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.

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.

Context. Type Ia supernovae (SNe Ia) are a key probe in modern cosmology, as they can be used to measure luminosity distances at gigaparsec scales. Models of their light curves are used to project heterogeneous observed data onto a common basis for analysis. Aims. The SALT model currently used for SN Ia cosmology describes SNe as having two sources of variability, accounted for by a color parameter c, and a “stretch” parameter x1. We extend the model to include an additional parameter we label x2, to investigate the cosmological impact of currently unaddressed light-curve variability. Methods. We constructed a new SALT model, that we dub “SALT3+”. This model was trained by an improved version of the SALTshaker code, using training data combining a selection of the second data release of cosmological SNe Ia from the Zwicky Transient Facility and the existing SALT3 training compilation. Results. We find additional, coherent variability in supernova light curves beyond SALT3. Most of this variation can be described as phase-dependent variation in g − r and r − i color curves, correlated with a boost in the height of the secondary maximum in i-band. These behaviors correlate with spectral differences, particularly in line velocity. We find that fits with the existing SALT3 model tend to address this excess variation with the color parameter, leading to less informative measurements of supernova color. We find that neglecting the new parameter in light-curve fits leads to a trend in Hubble residuals with x2 of 0.039 ± 0.005 mag, representing a potential systematic uncertainty. However, we find no evidence of a bias in current cosmological measurements. Conclusions. We conclude that extended SN Ia light-curve models promise mild improvement in the accuracy of color measurements, and corresponding cosmological precision. However, models with more parameters are unlikely to substantially affect current cosmological results.

Stars with zero-age main sequence masses between 140 and 260 M_⊙ are thought to explode as pair-instability supernovae (PISNe). During their thermonuclear runaway, PISNe can produce up to several tens of solar masses of radioactive nickel, resulting in luminous transients similar to some superluminous supernovae (SLSNe). Yet, no unambiguous PISN has been discovered so far. SN 2018ibb is a hydrogen-poor SLSN at z = 0.166 that evolves extremely slowly compared to the hundreds of known SLSNe. Between mid 2018 and early 2022, we monitored its photometric and spectroscopic evolution from the UV to the near-infrared (NIR) with 2–10 m class telescopes. SN 2018ibb radiated > 3 × 10⁵¹ erg during its evolution, and its bolometric light curve reached > 2 × 10⁴⁴ erg s⁻¹ at its peak. The long-lasting rise of > 93 rest-frame days implies a long diffusion time, which requires a very high total ejected mass. The PISN mechanism naturally provides both the energy source (⁵⁶Ni) and the long diffusion time. Theoretical models of PISNe make clear predictions as to their photometric and spectroscopic properties. SN 2018ibb complies with most tests on the light curves, nebular spectra and host galaxy, and potentially all tests with the interpretation we propose. Both the light curve and the spectra require 25–44 M_⊙ of freshly nucleosynthesised ⁵⁶Ni, pointing to the explosion of a metal-poor star with a helium core mass of 120–130 M_⊙ at the time of death. This interpretation is also supported by the tentative detection of [Co II] λ 1.025 μm, which has never been observed in any other PISN candidate or SLSN before. We observe a significant excess in the blue part of the optical spectrum during the nebular phase, which is in tension with predictions of existing PISN models. However, we have compelling observational evidence for an eruptive mass-loss episode of the progenitor of SN 2018ibb shortly before the explosion, and our dataset reveals that the interaction of the SN ejecta with this oxygen-rich circumstellar material contributed to the observed emission. That may explain this specific discrepancy with PISN models. Powering by a central engine, such as a magnetar or a black hole, can be excluded with high confidence. This makes SN 2018ibb by far the best candidate for being a PISN, to date.

Context. Transient astronomical events that exhibit no discernible association with a host galaxy are commonly referred to as hostless. These rare phenomena can offer unique insights into the properties and evolution of stars and galaxies. However, the sheer number of transients captured by contemporary high-cadence astronomical surveys renders the manual identification of all potential hostless transients impractical. Therefore, creating a systematic identification tool is crucial for studying these elusive events.

Aims. We present the ExtragaLactic alErt Pipeline for Hostless AstroNomical Transients (ELEPHANT), a framework for filtering hostless transients in astronomical data streams. It was designed to process alerts from the Zwicky Transient Facility (ZTF) presented in the Fink broker; however, its underlying concept can be applied to other data sources.

Methods. We used Fink to access all the ZTF alerts produced between January 2022 and December 2023, selecting alerts associated with extragalactic transients reported in SIMBAD or TNS, as well as those classified as supernovae (SNe) or kilonovae (KNe) by the machine learning (ML) classifiers within the broker. We then processed the associated stamps using a sequence of image analysis techniques to retrieve hostless candidates.

Results. We find that ≲2% of all analyzed transients are potentially hostless. Among them, only ~10% have a spectroscopic class reported on TNS, with type Ia SNe being the most common class, followed by superluminous SNe. In particular, among the host-less candidates retrieved by our pipeline, there is SN 2018ibb, which has been proposed to be a pair instability SN candidate, and SN 2022ann, one of only five known SNe Icn. When no class is reported on TNS, the dominant classes are quasi-stellar object (QSO) and SN candidates, with the former obtained from SIMBAD and the latter inferred using the Fink ML classifier.

Conclusions. ELEPHANT represents an effective strategy to filter extragalactic events within large and complex astronomical alert streams. There are many applications for which this pipeline will be useful, ranging from transient selection for follow-up to studies of transient environments. The results presented here demonstrate the feasibility of developing specially crafted pipelines that enable a variety of scientific studies based on large-scale surveys.

Context. Type II supernovae (SNe II) mark the endpoint in the lives of hydrogen-rich massive stars. Their large explosion energies and luminosities allow us to measure distances, metallicities, and star formation rates into the distant Universe. To fully exploit their use in answering different astrophysical problems, high-quality low-redshift data sets are required. Such samples are vital to understand the physics of SNe II, but also to serve as calibrators for distinct -and often lower-quality -samples. Aims. We present uBgVri optical and YJH near-infrared (NIR) photometry for 94 low-redshift SNe II observed by the Carnegie Supernova Project (CSP). A total of 9817 optical and 1872 NIR photometric data points are released, leading to a sample of high-quality SN II light curves during the first ≈150 days post explosion on a well-calibrated photometric system. Methods. The sample is presented and its properties are analysed and discussed through comparison to literature events. We also focus on individual SNe II as examples of classically defined subtypes and outlier objects. Making a cut in the plateau decline rate of our sample (s2), a new subsample of fast-declining SNe II is presented. Results. The sample has a median redshift of 0.015, with the nearest event at 0.001 and the most distant at 0.07. At optical wavelengths (V), the sample has a median cadence of 4.7 days over the course of a median coverage of 80 days. In the NIR (J), the median cadence is 7.2 days over the course of 59 days. The fast-declining subsample is more luminous than the full sample and shows shorter plateau phases. Of the non-standard SNe II highlighted, SN 2009A particularly stands out with a steeply declining then rising light curve, together with what appears to be two superimposed P-Cygni profiles of Hα in its spectra. We outline the significant utility of these data, and finally provide an outlook of future SN II science.

We present photometric and spectroscopic datasets for SN 2020pvb, a Type IIn-P supernova (SN) that is similar to SNe 1994W, 2005cl, 2009kn, and 2011ht, with a precursor outburst detected (PS1 w band ∼- 13.8 mag) around four months before the B-band maximum light. SN 2020pvb presents a relatively bright light curve that peaked at MB = -17.95 ± 0.30 mag and a plateau that lasted at least 40 days before going into solar conjunction. After this, the object was no longer visible at phases > 150 days above - 12.5 mag in the B band, suggesting that the SN 2020pvb ejecta interact with a dense, spatially confined circumstellar envelope. SN 2020pvb shows strong Balmer lines and a forest of Fe II lines with narrow P Cygni profiles in its spectra. Using archival images from the Hubble Space Telescope, we constrained the progenitor of SN 2020pvb to have a luminosity of log(L/L⊙)≤5.4, ruling out any single star progenitor over 50 M⊙. SN 2020pvb is a Type IIn-P whose progenitor star had an outburst*sim;0.5 yr before the final explosion; the material lost during this outburst probably plays a role in shaping the physical properties of the SN.

SN 2021adxl is a slowly evolving, luminous, Type IIn supernova with asymmetric emission line profiles, similar to the well-studied SN 2010jl. We present extensive optical, near-ultraviolet, and near-infrared photometry and spectroscopy covering ~1.5 years post discovery. SN 2021adxl occurred in an unusual environment, atop a vigorously star-forming region that is offset from its host galaxy core. The appearance of Lyα and O ii, as well as the compact core, would classify the host of SN 2021adxl as a “Blueberry” galaxy, analogous to higher redshift, low-metallicity, star-forming dwarf “Green Pea” galaxies. Using several abundance indicators, we find a metallicity of the explosion environment of only [Formula Presented], the lowest reported metallicity for a Type IIn SN environment. SN 2021adxl reaches a peak magnitude of Mr ≈ −20.2 mag and since discovery, SN 2021adxl has faded by only ~4 magnitudes in the r band with a cumulative radiated energy of ~1.5 × 1050 erg over 18 months. SN 2021adxl shows strong signs of interaction with a complex circumstellar medium, seen by the detection of X-rays, revealed by the detection of coronal emission lines, and through multicomponent hydrogen and helium profiles. In order to further understand this interaction, we model the Hα profile using a Monte Carlo electron scattering code. The blueshifted high-velocity component is consistent with emission from a radially thin spherical shell resulting in the broad emission components due to electron scattering. Using the velocity evolution of this emitting shell, we find that the SN ejecta collide with circumstellar material of at least [Formula Presented] assuming a steady-state mass-loss rate of ~4−6 × 10−3 M yr−1 for the first ~200 days of evolution. SN 2021adxl was last observed to be slowly declining at ~0.01 mag d−1, and if this trend continues, SN 2021adxl will remain observable after its current solar conjunction. Continuing the observations of SN 2021adxl may reveal signatures of dust formation or an infrared excess, similar to that seen for SN 2010jl.

We present SN 2023zaw—a subluminous (M_r = −16.7 mag) and rapidly evolving supernova (t_1/2,r = 4.9 days), with the lowest nickel mass (≈0.002 M_⊙) measured among all stripped-envelope supernovae discovered to date. The photospheric spectra are dominated by broad He i and Ca near-infrared emission lines with velocities of ∼10,000−12,000 km s⁻¹. The late-time spectra show prominent narrow He i emission lines at ∼1000 km s⁻¹, indicative of interaction with He-rich circumstellar material. SN 2023zaw is located in the spiral arm of a star-forming galaxy. We perform radiation-hydrodynamical and analytical modeling of the lightcurve by fitting with a combination of shock-cooling emission and nickel decay. The progenitor has a best-fit envelope mass of ≈0.2 M_☉ and an envelope radius of ≈50 R_⊙. The extremely low nickel mass and low ejecta mass (≈0.5 M_⊙) suggest an ultrastripped SN, which originates from a mass-losing low-mass He-star (zero-age main-sequence mass < 10 M_⊙) in a close binary system. This is a channel to form double neutron star systems, whose merger is detectable with LIGO. SN 2023zaw underscores the existence of a previously undiscovered population of extremely low nickel mass (<0.005 M_☉) stripped-envelope supernovae, which can be explored with deep and high-cadence transient surveys.

SN 2020zbf is a hydrogen-poor superluminous supernova (SLSN) at z = 0.1947 that shows conspicuous C II features at early times, in contrast to the majority of H-poor SLSNe. Its peak magnitude is M_g = −21.2 mag and its rise time (≲26.4 days from first light) places SN 2020zbf among the fastest rising type I SLSNe. We used spectra taken from ultraviolet (UV) to near-infrared wavelengths to identify spectral features. We paid particular attention to the C II lines as they present distinctive characteristics when compared to other events. We also analyzed UV and optical photometric data and modeled the light curves considering three different powering mechanisms: radioactive decay of ⁵⁶Ni, magnetar spin-down, and circumstellar medium (CSM) interaction. The spectra of SN 2020zbf match the model spectra of a C-rich low-mass magnetar-powered supernova model well. This is consistent with our light curve modeling, which supports a magnetar-powered event with an ejecta mass M_ej = 1.5 M_⊙. However, we cannot discard the CSM-interaction model as it may also reproduce the observed features. The interaction with H-poor, carbon-oxygen CSM near peak light could explain the presence of C II emission lines. A short plateau in the light curve around 35–45 days after peak, in combination with the presence of an emission line at 6580 Å, can also be interpreted as being due to a late interaction with an extended H-rich CSM. Both the magnetar and CSM-interaction models of SN 2020zbf indicate that the progenitor mass at the time of explosion is between 2 and 5 M_⊙. Modeling the spectral energy distribution of the host galaxy reveals a host mass of 10^8.7 M_⊙, a star formation rate of 0.24_−0.12^+0.41 M_⊙ yr⁻¹, and a metallicity of ∼0.4 Z_⊙.