Giorgos Leloudas

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.

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, and shows fast-rising, ultraviolet-bright emission peaking at M _r = −19.26 ± 0.09 mag and a peak blackbody temperature of T ≈29 000 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 ten days. In our SALT spectrum at 3.8 days, we observe a pronounced blueshift of the He II lines by 460 km s ⁻¹ 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 ⁻¹ 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 _⊙ and a corresponding high mass-loss rate of ≳10 ⁻¹ M _⊙ yr ⁻¹ . 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 _⊙ to explain SN 2025kzr, although a binary origin cannot be excluded.

We investigate the thermal emission and the extinction from dust associated with the nearby ( z = 0.0267) superluminous supernova (SLSN) 2018bsz. Our dataset combines daily cadence, simultaneous optical and near-infrared (NIR) coverage up to ∼100 days with late-time (+ 1.7 yr) mid-infrared (MIR) observations. At 230 days after the light-curve peak, the supernova (SN) is not detected in the optical, but shows a surprisingly strong NIR excess, with r − J > 3 mag and r − K _s > 5 mag. The temporal evolution of the infrared (IR) light curve allows us to assess whether the MIR emission originates from newly formed dust, either in the SN ejecta or in a postshock circumstellar shell, or instead from an IR echo produced by preexisting circumstellar or interstellar dust heated by the SN radiation. Our radiative-transfer modeling shows that the circumstellar and interstellar echo scenarios cannot simultaneously reproduce the observed IR luminosity and its temporal evolution, and we therefore disfavor these interpretations. In contrast, a scenario in which new dust forms in the SN ejecta at epochs ≳200 days provides a self-consistent explanation for the evolution of the SN flux. We can fit the spectral energy distribution well at +230 days with 5 × 10 ⁻⁴ M _⊙ of carbon dust, increasing over the following several hundred days to 10 ⁻² M _⊙ by +535 days. SN 2018bsz is the first SLSN showing evidence for dust formation within the SN ejecta, and appears to form 10 times more dust than normal core-collapse supernovae (SNe) at similar epochs. Together with their preference for low-mass, low-metallicity host galaxies, we suggest that superluminous SNe may be a significant contributor to dust formation in the early Universe.

Wide-field optical transient surveys collectively discover ∼50 new transients per night, making rapid, multi-filter and/or spectroscopic follow-up of all but the most exceptional events unfeasible. Consequently, the (early) light-curve properties of most supernovae (SNe), particularly sub-luminous events, are often poorly characterised. Here, we showcase recovery of key parameters by combining publicly available data from multiple, contemporaneous surveys. We built a catalogue of 125 SNe by cross-matching the MeerLICHT (ML) transient survey with the Transient Name Server list of spectroscopically classified SNe detected between 2017 August and 2022 October. For a subset of 14 SNe with good multi-survey photometric coverage, we combined ML- q , ZTF- g , r , and ATLAS- c , o data to construct composite early-time light curves. We then applied an expanding fireball ( t ² ) model to constrain their explosion dates and rest-frame rise times. We find that this simple model provides a reliable description for the early emission of most SNe in our sample and successfully constrains their rise time, a key parameter for characterising SN categories and probing progenitor origins. This multi-survey approach, only using standard survey products, provides a powerful method to characterise large numbers of SNe that would otherwise be lost, a technique that will be essential for maximising the scientific return from current and future surveys such as GOTO, BlackGEM, the Vera C. Rubin Observatory’s LSST, and Global Open Transient Telescope Array (GOTTA).

Type Ic broad-line supernovae (SNe Ic-BL) are often associated with energetic explosions that display a prompt outburst of high-energy emission. Since their progenitor lost the H and He envelopes before the explosion exposing the C/O core, their explosion dynamics and geometry can be seen in an unobscured and undistorted way. We present imaging polarimetry and spectropolarimetry of the Type Ic-BL SN 2026gzf obtained 4.6 and 16.5 days after the X-ray shock breakout, which was recorded by the Einstein Probe satellite as EP260321a, showing it to be one of the softest and intrinsically dimmest extragalactic fast X-ray transients. The persistent low continuum polarization indicates that the outer layer of SN 2026gzf is mostly spherical, suggesting the explosion did not significantly disrupt the progenitor envelope. At day 16.5, the calcium near-infrared triplet displays a peak polarization above 1.5%. The geometry of the associated line opacity is also compatible with an axisymmetric configuration. The spatial distribution of such oxygen-burning ashes thus indicates the presence of a symmetry axis of the excitation structure within the nearly spherical ejecta. The Ca , II triplet polarization is dominated by a primary component spanning ∼25,000–40,000 km s ⁻¹ , while a possible high-velocity polarization substructure of modest statistical significance may indicate a complex, nonaxisymmetric excitation geometry in the outer ejecta. By implementing a three-dimensional Monte Carlo calculation, we infer that a viewing angle of ∼40 ^∘ from the symmetry axis of the excitation structure could plausibly reproduce the observed spectral and polarization profiles of the Ca II triplet.

We report multiwavelength studies of the transient EP250916a, detected by the Einstein Probe on 2025 September 16. Located at low Galactic latitude, the source exhibited a rapid X-ray brightening, reaching an unabsorbed 0.5–10 keV flux of (6.4 ± 0.1) × 10 ⁻¹⁰ erg cm ⁻² s ⁻¹ , followed by a plateau and a two-stage decay lasting over 40 days. Swift/X-Ray Telescope (XRT) monitoring shows a persistently hard spectrum (Γ ≈ 1.6–2.2) with only modest softening during decay, while a Nuclear Spectroscopic Telescope Array (NuSTAR) observation confirms a hard-state continuum extending up to 70 keV. Timing analysis of XMM-Newton data reveals a weak quasiperiodic oscillation (QPO) at ∼13 Hz. No other coherent pulsations or thermonuclear bursts are detected. Broadband spectral modeling favors a nonthermal power-law continuum with partial-covering absorption and shows no significant thermal disk component. Optical imaging obtained with Nordic Optical Telescope, Las Campanas Observatory, and GaiaDR3 identifies two faint sources within the 2″ Swift/XRT positional uncertainty. A MeerKAT observation at 1.28 GHz yielded no radio counterpart, with a 3 σ upper limit of 60 μ Jy beam ⁻¹ . The combination of a long-lasting outburst, a hard nonthermal X-ray spectrum, a weak QPO detection, the absence of coherent timing features, and faint potential optical counterparts disfavors a stellar-flare or extragalactic origin and supports an accreting compact-object scenario. Comparisons with similar faint, hard-state transients place EP250916a within a growing population of low-luminosity, hard-state black hole X-ray binary candidates.

Context. Interacting transients probe mass loss in the final stages of stellar evolution; however, the geometry and timing of multi-episode mass loss remain poorly constrained. SN 2024abvb is a nearby interacting event with transitional Ibn/Icn spectroscopic properties and multi-epoch polarimetry, offering a rare opportunity to study structured circumstellar material (CSM). Aims. We aim to characterise the kinematics, composition, and geometry of the CSM around SN 2024abvb and to identify plausible progenitor and ejection scenarios that can produce the observed spectro-polarimetric evolution. Methods. We present high-resolution (VLT/UVES and VLT/X-Shooter) optical/NIR spectroscopy across several epochs, complemented by broadband polarimetry and spectropolarimetry (VLT/FORS2 and NOT/ALFOSC). Line identifications, velocity decompositions, and polarimetric time series were used to trace multiple kinematic components and changes in scattering geometry. Results. The high-resolution spectra reveal multiple narrow CSM components composed of He, C, and O with absorption minima at ∼150 − 400 km s ⁻¹ and additional faster material up to ∼2000 km s ⁻¹ . Low-velocity Balmer absorptions are present, indicating distant H-rich material, a first in Type Ibn/Icn supernovae. Polarimetry shows a marked evolution ( P ∼ 1% near the peak, ≲0.5% after ∼1 week, rising to ∼1.5% after ∼20 d with ∼50° position-angle rotation and to ∼4% after ∼30 d, stronger in the blue), implying a time-variable, wavelength-dependent scattering and obscuration component. The combination of kinematics and polarimetric behaviour is consistent with multiple, concentric toroidal shells with differing orientations and partial dust content.

Type Ia Supernovae (SNe Ia) are well known for their role as standardizable cosmological candles. Their uniformity is credited to their single origin as thermonuclear explosions of White dwarf (WD) stars. Nevertheless, some SNe Ia break this regularity. Prominently, the Iax subclass are less energetic and remarkably diverse, raising questions about their progenitor systems. While no progenitor system of a normal SN Ia has ever been detected, a luminous blue star was identified in pre-explosion images of the site of the bright SN Iax SN 2012Z, suggested to be a helium giant companion star acting as a mass donor to a WD SN progenitor. This is in line with models of weak mass accretion of a WD from a binary companion, producing an explosion that does not fully disrupt the star. However, these models fail to explain the properties of the faintest Type Iax explosions, suggesting either they originate from other WD binary systems, or even from massive progenitor stars. Here, we present the faint SN Iax SN 2024vjm - possibly the faintest supernova observed to date. Using a deep pre-explosion image taken by the recently launched Euclid space mission, we show that its progenitor system must be fainter than the helium giant SN Iax progenitor candidate of SN 2012Z, as well as that of the luminous red companion or remnant of the faint SN 2008ha, and may require a subdwarf helium star as a mass donor. The deep image also provides strong arguments against a massive star origin for this faint supernova. Our observations argue that SN 2024vjm is a WD explosion, but we find that remarkably faint SNe Iax fade more slowly than bright ones, i.e., they evolve in an opposite manner from the famous Phillips relation that makes regular SNe Ia cosmological candles.

We present spectroscopic and photometric observations and analysis of SN 2024abvb, a peculiar transitional Type Ibn/Icn supernova located at an unusually large projected distance from its host galaxy (21.5 kpc). SN 2024abvb displays an extended rise time in the g- and o-bands (10.1 and 10.6 days respectively), followed by a linear decline in all photometric bands. Comparisons with other supernova subclasses show that the photometric and spectroscopic evolution of SN 2024abvb are distinct from Type Ibn and Type Icn events, with a higher peak r-band luminosity and lower blackbody temperatures. Spectra reveal an initial blue continuum and narrow P-Cygni profiles, with C ii λ5890 dominating in emission, persisting at late phases, and showing a rapid decline in the expansion velocity. Weak He i λ5876 features are tentatively detected at early times. Analysis of progenitor scenarios rules out thermonuclear origins based on incompatible light curve shapes and spectral signatures. A rare massive star progenitor appears unlikely given the low local star formation rate. The most plausible origin is an ultra-stripped supernova scenario involving a binary system; this best explains the observed separation from the host, the low circumstellar material mass, the fast photometric evolution and the low nickel production, although a discrepancy in model versus observed ejecta mass remains. These results reinforce the classification of SN 2024abvb as a distinctive Type Ibn/Icn event and highlight the diversity of progenitor channels for interacting supernovae.

We present spectroscopic and photometric observations and analysis of SN 2024abvb, a peculiar transitional Type Ibn/Icn supernova located at an unusually large projected distance from its host galaxy (21.5 kpc). SN 2024abvb displays an extended rise time in the $g$- and $o$-bands (10.1 and 10.6 days respectively), followed by a linear decline in all photometric bands. Comparisons with other supernova subclasses show that the photometric and spectroscopic evolution of SN 2024abvb are distinct from Type~Ibn and Type Icn events, with a higher peak $r$-band luminosity and lower blackbody temperatures. Spectra reveal an initial blue continuum and narrow P-Cygni profiles, with C {\sc ii} $λ$5890 dominating in emission, persisting at late phases, and showing a rapid decline in the expansion velocity. Weak He {\sc i} $λ$5876 features are tentatively detected at early times. Analysis of progenitor scenarios rules out thermonuclear origins based on incompatible light curve shapes and spectral signatures. A rare massive star progenitor appears unlikely given the low local star formation rate. The most plausible origin is an ultra-stripped supernova scenario involving a binary system; this best explains the observed separation from the host, the low circumstellar material mass, the fast photometric evolution and the low nickel production, although a discrepancy in model versus observed ejecta mass remains. These results reinforce the classification of SN 2024abvb as a distinctive Type Ibn/Icn event and highlight the diversity of progenitor channels for interacting supernovae.