches. We find that, given the nominal up-to-date distance to this event of 373 ± 104 Mpc, our efficiencies are above 10% for KNe brighter than −17.5 absolute magnitude. Assuming the optical counterpart known as kilonova (KN) lies within the ZTF footprint, our limits constrain the brightest end of the KN parameter space. Through dedicated radiative transfer simulations of KNe from binary neutron star (BNS) and black hole–neutron star mergers, we exclude parts of the BNS KN parameter space. Up to 35% of the models with high wind ejecta mass ( M _wind ≈ 0.13 M _⊙ ) are ruled out when viewed face-on ( cos θ obs = 1.0 ). Finally, we present a joint analysis using the combined coverage from ZTF and the Gravitational Wave Multimessenger Dark Energy Camera Survey. The joint observations cover 73% of the Bilby localization region, and the combined efficiency has a stronger impact on rising and slowly fading models, allowing us to rule out 55% of the high-mass KN models viewed face-on.