Daniel Steeghs

The local environment of a Type Ia supernova (SN Ia) traces the local stellar-population age (a statistical proxy for the progenitor age), star formation, dust, and chemical enrichment that can influence both its light-curve properties and its standardized luminosity. We assembled a homogeneous explosion-site dataset to test how near-ultraviolet (UV) imaging combined with optical integral-field spectroscopy improves the characterization of SN Ia environments and their connection to SN Ia observables. We analyzed 340 low-redshift SN Ia explosion sites observed with HST/WFC3 in F275W together with their matched VLT/MUSE spectroscopy. For each site, we measured local photometry and extracted a 1 kpc-aperture spectrum. We then modeled the stellar continuum with STARLIGHT in a joint UV+optical fit, derived local stellar and gas-phase properties, and fit the SN light curves with sncosmo /SALT3-NIR. We find that UV emission is detected at 235 of the 337 sites with valid HST imaging. The UV- and H α -based star-formation-rate surface densities are strongly correlated, with the UV estimates systematically higher by about 0.5 dex. Including the UV constraint in the STARLIGHT fit shifts local luminosity-weighted ages toward older values by a median of +0.20 dex for UV detections and +0.45 dex for UV nondetections, showing that UV data help break age-dust-metallicity degeneracies and substantially alter and tighten the inferred local stellar-population ages within the adopted modeling framework. Consistently, F275W− r is found to be very strongly correlated with the joint-fit stellar age. Within the 169-SN Ia calibration sample, the joint stellar age, the H α specific star-formation rate (sSFR), and the UV sSFR all correlate strongly with the SALT3 stretch parameter, x ₁ , with significances above 6.8 σ . After standardization, the residual Hubble-residual trends are weaker: the strongest trend is found for the local mass-weighted age, while the UV and H α sSFRs show residual offsets of comparable sign and amplitudes of order ∼0.05 mag. None of the environmental dependencies of the Hubble residual exceeds 3 σ . The results point to a dominant empirical association between light-curve stretch and a covariant young-stellar-population axis, largely absorbed by the standard light-curve corrections, while any residual luminosity dependence on the local environment is weaker and more subtle. If confirmed with larger samples, these residual trends may still become relevant for the younger SN Ia populations expected at higher redshift. Therefore, they should be tested in cosmology analyses with forthcoming Roman and JWST SN samples.

Context . Time-domain photometric monitoring is essential for characterizing cometary evolution, particularly for rare interstellar objects with limited observing opportunities. Aims . We aimed to characterize the pre-perihelion photometric behavior and dust activity of the interstellar comet 3I/ATLAS and to test the capability of the Black Hole Target and Observation Manager (BHTOM) platform and telescope network for coordinated high-cadence non-sidereal observations. Methods . We obtained 70 days of time-series photometry of 3I/ATLAS from 2025 July 4 to September 11 using 16 telescopes and 1554 images. The data were processed and calibrated with the BHTOM pipeline. High-cadence multiband imaging was used to measure the rotation period and color evolution, while the dust activity was quantified via Afρ measurements. Results . We present a pre-perihelion light curve of 3I/ATLAS from R _h = 3.18–2.19 au, which exhibited a steady brightening by ∼3 magnitudes with no evidence of anomalous behavior. We measured a rotation period of P _rot = 15.98 ± 0.08 h. The colors were statistically non-changing, with only a weak, non-significant tendency for 3I/ATLAS to become bluer at 3.5 > R _h > 2.2 au. The relative dust production increased from A (0°) fρ ∼600 to 1100 cm, and the upper limit on the dust mass-loss rate increased from ≤217 kg s ⁻¹ to ≤328 kg s ⁻¹ . We measured an activity index of n = −1.24 ± 0.02, which is consistent with a well-developed dust coma.

The candidate gravitational wave (GW) event S250818k was one of only three non-retracted LIGO-Virgo-KAGRA public alerts issued during the fourth observing run of the network (O4) with a binary neutron star (BNS) merger classification probability exceeding one percent. This triggered a prompt search for a potential electromagnetic (EM) counterpart in the large localization error region (949 deg ² projected in the sky at the 90% credible level). The transient supernova (SN) 2025ulz discovered by the Zwicky Transient Facility (ZTF) during the search, attracted a great deal of attention due to a potential spatial and temporal coincidence, and due to its initial fast decay and featureless spectrum. Here, we report on the follow-up of this transient by the Electromagnetic counterparts of gravitational wave sources at the Very Large Telescope (ENGRAVE) Collaboration. We conducted an extensive multi-wavelength observational campaign, which led to the spectral classification of the transient as a type IIb SN, indicating that it is unrelated to the candidate GW event. In this article, we describe our observing strategies, data reduction, and interpretation. All of our results confirm and strengthen our classification of the source, and also show that shock-cooling tails associated with type IIb SNe are one of the most prominent contaminants in kilonova searches.

We present the results of a search for continuous gravitational waves (GWs) from the low-mass X-ray binary Scorpius X-1 using LIGO data from the first part of the fourth LIGO–Virgo–KAGRA observing run. By applying the resampling version of the cross-correlation pipeline to search for signal frequencies f ₀ between 25 and 200 Hz (corresponding to neutron star spin frequencies of 12.5–100 Hz for GWs due to triaxiality, or ∼15–20 to ∼120–150 Hz for GWs due to r -modes), we are able to achieve longer coherence times than before, and for the first time set upper limits below the standard torque balance level, independent of neutron star spin inclination, for 50 Hz ≲ f ₀ ≲ 200 Hz. The most sensitive upper limits on the gravitational wave amplitude h ₀ , at the upper end of the frequency band searched, approach 5 × 10 ⁻²⁶ marginalized over inclination angle and 2 × 10 ⁻²⁶ assuming the most favorable inclination. The marginalized upper limits correspond to a sensitivity depth of 70–75 Hz ^−1/2 , improving sensitivity considerably over previous searches. Expressed as constraints on the triaxial deformation of the neutron star, the limits correspond to an ellipticity of 3 × 10 ⁻⁵ if the GW frequency f ₀ is 75 Hz and 3 × 10 ⁻⁶ if f ₀ = 200 Hz, approaching deformations that could be supported by ordinary nuclear matter. Outliers from the search were ruled out as potential signals by a combination of hierarchical follow-up and analysis of additional data from later in the observing run.

The study of young and fast astronomical transients is often limited by the difficulty of acquiring data, particularly spectra, within hours to days of explosion. Historically, follow-up programmes have rarely obtained spectra until many days after discovery; coupled with lower-cadence discovery observations, this has meant that the early evolution of transients has not been systematically probed. The Fast Analysis and Spectroscopy of Transients (FAST) programme aims to build a rich data set on the early evolution of a range of transients by linking a high-cadence discovery survey on the Gravitational-wave Optical Transient Observer (GOTO) with immediate visitor-mode spectroscopic follow-up. We report results from two prototype runs, FAST 23 and 24, carried out in August 2023 and February 2024 using the 2.54 m Isaac Newton Telescope. Between these, 51 transient classifications were reported, 19 of which were discovered by GOTO with a median time between discovery and first spectrum of < 2 d. We highlight the discovery and characterization of SN 2024cld, enabled by spectroscopic observations beginning within 30 min of discovery, which reveals the complex mass-loss history of an evolved supergiant star in its final years prior to supernova. Further GOTO–FAST runs are ongoing at the Nordic Optical Telescope, and we hope this paper will be informative for those considering the creation of similar programmes.

Gamma-ray bursts (GRBs) and superluminous supernovae (SLSNe) are the most energetic explosions known in nature. Both arise from the collapse of massive stars in low metallicity environments [1–4], and are likely driven to their extreme energetics by a black hole [5] or neutron star central engine [6, 7]. Their excep- tional brightness has enabled us to probe stellar death from the nearby to very distant Universe [8, 9]. However, the relationship, if any, between these two extreme populations has been unclear. Here we report observations with the James Webb Space Telescope (JWST) and Hubble Space Telescope (HST) that uncover a superluminous supernova (SLSN) following a GRB (GRB 230818A) at z = 2.375. The supernova (SN) reaches a peak of MB ∼ −21 at around 50 rest-frame days and decays slowly. These observations clearly demonstrate that, at least in some cases, the same stellar collapse can drive both of the most extreme explosions in nature. A magnetar model cannot simultaneously explain both the GRB and supernova properties with consistent parameters for standard spin-down assumptions, but reasonable fits to the SN are found for both the radioactive power and circumstellar interaction. Phenomenologically, the SN is similar to SN 2007bi, suggested to be radioactively powered [10]. If GRB 230818A is radioactively powered, then massive core collapse can produce both a black hole and a very high ejecta and nickel mass. This would have substantial implica- tions for the study of SN and GRBs from first-generation stars, as well as for the production of the binary black hole population observed in gravitational waves.

Observations of core-collapse supernovae (CCSNe) suggest that some massive stars undergo intense mass loss shortly before explosion, but the underlying mechanisms remain unknown. Here, we report evidence of giant outbursts of clumpy material from a massive star in the final decades before explosion. Photometric, spectroscopic, and polarimetric data of SN 2024qiw reveal a bumpy light curve, a broad H α profile, and variable polarization, all consistent with interactions taking place between SN ejecta and clumpy circumstellar material, implying a mass-loss rate of ≳10 ⁻² M _⊙ yr ⁻¹ . Taken together, the most likely explanation is multiple major eruptions, similar to those of luminous blue variables (LBVs), but occurring shortly before explosion. This challenges standard stellar evolution theory by requiring either that LBVs explode terminally or that other evolutionary phases produce eruptive episodes. In spite of very high pre-SN mass loss, the resulting SN is classed as Type II, rather than Type IIn, highlighting diverse and previously unrecognized late-stage mass-loss processes.

Context. AM Canum Venaticorum (AM CVn) stars are ultra-compact binary systems composed of a white dwarf primary accreting from a hydrogen-deficient donor. They play a crucial role in astrophysics as potential progenitors of Type Ia supernovae and as laboratories for gravitational wave studies. However, their formation and evolutionary history remain incomplete. Three formation channels have been discussed in the literature: the white dwarf, He-star, and cataclysmic variable channels. Aims. The chemical composition of the accretor atmosphere reflects the material transferred from the donor. In this work we took the first accurate measurements of the fundamental parameters of the accreting white dwarf in ZTF J225237.05−051917.4, including the abundances of key elements such as carbon, nitrogen, and silicon, by analysing ultraviolet spectra obtained with the Hubble Space Telescope (HST). These measurements provide new insight into the evolutionary history of the system and, together with existing optical observations, establish it as a benchmark to develop our pipeline, paving the way for its application to a larger sample of AM CVn systems. Methods. We determined the binary parameters through photometric analysis and constrained the atmospheric parameters of the white dwarf accretor, including its effective temperature, surface gravity, and chemical abundances, by fitting the HST ultraviolet spectrum with synthetic spectral models. We then inferred the system’s formation channel by comparing the results with theoretical evolutionary models. Results. According to our measurements, the accretor’s effective temperature ( T _eff ) is 23 300 ± 600 K and the surface gravity (log g ) is 8.4 ± 0.3, which imply an accretor mass ( M _WD ) of 0.86 ± 0.16 M _⊙ . We find a high nitrogen-to-carbon abundance ratio by mass of > 153. Conclusions. The accretor is significantly hotter than previous estimates based on simplified blackbody fits to the spectral energy distribution, underscoring the importance of detailed spectral modelling for accurately determining system parameters. Our results show that ultraviolet spectroscopy is well suited to constraining the formation channels of AM CVn systems. Of the three proposed formation channels, the He-star channel can be excluded given the high nitrogen-to-carbon ratio. Our results are consistent with both the white dwarf and cataclysmic variable channels.