Tomás Cabrera

The association of broad-lined Type Ic (Ic-BL) supernovae with long-duration gamma-ray bursts (GRBs) has been known for 28 yr. However, only about 70 GRB supernovae (GRB-SNe) have been identified, of which only one-half have spectroscopic classifications. At z = 0.153, GRB 260310A is the twelfth spectroscopically confirmed GRB-SN discovered within 1 Gpc, offering a critical opportunity to follow one of these rare supernovae in detail. We present optical to near-infrared imaging and spectroscopy of GRB 260310A and SN 2026fgk out to 65 days after discovery. The optical afterglow is among the brightest ever observed from a GRB. Spectra obtained more than 2 weeks after the explosion reveal broad absorption features that securely identify SN 2026fgk as a Type Ic-BL supernova (SN). Modeling of the multiwavelength ( grizJK _s ) lightcurve shows that the SN is approximately one-half the luminosity ( k _98bw = 0.4–0.6) of the canonical GRB-SN 1998bw. We derive a nickel mass of M _Ni = 0.4–0.5 M _⊙ with a total ejected mass of M _ej ≈ 4–6 M _⊙ and kinetic energy E _K = (3–8) × 10 ⁵¹ erg. The GRB exploded at an extremely large offset of 15 kpc from its host galaxy. Long-slit spectra reveal a “bridge” of nebular emission extending along the galaxy’s disk to the GRB location, which has a subsolar metallicity (∼0.4 Z _⊙ ), compared to a near-solar metallicity for the host galaxy. This indicates that the large offset arises from the galaxy’s extended light profile rather than an isolated environment.

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.

While the LIGO/Virgo/KAGRA (LVK) gravitational-wave (GW) detectors have detected over 300 binary black hole (BBH) mergers to date, the first confirmation of an electromagnetic (EM) counterpart to such an event remains elusive. Previous works have performed searches for counterpart candidates in transient catalogs and have identified active galactic nucleus (AGN) flares coincident with GW events; existing theory predicts that such flares may arise from the interaction of the merger remnant with the embedding accretion disk environment. We apply a statistical formalism to measure the significance of coincidence for the catalog as a whole, measuring that less than 3% (90% credible interval) of LVK BBH mergers give rise to observable AGN flares. This result still allows up to ∼40% of BBH mergers to originate in AGN disks. We also examine the individual coincidences of each merger–flare pairing, determining that in all cases the flares are more likely to belong to a background population of flares not associated with GW events. Our results are consistent with theoretical predictions accounting for the observability of EM counterparts in AGN disks, as well as based on the fact that the most massive/luminous AGNs (such as those included in the search) are not expected to harbor the majority of the BBHs. We emphasize that developing both the means to distinguish BBH counterpart flares from background AGN flares and an understanding of which BBHs are most likely to produce AGN flares as counterparts is critical to optimizing the use of follow-up resources.

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.

On 2025 August 18, the LIGO–Virgo–KAGRA collaboration reported gravitational waves from a subthreshold binary neutron star merger. If astrophysical, this event would have a surprisingly low chirp mass, suggesting that at least one neutron star was below a solar mass. The Zwicky Transient Facility mapped the coarse localization and discovered a transient, ZTF 25abjmnps (AT2025ulz), which was spatially and temporally coincident with the gravitational-wave trigger. The first week of follow-up suggested properties reminiscent of a GW170817-like kilonova. Subsequent follow-up suggests properties most similar to a young, stripped-envelope, Type IIb supernova. Although we cannot statistically rule out chance coincidence, we undertake due diligence analysis to explore the possible association between ZTF 25abjmnps and S250818k. Theoretical models have been proposed wherein subsolar neutron star(s) may form (and subsequently merge) via accretion-disk fragmentation or core fission inside a core-collapse supernova—i.e., a “superkilonova.” Here, we qualitatively discuss our multiwavelength dataset in the context of the superkilonova picture. Future higher-significance gravitational-wave detections of subsolar neutron star mergers with extensive electromagnetic follow-up would conclusively resolve this tantalizing multimessenger association.

The first joint electromagentic (EM) and gravitational-wave (GW) detection, known as GW170817, marked a critical juncture in our collective understanding of compact object mergers. However, it has now been 8 yr since this discovery, and the search for a second EM-GW detection has yielded no robust discoveries. Recently, on 2025 August 18, the LIGO–Virgo–KAGRA Collaboration reported a low-significance (high-false-alarm rate) binary neutron star merger candidate S250818k. Rapid optical follow-up revealed a single optical candidate AT2025ulz ( z = 0.08484) that initially appeared consistent with kilonova emission. We quickly initiated a set of observations with Swift, XMM-Newton, Chandra, and the Very Large Array to search for nonthermal afterglow emission. Our deep X-ray and radio search rules out that the optical rebrightening of AT2025ulz is related to the afterglow onset, reinforcing its classification as a stripped-envelope supernova (SN 2025ulz). We derive constraints on the afterglow parameters for a hypothetical binary neutron star merger at the distance of AT2025ulz (≈400 Mpc) based on our X-ray and radio limits. We conclude that our observational campaign could exclude a GW170817-like afterglow out to viewing angles of θ _v ≈ 12 . ° 5. We briefly discuss the prospects for the future discovery of off-axis afterglows.

With GW170817 being the only multimessenger gravitational-wave (GW) event with an associated kilonova detected so far, there exists a pressing need for realistic estimation of the GW localization uncertainties and rates, as well as optimization of available telescope time to enable the detection of new kilonovae. We simulate GW events assuming a data-driven distribution of binary parameters for the LIGO/Virgo/KAGRA fourth and fifth observing runs (O4 and O5). We map the binary neutron star (BNS) and neutron star–black hole (NSBH) properties to the kilonova optical light curves. We use the simulated population of kilonovae to generate follow-up observing plans, with the primary goal of optimizing detection with the Gravitational Wave Multi-Messenger Astronomy DECam Survey. We explore the dependence of kilonova detectability on the mass, distance, inclination, and spin of the binaries. Assuming that no BNS was detected during O4 until the end of 2024, we present updated GW BNS (NSBH) merger detection rates. We expect to detect BNS (NSBH) kilonovae with DECam at a per-year rate of 0–2.0 (0) in O4 and 2.0–19 (0–1.0) in O5. We expect the majority of BNS detections and also those accompanied by a detectable kilonova to produce a hypermassive NS remnant, with a significant fraction of the remaining BNSs promptly collapsing to a BH. We release GW simulations and depths required to detect kilonovae based on our predictions to support the astronomical community in their multimessenger follow-up campaigns and analyses.

The recent detection of fast-moving stars in the core of Omega Centauri (ω Cen), the most massive globular cluster (GC) in the Milky Way, has provided strong evidence for the presence of an intermediate-mass black hole (IMBH). As ω Cen is likely the accreted nucleus of a dwarf galaxy, this IMBH also represents a unique opportunity to study black hole (BH) seeding mechanisms and their potential role in the formation of supermassive BHs. We present Monte Carlo N-body models of ω Cen with detailed treatments for the loss-cone dynamics involving stars, binaries, and compact objects. Starting with BH seeds of 500–5000 M _⊙ (consistent with runaway collisions of massive stars), our cluster models grow IMBHs with masses of ∼50,000 M _⊙ after 12 Gyr, while successfully reproducing the present-day surface brightness and velocity dispersion profiles of ω Cen. We find a population of fast stars similar to those observed in the core of ω Cen, with the fastest stars originating from binaries that were tidally disrupted by the IMBH. The IMBH growth is primarily driven by mergers with 30–40 M _⊙BHs, suggesting a present-day IMBH–BH merger rate of ∼(4–8) × 10⁻⁸ yr⁻¹ in ω Cen–like GCs. Our models also predict a similar rate of tidal disruption events (TDEs; ∼5 × 10⁻⁸ yr⁻¹), which, depending on the frequency of ω Cen–like GCs per galaxy, may represent anywhere from 0.1% to 10% of the observed TDE rate.