We present X-shooter spectroscopic and photometric observations of a sample of 21 hydrogen-poor superluminous supernovae (SLSNe-I), spanning a redshift range of z = 0.13 − 0.95, aimed at searching for shells of circumstellar material (CSM). Specifically, we focused on identifying broad Mg II absorption features that are blueshifted by several thousand kilometers per second relative to the narrow absorption lines associated with the host galaxy. These broad features have previously been interpreted to arise from resonance line scattering of the SLSN continuum by rapidly expanding CSM ejected shortly before explosion. Utilizing high-quality near-ultraviolet spectra, we modeled the region around 2800 Å to characterize the Mg II line profiles, enabling us to either confirm their presence or place constraints on undetected CSM shells. We identified five objects in our sample that show broad Mg II absorption features consistent with the presence of CSM. While SN 2018ibb, SN 2020xga, and SN 2022xgc have been previously reported, we identified previously undiscovered CSM shells in DES15S2nr and DES16C3ggu. In the case of DES15S2nr, the CSM shell is located at ∼3.4 × 10¹⁵ cm and is moving with a maximum velocity of ∼4800 km s⁻¹. For DES16C3ggu, the shell lies at ∼4.8 × 10¹⁵ cm and reaches up to ∼4700 km s⁻¹. These shells were likely expelled approximately two and three months before the explosion of their respective associated SNe on timescales consistent with late-stage eruptive mass-loss episodes. We further found evidence that the velocities of the CSM shells in all objects lie within 3000 − 5000 km s⁻¹, which may reflect an intrinsic property and could hint at a similar mass-ejection mechanism. We did not find any correlations between the shell properties and the SN properties, except for a marginal correlation between the light curve decline timescale and the shell velocities. This correlation needs further work; however, if it applies, it is a powerful link between the late-time mass ejection and eventual explosion. We further demonstrate that CSM configurations similar to the majority of the detected shells would have been observable in spectra with a signal-to-noise > 5 per resolution element, and that the lines from a shell are, in general, detectable except in cases where the shell is either very geometrically and/or optically thin. Therefore, we conclude that the non-detections are unlikely to arise from selection effects but they may instead point to the existence of a subclass of SLSN-I progenitors undergoing late-stage shell ejections shortly before explosion.

Context. Superluminous supernovae (SLSNe) are a rare class of transients with peak luminosities 10–100 times greater than those of standard core-collapse supernovae (SNe). The mechanisms powering their extreme brightness remain debated, with circumstellar medium (CSM) interaction, or energy injection from a central engine like a magnetar wind nebula being the most plausible scenarios. While the optical properties of SLSNe are extensively studied, their γ-ray signatures remain poorly constrained.

Aims. To further constrain the underlying mechanism, we carried out a systematic search for giga-electronvolt γ-ray emission using the Fermi Large Area Telescope (LAT) from a sample of nearby hydrogen-poor (Type I) and hydrogen-rich (Type II) SLSNe over the past 16 years. Our objective is to test predictions from CSM and magnetar models, and to assess the prospects for future detections with the Cherenkov Telescope Array Observatory (CTAO).

Methods. For the six targets of this sample, we studied the time variability of a putative γ-ray signal at the optical position of the SLSN on a six-month timescale, and in the case of SN 2017egm, we further investigated variability on 15-day intervals and applied a Bayesian block algorithm to characterize the time variability of the signal. We then compared the temporal evolution and spectral properties to the predictions from a magnetar and CSM interaction model.

Results. Among the sample, only SN 2017egm shows significant γ-ray emission, with likelihood test statistic (TS) values of 26–33 (i.e., > 5σ) depending on the adopted time window. The signal arises between 50 and 160 days after explosion and is well described by a power-law spectrum with index Γ = 2.17 ± 0.23. The emission is consistent both in terms of its light curve and its spectrum, with predictions from magnetar models requiring either low nebular magnetization or faster spin-down than dipole losses. The CSM shell interaction scenario can reproduce the observed flux level but not the observed timing of the γ-ray signal. In addition, the observed ratio, L_γ/L_opt ∼ 1, is inconsistent with theoretical expectations and not in line with ratio measurements in other interacting CSM-dominated objects (e.g., novae or SNe) where this ratio is less than 10⁻².

Conclusions. Our study strongly suggests that a central engine like a magnetar plays a key role in this SLSN and could explain the bulk of the optical and γ-ray light curves properties. In order to explain the observed late-time bumps in the optical light curve of SN 2017egm, we require either: a hybrid picture combining magnetar and multiple CSM shells for the optical bumps or a pure magnetar model with infalling matter on an accretion disk. Finally, simulations of 50 hours of CTAO observations indicate that a SN 2017egm-like event would be detectable up to ∼140 Mpc in the magnetar model but not in the CSM model due to strong γ − γ absorption.

Although historically classified into discrete subclasses, there is growing evidence that indicates that core-collapse supernovae (CCSNe) categories often overlap, reflecting continuous variations in progenitor structure, mass-loss history, and circumstellar environments rather than strictly distinct channels. In this review, we explore the proposed continua that link hydrogen-rich Type II SNe to stripped-envelope explosions (IIb→Ib→Ic), and that extend further into interaction-dominated and superluminous events. We discuss the physical processes—stellar winds, binary interaction, eruptive outbursts, and circumstellar interaction—that may produce graded outcomes across classes, while highlighting where observational evidence supports or challenges smooth transitions. We propose that CCSNe are better viewed as a multidimensional continuum of explosion outcomes, where traditional subclasses act as reference points rather than strict boundaries. Future progress will rely on large, homogeneous datasets and advanced modeling to disentangle true evolutionary sequences from apparent overlaps, ultimately connecting progenitor pathways to the observed diversity of explosions.

We present photometric and spectroscopic observations of SN 2021aaev, a hydrogen-rich, superluminous supernova with persistent (at least ∼100 days) narrow Balmer lines (SLSN-IIn) at redshift z = 0.1557. SN 2021aaev rose over 28.1  ±  1.0 rest-frame days after explosion, reaching a peak absolute magnitude of −21.46 ± 0.01 in the ATLAS o band. The prepeak spectra resemble those of typical Type IIn SNe with flash-ionization features arising from the interaction with a dense, confined circumstellar medium (CSM), albeit the flash timescale is longer than usual (>20 days). Postpeak, the narrow emission lines evolve slowly, and the absence of ejecta features indicates strong deceleration by the CSM. The total radiated energy (about 1.41 × 10⁵¹ erg) is possibly explained by a low-mass (1–2 M_⊙) ejecta ploughing into a massive (9–19 M_⊙), extended (outer radius >1 × 10¹⁶ cm) H-rich CSM, or alternatively by magnetar-powered models. Interestingly, the host environment consists of a spiral galaxy with a red substructure in the southeastern part, and the SN’s exact location coincides with this quiescent red region (star formation rate yr⁻¹). Given the atypical environment and the obscuring effect of the massive CSM, a thermonuclear (Type Ia-CSM) origin cannot be ruled out. Altogether, SN 2021aaev is a compelling case to study the diversity of SLSN-IIn features and their host environments.

Context. Current large-scale, high-cadence surveys, such as the Zwicky Transient Facility (ZTF), provide detections of new and rare types of transients and supernovae (SNe) whose physical origins are not well understood.

Aims. We aim to investigate the nature of SN 2021lwz at a redshift z = 0.065, an over-luminous SN with an absolute magnitude of M_g ∼ −20.1 AB that falls in the lower range of superluminous supernovae (SLSNe) luminosities and was discovered in a faint dwarf galaxy with an absolute magnitude of M_g ≃ −14.5 AB.

Methods. We studied SN 2021lwz using optical spectroscopy and photometry and by imaging linear polarimetry obtained during several follow-up campaigns. All the data were used to analyse and model the evolution of the explosion. Comparisons with other SNe of well-known or rarer types were investigated.

Results. SN 2021lwz belongs to the rare class of rapidly evolving transients. The bolometric light curve rises in about seven days to a peak luminosity of about 5 × 10⁴³ erg/s, at a rate of 0.2 mag day⁻¹ close to the peak. Spectroscopy modelling reveals more similarities with a normal Type Ic-like SN than with an SLSN before peak, showing slightly broadened lines after peak. Light curve modelling shows that the Arnett model of the bolometric light curve using a radioactive source (⁵⁶Ni) is not able to reasonably explain the light curve evolution. A magnetar model seems more appropriate, suggesting that the explosion of low ejecta mass (M_ej ∼ 0.24 M_⊙) took place in a low-mass (M ∼ 10^6.66 M_⊙) dwarf galaxy of specific star formation rate about ten times larger than typical star-forming galaxies.

Conclusions. SN 2021lwz is an uncommon transient showing many similarities with several classes of transients, including rare transients. It may be an interesting example of how differences in ejecta mass and engine parameters could produce a wide range of engine-driven stripped-envelope SNe.

We present observations of SN 2023xgo, a transitional Type Ibn/Icn SN, from −5.6 to 63 d relative to r-band peak. Early spectra show C iii  λ5696 emission like Type Icn SNe, shifting to Type Ibn features. The He i velocities (1800–10 000 km s⁠⁻¹) and pseudo-equivalent widths are among the highest in the Ibn/Icn class. The light curve declines at 0.14 mag d⁻¹ until 30 d, matching SNe Ibn/Icn but slower than fast transients. SN 2023xgo is the faintest in our SN Ibn sample (M_r = −17.65 ± 0.04⁠⁠) but shows typical colour and host properties. Semi-analytical modelling of the light curve suggests a compact CSM shell (∼ 10¹²–10¹³ cm), mass-loss rate between 10⁻⁴ and 10⁻³ M_☉ yr⁻¹ with CSM and ejecta masses of ∼0.22 and 0.12 M_⁠☉, respectively. Post-maximum light-curve, spectral modelling favours a ∼3 M_☉ helium star progenitor with extended (⁠∼ 10¹⁵ cm), stratified CSM (density exponent of 2.9) and mass-loss rate of 0.1 − 2.7 M_☉ yr⁠⁻¹. These two mass-loss regimes imply a radially varying CSM, shaped by asymmetry or changes in the progenitor’s mass-loss over time. This mass-loss behaviour fits both binary and single-star evolution. Early Icn-like features stem from hot carbon ionization, fading to Ibn-like with cooling. SN 2023xgo thus offers rare insight into the connection between SNe Icn, Ibn, and SNe Ibn with ejecta signatures.

Type Ia supernovae (SNe Ia) provide the robustest means of measuring extragalactic distances. While increasing the number of SNe Ia observed in the optical has received most effort so far, near-infrared (NIR) observations remain scarce despite their advantages. The dust extinction in NIR observations is lower and the behavior of standard candles is more intrinsic and therefore requires little to no empirical corrections. We present the ANDICAM-SOFI Near-infrared and Optical type Ia Supernova (ASNOS) dataset with sample size of 1482 epochs in the BVRIYJH filters from the ANDICAM instrument on the 1.3-meter SMARTS telescope at Cerro Tololo Inter-American Observatory, along with 125 JHK epochs from the SOFI instrument on the 3.58-meter New Technology Telescope on the La Silla Observatory. Additionally, we incorporate optical forced photometry from the Zwicky Transient Facility and the Asteroid Terrestrial-impact Last Alert System. The sample comprises 41 SNe Ia in total, including 29 normal events, eight 1991T-like objects, and four peculiar subtypes, all located at redshifts z<0.085. We provide a detailed overview of the ASNOS sample selection, data reduction, SN photometry, construction of the global and local host-galaxy spectral energy distribution, and SN light-curve fitting using three methods: SALT3-NIR, SNooPy, and BayeSN. A companion paper will present the cosmological analysis.

Although all Type II supernovae (SNe) originate from massive stars possessing a hydrogen-rich envelope, their light-curve morphology is diverse, reflecting poorly characterized heterogeneity in the physical properties of their progenitor systems. Here, we present a detailed light-curve analysis of a magnitude-limited sample of 639 Type II SNe from the Zwicky Transient Facility Bright Transient Survey. Using Gaussian processes, we systematically measure empirical light-curve features (e.g. rise times, peak colours, and luminosities) in a robust sampling-independent manner. We focus on rise times as they are highly sensitive to pre-explosion progenitor properties, especially the presence of a dense circumstellar medium (CSM) shed by the progenitor in the years immediately pre-explosion. By correlating our feature measurements with physical parameters from an extensive grid of ꜱᴛᴇʟʟᴀ hydrodynamical models with varying progenitor properties (CSM structure, ⁠Ṁ, R_CSM⁠, and M_ZAMS⁠), we quantify the proportion of events with sufficient pre-explosion mass loss to significantly alter the initial light curve (roughly ⁠) in a highly complete sample of 377 spectroscopically classified Type II SNe. We find that 67  6 per cent of observed SNe in our magnitude-limited sample show evidence for substantial CSM (⁠⁠) close to the progenitor (⁠R_CSM < 10¹⁵ cm) at the time of explosion. After applying a volumetric-correction, we find per cent of all Type II SN progenitors possess substantial CSM within 10¹⁵ cm at the time of explosion. This high fraction of progenitors with dense CSM, supported by photometric and spectroscopic evidence of previous SNe, reveals mass-loss rates significantly exceeding those measured in local group red supergiants or predicted by current theoretical models.

We present the luminosity function and volumetric rate of a sample of Type IIP supernovae (SNe) from the Zwicky Transient Facility Census of the Local Universe survey (CLU). This is the largest sample of Type IIP SNe from a systematic volume-limited survey to-date. The final sample includes 330 Type IIP SNe and 36 low-luminosity Type II (LLIIP) SNe with M_r,peak > −16 mag, which triples the literature sample of LLIIP SNe. The fraction of LLIIP SNe is 19−4+3% of the total CLU Type IIP SNe population (8−2+1% of all core-collapse SNe). This implies that while LLIIP SNe likely represent the fate of core-collapse SNe of 8–12 M_⊙ progenitors, they alone cannot account for the fate of all massive stars in this mass range. To derive an absolute rate, we estimate the ZTF pipeline efficiency as a function of the apparent magnitude and the local surface brightness. We derive a volumetric rate of (3.9−0.4+0.4)×104Gpc−3yr−1 for Type IIP SNe and (7.3−0.6+0.6)×103Gpc−3yr−1 for LLIIP SNe. Now that the rate of LLIIP SNe is robustly derived, the unresolved discrepancy between core-collapse SN rates and star formation rates cannot be explained by LLIIP SNe alone.

We present an analysis of the optical and near-infrared properties of SN 2022lxg, a bright (Mg peak=-19.41 mag) and rapidly evolving supernova (SN). It was discovered within a day of explosion, and rose to peak brightness in ∼10 d. Two distinct phases of circumstellar interaction are evident in the data. The first is marked by a steep blue continuum (T > 15 000 K) with flash-ionisation features due to hydrogen and HēII. The second, weaker phase is marked by a change in the colour evolution accompanied by changes in the shapes and velocities of the spectral line profiles. Narrow P-Cygni profiles (150 km s-1) of HēI further indicate the presence of slow-moving, unshocked material and suggest partial stripping of the progenitor. The fast decline of the light-curve from the peak (3.48 ± 0.26 mag (50 d)-1 in g band) implies that the ejecta mass must be low. Spectroscopically, until +35 d there are similarities with some Type IIb SNe but then there is a transition to spectra that are more reminiscent of an interacting SN II. However, metal lines are largely absent in the spectra, even at epochs of ∼80 d. Its remote location (∼4.6 kpc projected offset) from the presumed host galaxy, a dwarf with MB ∼-14.4 mag, is consistent with our metallicity estimate close to the values of the Small Magellanic Cloud obtained from scaling relations. Furthermore, several lines of evidence (including intrinsic polarisation of p ∼(0.5-1.0)%) point to deviations from spherical symmetry. We suggest that a plausible way of uniting the observational clues is to consider a binary system that underwent case C mass transfer. This failed to remove the entire H envelope of the progenitor before it underwent core collapse. In this scenario, the progenitor itself would be more compact and perhaps straddle the boundary between blue and yellow supergiants, which ties in with the early spectroscopic similarity to Type IIb SNe.