Type Ia supernovae (SNe Ia) arise from the thermonuclear explosions of white dwarfs in multiple-star systems. A rare subclass of SNe Ia exhibit signatures of interaction with circumstellar material (CSM), allowing for direct constraints on companion material. While most known events show evidence for dense nearby CSM identified via peak-light spectroscopy (as SNe Ia-CSM), targeted late-time searches have revealed a handful of cases exhibiting delayed CSM interaction with detached shells. Here we present the first all-sky search for late CSM interaction in SNe Ia using a new image subtraction pipeline for mid-infrared data from the NEOWISE space telescope. Analyzing a sample of  ≈8500 SNe Ia, we report evidence for late-time mid-infrared brightening in five previously overlooked events spanning subtypes SNe Iax, SNe Ia-91T, and super-Chandra SNe Ia. Our systematic search doubles the known sample and suggests that ≳0.05% of SNe Ia exhibit mid-infrared signatures of delayed CSM interaction. The mid-infrared light curves ubiquitously indicate the presence of multiple (or extended) detached CSM shells located at ≳10¹⁶–10¹⁷ cm, containing 10⁻⁶ to 10⁻⁴ M_⊙ of dust, with some sources showing evidence for new dust formation, possibly within the cold, dense shell of the ejecta. We do not detect interaction signatures in spectroscopic and radio follow-up; however, the limits are largely consistent with previously confirmed events given the sensitivity and observation phase. Our results highlight that CSM interaction is more prevalent than previously estimated from optical and ultraviolet searches and that mid-infrared synoptic surveys provide a unique window into this phenomenon.

Aims. We investigate the spectroscopic characteristics of intermediate-luminosity Red Transients (ILRTs), a class of elusive objects with peak luminosity between that of classical novae and standard supernovae. Our goal is to provide a stepping stone in the path to unveiling the physical origin of these events based on the analysis of the collected datasets. Methods. We present the extensive optical and near-infrared (NIR) spectroscopic monitoring of four ILRTs, namely NGC 300 2008OT-1, AT 2019abn, AT 2019ahd and AT 2019udc. First we focus on the evolution of the most prominent spectral features observed in the low-resolution spectra. We then present a more detailed description of the high-resolution spectrum collected for NGC 300 2008OT-1 with the Very Large Telescope equipped with UVES. Finally, we describe our analysis of late-time spectra of NGC 300 2008OT-1 and AT 2019ahd through comparisons with both synthetic and observed spectra. Results. Balmer and Ca lines dominate the optical spectra, revealing the presence of slowly moving circumstellar medium (CSM) around the objects. The line luminosity of Hα, Hβ, and Ca II NIR triplet presents a double peaked evolution with time, possibly indicative of interaction between fast ejecta and the slow CSM. The high-resolution spectrum of NGC 300 2008OT-1 reveals a complex circumstellar environment, with the transient being surrounded by a slow (∼30 km s-1) progenitor wind. At late epochs, optical spectra of NGC 300 2008OT-1 and AT 2019ahd show broad (∼2500 km s-1) emission features at ∼6170 Å and ∼7000 Å which are unprecedented for ILRTs. We find that these lines originate most likely from the blending of several narrow lines, possibly of iron-peak elements.

Supernovae (SNe) come in various flavors and are classified into different types based on emission and absorption lines in their spectra. SN candidates are now abundant with the advent of large systematic sky surveys like the Zwicky Transient Facility (ZTF), however, the identification bottleneck lies in their spectroscopic confirmation and classification. Fully robotic telescopes with dedicated spectrographs optimized for SN follow-up have eased the burden of data acquisition. However, the task of classifying the spectra still largely rests with the astronomers. Automating this classification step reduces human effort and can make the SN type available sooner to the public. For this purpose, we have developed a deep-learning based program for classifying core-collapse supernovae (CCSNe) with ultra-low resolution spectra from the SED-machine spectrograph on the Palomar 60 inch telescope. The program consists of hierarchical classification task layers, with each layer composed of multiple binary classifiers running in parallel to produce a reliable classification. The binary classifiers utilize recurrent neural networks and convolutional neural networks architecture and are designed to take multiple inputs to supplement spectra with g- and r-band photometry from ZTF. On non-host-contaminated and good quality SEDM spectra (“gold” test set) of CCSNe, CCSNscore is ∼94% accurate in distinguishing between hydrogen-rich (Type II) and hydrogen-poor (Type Ibc) CCSNe. With light curve input, CCSNscore classifies ∼83% of the gold set with high confidence (score ≥0.8 and score-error < 0.05), with ∼98% accuracy. Based on SNIascore’s and CCSNscore’s real-time performance on bright transients (mpk ≤ 18.5) and our reporting criteria, we expect ∼0.5% (∼4) true SNe Ia to be misclassified as SNe Ibc and ∼6% (∼17) of true CCSNe to be misclassified between Type II and Type Ibc annually on the Transient Name Server.

We present photometric and spectroscopic observations of SN 2020xga and SN 2022xgc, two hydrogen-poor superluminous supernovae (SLSNe-I) at z=-0.4296 and z = 0.3103, respectively, which show an additional set of broad Mg II absorption lines, blueshifted by a few thousands kilometer second-1 with respect to the host galaxy absorption system. Previous work interpreted this as due to resonance line scattering of the SLSN continuum by rapidly expanding circumstellar material (CSM) expelled shortly before the explosion. The peak rest-frame g-band magnitude of SN 2020xga is -22.30 ± 0.04 mag and of SN 2022xgc is -21.97 ± 0.05 mag, placing them among the brightest SLSNe-I. We used high-quality spectra from ultraviolet to near-infrared wavelengths to model the Mg II line profiles and infer the properties of the CSM shells. We find that the CSM shell of SN 2020xga resides at ∼1.3×1016 cm, moving with a maximum velocity of 4275 km s-1, and the shell of SN 2022xgc is located at ∼0.8×1016 cm, reaching up to 4400 km s-1. These shells were expelled ∼11 and ∼5 months before the explosions of SN 2020xga and SN 2022xgc, respectively, possibly as a result of luminous-blue-variable-like eruptions or pulsational pair instability (PPI) mass loss. We also analyzed optical photometric data and modeled the light curves, considering powering from the magnetar spin-down mechanism. The results support very energetic magnetars, approaching the mass-shedding limit, powering these SNe with ejecta masses of ∼7-9M⊙. The ejecta masses inferred from the magnetar modeling are not consistent with the PPI scenario pointing toward stars > 50M⊙ He-core; hence, alternative scenarios such as fallback accretion and CSM interaction are discussed. Modeling the spectral energy distribution of the host galaxy of SN 2020xga reveals a host mass of 107.8 M⊙, a star formation rate of 0.96-0.26+0.47 M⊙ yr-1, and a metallicity of ∼0.2 Z⊙

We present multiwavelength analysis of ZTF23abelseb (AT 2023sva), an optically discovered fast-fading (∆mr = 2.2 mag in ∆t = 0.74 d), luminous (Mr ∼ −30.0 mag), and red (g − r = 0.50 mag) transient at z = 2.28 with accompanying luminous radio emission. AT 2023sva does not possess a γ -ray burst (GRB) counterpart to an isotropic equivalent energy limit of Eγ,iso < 1.6 × 1052 erg, determined through searching γ -ray satellite archives between the last non-detection and first detection, making it the sixth example of an optically discovered afterglow with a redshift measurement and no detected GRB counterpart. We analyse AT 2023sva’s optical, radio, and X-ray observations to characterize the source. From radio analyses, we find the clear presence of strong interstellar scintillation (ISS) 72 d after the initial explosion, allowing us to place constraints on the source’s angular size and bulk Lorentz factor. When comparing the source sizes derived from ISS of orphan events to those of the classical GRB population, we find orphan events have statistically smaller source sizes. We also utilize Bayesian techniques to model the multiwavelength afterglow. Within this framework, we find evidence that AT 2023sva possesses a shallow power-law structured jet viewed slightly off-axis (θv = 0.07 ± 0.02) just outside of the jet’s core opening angle (θc = 0.06 ± 0.02). We determine this is likely the reason for the lack of a detected GRB counterpart, but also investigate other scenarios. AT 2023sva’s evidence for possessing a structured jet stresses the importance of broadening orphan afterglow search strategies to a diverse range of GRB jet angular energy profiles, to maximize the return of future optical surveys.

Context. Hydrogen-rich superluminous supernovae (SLSNe II) are rare. The exact mechanism producing their extreme light curve peaks is not understood. Analysis of single events and small samples suggest that circumstellar material (CSM) interaction is the main mechanism responsible for the observed features. However, other mechanisms cannot be discarded. Large sample analysis can provide clarification.

Aims. We aim to characterize the light curves of a sample of 107 SLSNe II to provide valuable information that can be used to validate theoretical models.

Methods. We analyzed the gri light curves of SLSNe II obtained through ZTF. We studied the peak absolute magnitudes and characteristic timescales. When possible, we computed the g − r colors and pseudo-bolometric light curves, and estimated lower limits for their total radiated energy. We also studied the luminosity distribution of our sample and estimated the fraction that would be observable by the LSST. Finally, we compared our sample to other H-rich SNe and to H-poor SLSNe I.

Results. SLSNe II are heterogeneous. Their median peak absolute magnitude is ∼ − 20.3 mag in optical bands. Their rise can take from ∼two weeks to over three months, and their decline times range from ∼twenty days to over a year. We found no significant correlations between peak magnitude and timescales. SLSNe II tend to show fainter peaks, longer declines, and redder colors than SLSNe I.

Conclusions. We present the largest sample of SLSN II light curves to date, comprising 107 events. Their diversity could be explained by different CSM morphologies, although theoretical analysis is needed to explore alternative scenarios. Other luminous transients, such as active galactic nuclei, tidal disruption events or SNe Ia-CSM, can easily become contaminants. Thus, good multiwavelength light curve coverage becomes paramount. LSST could miss ∼30% of the ZTF events in its gri band footprint.

Aims. Stripped envelope (SE) supernovae are explosions of stars that have somehow lost most of their outer envelopes. We present the discovery and analyse the observations of the Type Ib supernova 2019odp (a.k.a. ZTF19abqwtfu) covering epochs within days of the explosion to late nebular phases at 360 d post-explosion.Methods. Our observations include an extensive set of photometric observations and low- to medium-resolution spectroscopic observations, both covering the complete observable time range. We analysed the data using analytic models for the recombination cooling emission of the early excess emission and the diffusion of the peak light curve. We expanded on existing methods to derive oxygen mass estimates from nebular phase spectroscopy, and briefly discuss progenitor models based on this analysis.Results. Our spectroscopic observations confirm the presence of He in the supernova ejecta and we thus (re)classify SN 2019odp as a Type Ib supernova. From the pseudo-bolometric light curve, we estimate a high ejecta mass of Mej ∼ 4 − 7 M⊙. The high ejecta mass, large nebular [O I]/[Ca II] line flux ratio (1.2 − 1.9), and an oxygen mass above ⪆0.5 M⊙ point towards a progenitor with a pre-explosion mass higher than 18 M⊙. Whereas a majority of analysed SE supernovae in the literature seem to have low ejecta masses, indicating stripping in a binary star system, SN 2019odp instead has parameters that are consistent with an origin in a single massive star. The compact nature of the progenitor (≲10 R⊙) suggests that a Wolf-Rayet star is the progenitor.

We present the long-term photometric and spectroscopic analysis of a transitioning SN IIn/Ibn from –10.8 d to 150.7 d post V-band maximum. SN 2021foa shows prominent He I lines comparable in strength to the H α line around peak, placing SN 2021foa between the SN IIn and SN Ibn populations. The spectral comparison shows that it resembles the SN IIn population at pre-maximum, becomes intermediate between SNe IIn/Ibn, and at post-maximum matches with SN IIn 1996al. The photometric evolution shows a precursor at –50 d and a light curve shoulder around 17 d. The peak luminosity and colour evolution of SN 2021foa are consistent with most SNe IIn and Ibn in our comparison sample. SN 2021foa shows the unique case of an SN IIn where the narrow P-Cygni in H α becomes prominent at 7.2 d. The H α profile consists of a narrow (500–1200 km s-1) component, intermediate width (3000–8000 km s-1) and broad component in absorption. Temporal evolution of the H α profile favours a disc-like CSM geometry. Hydrodynamical modelling of the light curve well reproduces a two-component CSM structure with different densities (ρ ∝ r-2–ρ ∝ r-5), mass-loss rates (10-3–10-1 M☉ yr-1) assuming a wind velocity of 1000 km s-1 and having a CSM mass of 0.18 M☉. The overall evolution indicates that SN 2021foa most likely originated from an LBV star transitioning to a WR star with the mass-loss rate increasing in the period from 5 to 0.5 yr before the explosion or it could be due to a binary interaction.

SN 2023tsz is a Type Ibn supernova (SN Ibn), an uncommon subtype of stripped-envelope core-collapse supernovae (SNe), discovered in an extremely low-mass host. SNe Ibn are characterized by narrow helium emission lines in their spectra and are believed to originate from the collapse of massive Wolf–Rayet (WR) stars, though their progenitor systems still remain poorly understood. In terms of energetics and spectrophotometric evolution, SN 2023tsz is largely a typical example of the class, although line profile asymmetries in the nebular phase are seen, which may indicate the presence of dust formation or unshocked circumstellar material. Intriguingly, SN 2023tsz is located in an extraordinarily low-mass host galaxy that is in the second percentile for stripped-envelope SN host masses and star formation rates (SFRs). The host has a radius of 1.0 kpc, a g-band absolute magnitude of <ani:tex-math>-12.72 ± 0.05$</ani:tex-math>, and an estimated metallicity of log (Z*/ Z⊙)≈ -1.6. The SFR and metallicity of the host galaxy raise questions about the progenitor of SN 2023tsz. The low SFR suggests that a star with sufficient mass to evolve into a WR would be uncommon in this galaxy. Further, the very low metallicity is a challenge for single stellar evolution to enable H and He stripping of the progenitor and produce an SN Ibn explosion. The host galaxy of SN 2023tsz adds another piece to the ongoing puzzle of SNe Ibn progenitors, and demonstrates that they can occur in hosts too faint to be observed in contemporary sky surveys at a more typical SN Ibn redshift.

Context. At late stages, massive stars experience strong mass-loss rates, losing their external layers and thus producing a dense H-rich circumstellar medium (CSM). After the explosion of a massive star, the collision and continued interaction of the supernova (SN) ejecta with the CSM power the SN light curve through the conversion of kinetic energy into radiation. When the interaction is strong, the light curve shows a broad peak and high luminosity that lasts for several months. For these SNe, the spectral evolution is also slower compared to non-interacting SNe. Notably, energetic shocks between the ejecta and the CSM create the ideal conditions for particle acceleration and the production of high-energy (HE) neutrinos above 1 TeV. Aims. We study four strongly interacting Type IIn SNe, 2021acya, 2021adxl, 2022qml, and 2022wed, in order to highlight their peculiar characteristics, derive the kinetic energy of their explosion and the characteristics of the CSM, infer clues on the possible progenitors and their environment, and relate them to the production of HE neutrinos. Methods. We analysed spectro-photometric data of a sample of interacting SNe to determine their common characteristics and derive the physical properties (radii and masses) of the CSM and the ejecta kinetic energies and compare them to HE neutrino production models. Results. The SNe analysed in this sample exploded in dwarf star-forming galaxies, and they are consistent with energetic explosions and strong interaction with the surrounding CSM. For SNe 2021acya and 2022wed, we find high CSM masses and mass-loss rates, linking them to very massive progenitors. For SN 2021adxl, the spectral analysis and less extreme CSM mass suggest a stripped-envelope massive star as a possible progenitor. SN 2022qml is marginally consistent with being a Type Ia thermonuclear explosion embedded in a dense CSM. The mass-loss rates for all the SNe are consistent with the expulsion of several solar masses of material during eruptive episodes in the last few decades before the explosion. Finally, we find that the SNe in our sample are marginally consistent with HE neutrino production.