Enrico Cappellaro

We present spectrophotometric observations of the Type Ibn supernova SN 2020able, along with a comprehensive analysis and modelling of its evolution. The transient occurred in SDSS J092602.93+243115.1, a faint host galaxy characterised by significantly subsolar metallicity (12 + log(O/H) ≃ 8.28 dex). The early photometric evolution and high peak luminosity ( L _peak ≃ 4.4 × 10 ⁴³ erg s ⁻¹ ) indicate an efficient conversion of ejecta kinetic energy into radiation via interaction with the circumstellar medium. Light-curve modelling suggests that this interaction involves ejecta with a mass of M _ej ≃ 1.3 M _☉ impacting a shell of M _CSM ≃ 0.6 M _☉ . However, interaction is found to be geometrically and temporally confined within a few days after explosion, with the later evolution of the transient being dominated by the free expansion of the ejecta. Spectroscopic analysis reveals a complex, stratified, circumstellar environment. In addition to the innermost shocked region, we identify two additional He-rich shells: a dense inner one ( n _e ≃ 1.4 × 10 ⁹ cm ⁻³ ) located at R ≳ 9.3 × 10 ¹⁴ cm, and a more extended outer component reaching R _out ≃ 3.3 × 10 ¹⁵ cm and expanding at approximately 1.2 × 10 ³ km s ⁻¹ . The physical parameters of the former suggest it was produced by a discrete, pre-supernova eruptive event similar to that of SN 2006jc, which expelled approximately 1.8 × 10 ⁻² M _☉ of material. Conversely, the expansion velocity of the outermost shell is consistent with the escape velocity of Wolf–Rayet stars, pointing towards a late WN- to WC-type progenitor as the most viable candidate for SN 2020able.

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.

Aims. We present and interpret the data of the nearby hydrogen-deficient but helium-rich superluminous supernova SN 2021bnw, which reached a magnitude of –20.7 at maximum luminosity in g band. Methods. We discuss the light curves and spectra of SN 2021bnw based on its spectro-photometric follow-up, exploiting different observational facilities. We reproduced the near-IR spectrum of SN 2021bnw with TARDIS to inspect the chemical composition at late photospheric phases and identify helium features. We also used a STELLA model coupling hydrodynamics and radiation transport to constrain the physical parameters of the explosion assuming a ⁵⁶ Ni+CSM scenario. Results. We suggest that SN 2021bnw was mainly powered by the interaction of the ejecta with a previously lost He-rich circumstellar material, coupled with a central power source. Conclusions. This work expands the data sample of He-rich superluminous supernovae (SLSNe Ib), and for a single-progenitor scenario, it can constrain the masses and physics of their progenitors.

We report the results of a photometric and spectroscopic follow-up campaign of the unusual interacting supernova (SN) 2022pda. Precursor variability lasting ∼100 days is observed before the explosion. The SN light curve has a double-peak shape. It reached a first maximum of M _r = −19.6 ± 0.2 mag, followed by an initial 2 month decline and a second, broad peak lasting about 6 months. The early spectra show a blue continuum with dominant H and He emission lines. A high-resolution pre-maximum spectrum shows that the profile of the He i λ 5876 line consists of a moderately narrow (∼1900 km s ⁻¹ ) P Cygni absorption superposed on a broader (∼3300 km s ⁻¹ ) component. In the blue region, several spectral features are identified, including C iii /N iii /O ii blends. Two broad bumps at 4600–5200 Å and 6400–6800 Å regions reveal a complex profile, which are likely due to blends of H, He, and other emission lines. Late-time spectra are still dominated by prominent and broad H and He lines in emission. Shock-driven model fits to the bolometric light curve suggest that the SN is powered by interaction with a massive CSM with enhanced mass-loss rates ∼5 M _⊙ yr ⁻¹ , expelled during two events that occurred ∼1 and ∼0.2 yr before the explosion. The overall SN evolution indicates that SN 2022pda is a transitional event between an H-rich SN IIn (SN 2009ip-like) and an He-rich SN Ibn. Our findings suggest that the progenitor was likely a luminous blue variable transitioning towards a Wolf–Rayet stage.

We present optical photometric and spectroscopic observations of the Type IIb supernova (SN) 2017ati. It reached maximum light about 27 d after explosion, and the light curve shows a broad, luminous peak with an absolute r -band magnitude of M _r = −18.48 ± 0.16 mag. At about 50 d after maximum light, SN 2017ati exhibited a decline rate close to that expected from the ⁵⁶ Co → ⁵⁶ Fe radioactive decay, at 0.98 mag per 100 days, as is usually observed in SNe IIb. However, it remains systematically brighter at late times by about 1–2 mag, exceeding the usual upper luminosity range of this class. As a result, modelling the light curve of SN 2017ati with a standard ⁵⁶ Ni decay scenario requires a large nickel mass of up to ∼0.37 M _⊙ and still fails to reproduce the early-time light curve adequately. In contrast, incorporating additional energy input from a magnetar yields a significantly improved fit to the light curve of SN 2017ati, which would reduce the nickel mass to ∼0.21 M _⊙ , which is still close to the upper end of the range typically inferred for SNe IIb. Comparing the fitted results of SN 2017ati with the known sample of SNe IIb indicates that its luminosity evolution is best explained by a combination of neutron star spin-down energy and radioactive nickel deposition. From late-time nebular spectra of SN 2017ati, the luminosity of the [O I ] λλ 6300, 6364 doublet implies an oxygen mass of ∼1.82 − 3.34 M _⊙ , and the combination of a [Ca II ]/[O I ] flux ratio of ∼0.5 with nebular spectral model comparisons favours a progenitor zero-age main-sequence mass of ≥17 M _⊙ .

We present a photometric and spectroscopic analyses of the Type Ibn supernova (SN) 2024acyl. It rises to an absolute magnitude peak of M _o = −17.58 ± 0.15 mag in 10.6 days, and displays a rapid linear post-peak light-curve decline in all bands (e.g. γ _0 − 60 ( V ) = 0.097 ± 0.002 mag day ⁻¹ ), similar to most SNe Ibn. The optical pseudobolometric light curve peaks at (3.5 ± 0.8)×10 ⁴² erg s ⁻¹ , with a total radiated energy of (5.0 ± 0.4)×10 ⁴⁸ erg. The spectra are dominated by a blue continuum at early stages, with narrow P-Cygni He I lines and flash-ionisation emission lines of C III , N III , and He II . The P-Cygni He I features gradually evolve and become emission-dominated in late-time spectra. The H α line is detected throughout the entire spectral evolution, which indicates that the circumstellar material (CSM) is helium-rich with some residual amount of hydrogen. Our multi-band light-curve modelling yields estimates of the ejecta mass of M _ej = 0.49 ^+0.11 _−0.09 M _⊙ with a kinetic energy of E _k = 0.06 ^+0.01 _−0.01 × 10 ⁵¹ erg, and a ⁵⁶ Ni mass of M _Ni = 0.018 M _⊙ . The inferred CSM properties are characterised by a mass of M _CSM = 0.51 ^+0.05 _−0.04 M _⊙ , an inner radius of R ₀ =17.8 ^+3.6 _−3.0 AU, and a density of ρ _CSM = (8.3 ^+2.7 _−1.2 ) × 10 ⁻¹² g cn ⁻³ . The multi-epoch spectra are well reproduced by the CMFGEN/ he4p0 model, corresponding to a He-ZAMS mass of 4 M _⊙ (H-ZAMS mass 18.11 M _⊙ , pre-SN mass 3.16 M _⊙ ). These findings are consistent with a scenario of an SN powered by ejecta-CSM interaction originating from a low-mass helium star that evolved within an interacting binary system where the CSM with some residual hydrogen may originate from the mass-transfer process. We also discuss an extreme scenario involving the possible merger of a helium white dwarf. In addition, a channel of core-collapse explosion of a late-type Wolf-Rayet (WR) star with hydrogen, or a transitional star between an Of and a WR type (e.g. an Ofpe/WN9 star) with fallback accretion cannot be entirely ruled out.

Einstein Telescope (ET) is the European project for a gravitational-wave (GW) observatory of third-generation. In this paper we present a comprehensive discussion of its science objectives, providing state-of-the-art predictions for the capabilities of ET in both geometries currently under consideration, a single-site triangular configuration or two L-shaped detectors. We discuss the impact that ET will have on domains as broad and diverse as fundamental physics, cosmology, early Universe, astrophysics of compact objects, physics of matter in extreme conditions, and dynamics of stellar collapse. We discuss how the study of extreme astrophysical events will be enhanced by multi-messenger observations. We highlight the ET synergies with ground-based and space-borne GW observatories, including multi-band investigations of the same sources, improved parameter estimation, and complementary information on astrophysical or cosmological mechanisms obtained combining observations from different frequency bands. We present advancements in waveform modeling dedicated to third-generation observatories, along with open tools developed within the ET Collaboration for assessing the scientific potentials of different detector configurations. We finally discuss the data analysis challenges posed by third-generation observatories, which will enable access to large populations of sources and provide unprecedented precision.

Type Ia supernovae (SNe Ia) are among the most precise cosmological distance indicators used to study the expansion history of the Universe. The vast increase in SN Ia data due to large-scale astrophysical surveys has led to the discovery of a wide variety of SN Ia sub-classes, such as transitional and fast-declining SNe Ia. However, their distinct photometric and spectroscopic properties differentiate them from the population of normal SNe Ia such that their use as cosmological tools remains challenged. Here, we present a high-cadenced photometric and spectroscopic dataset of two SNe Ia, SNe 2020ue and 2020nlb, which were discovered in the nearby Virgo cluster of galaxies. Our study shows that SN 2020nlb is a normal SN Ia whose unusually red colour is intrinsic, arising from a lower photospheric temperature rather than interstellar reddening, providing clear evidence that colour diversity among normal SNe Ia can have a physical origin. In contrast, SN 2020ue has photometric properties, such as colour evolution and light curve decay rate, similar to those of transitional SNe. It is hence more spectroscopically aligned with normal SNe Ia. This is evident from spectroscopic indicators such as the pseudo-equivalent width of Si II lines. Thus, such SNe Ia, which lie photometrically at the edge of the standard normal SNe Ia range, may be missed in cosmological SNe Ia samples. Our results highlight that a spectroscopic analysis of SNe Ia around peak brightness is crucial for identifying intrinsic colour variations and constructing a more complete and physically homogeneous SN Ia sample for precision cosmology.

Context. Type IIb supernovae (SNe IIb) are stellar explosions whose spectra reveal transitional features between hydrogen-rich (Type II) and helium-rich (Type Ib) SNe. Their progenitors are massive stars that were mostly stripped of their hydrogen envelope, likely through binary interaction and/or strong stellar winds. This makes such stars key tools in studies of the late stages of the evolution of massive stars. Aims. We present an extensive photometric and spectroscopic follow-up campaign of the Type IIb SN 2022ngb. Through the detailed modeling of this dataset, we aim to constrain the key physical parameters of the explosion, infer the nature of the progenitor star and its environment, and probe the dynamical properties of the ejecta. Methods. We analyzed photometric and spectroscopic data of SN 2022ngb. By constructing and modeling the bolometric light curve with semi-analytic models, we were able to estimate the primary explosion parameters. The spectroscopic data were compared with those of well-studied SNe IIb and NLTE models to constrain the properties of the progenitor and the structure of the resulting ejecta. Results. SN 2022ngb is a low-luminosity SN IIb with a peak bolometric luminosity of L _Bol = 7.76 ^+1.15 _−1.00 × 10 ⁴¹ erg s ⁻¹ and a V -band rising time of 24.32 ± 0.50 days. The light curve modeling indicates an ejecta mass of ∼2.9 − 3.2 M _⊙ , an explosion energy of ∼1.4 × 10 ⁵¹ erg, and a low synthesized ⁵⁶ Ni mass of ∼0.045 M _⊙ . The nebular phase spectra exhibit asymmetric line profiles, pointing to a nonspherical explosion and an anisotropic distribution of radioactive material. Our analysis reveals a relatively compact stripped-envelope progenitor with a pre-SN mass of approximately 4.7 M _⊙ (corresponding to a 15–16 M _⊙ ZAMS star). Conclusions. Our analysis suggests that SN 2022ngb originated from the explosion of a moderate-mass relatively compact, stripped-envelope star in a binary system. The asymmetries inferred from the nebular phase spectral line features indicate the occurrence of a nonspherical explosion.

We present optical photometric and spectroscopic observations of the rapidly declining Type IIL supernova (SN) 2016iog. SN 2016iog reached its peak ∼14 days after explosion, with an absolute magnitude in the V band of −18.64 ± 0.15 mag, followed by a steep decline of 8.85 ± 0.15 mag (100 d) ⁻¹ post-peak. Such a high decline rate makes SN 2016iog one of the fastest-declining Type IIL SNe observed to date. The rapid rise in the light curve, combined with the nearly featureless continuum observed in the spectrum at +9.3 days, suggests the presence of interaction. In the recombination phase, we observed broad H α lines that persist at all epochs. In addition, the prominent double-peaked H α feature observed in the late-time spectrum (+190.8 days) is likely attributable either to significant dust formation within a cool dense shell or to asymmetric circumstellar material. These features suggest the presence of a sustained interaction around SN 2016iog. We propose that the observed characteristics of SN 2016iog can be qualitatively explained by assuming a low-mass H-rich envelope surrounding a red supergiant progenitor star with low-density circumstellar material.