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We present the analysis of the luminous Type II Supernova (SN) 2021tsz, which exploded in a low-luminosity galaxy. It reached a peak magnitude of −18.88 ± 0.13 mag in therband and exhibited an initial rapid decline of 4.05 ± 0.14 mag (100 d)⁻¹from peak luminosity till ∼30 d. The photospheric phase is short, with the SN displaying bluer colours and a weak Hαabsorption component–features consistent with other luminous, short-photospheric phase Type II SNe. A distinct transition from the photospheric to the radioactive tail phase in theVband–as is common in hydrogen-rich Type II SNe–is not visible in SN 2021tsz, although a modest ∼1 mag drop is apparent in the redder filters. Hydrodynamic modelling suggests the luminosity is powered by ejecta-circumstellar material (CSM) interaction during the early phases (< 30 days). Interaction with 0.6 M_⊙of dense CSM extending to 3100 R_⊙reproduces the observed luminosity, with an explosion energy of 1.3 × 10⁵¹erg. The modelling indicates a pre-SN mass of 9 M_⊙, which includes a hydrogen envelope of 4 M_⊙, and a radius of ∼1000 R_⊙. Spectral energy distribution analysis and strong-line diagnostics revealed that the host galaxy of SN 2021tsz is a low-metallicity, dwarf galaxy. The low-metallicity environment and the derived high mass loss from the hydrodynamical modelling strongly support a binary progenitor system for SN 2021tsz. 

We present optical photometric and spectroscopic observations of the peculiar Type Ia supernovae (SNe Ia) ASASSN-20jq/SN 2020qxp. It is a low-luminosity object, with a peak absolute magnitude ofM_B = −17.1 ± 0.5 mag, while its post-peak light-curve decline rate of Δm₁₅(B) = 1.35 ± 0.09 mag and color-stretch parameter ofs_BV ⪆ 0.82 is similar to that of normal luminosity SNe Ia. That makes it a prevalent outlier in both the SN Ia luminosity-width and the luminosity-color-stretch relations. The analysis of the early light curves indicates a possible “bump” during the first ≈1.4 days of explosion. ASASSN-20jq synthesized a low radioactive⁵⁶Ni mass of 0.09 ± 0.01 M_⊙. The near-maximum light spectra of the supernova show strong Si IIabsorption lines, indicating a cooler photosphere than normal SNe Ia; however, it lacks Ti IIabsorption lines. Additionally, it shows unusually strong absorption features of O Iλ7773 and the Ca IInear-infrared triplet. The nebular spectra of ASASSN-20jq show a remarkably strong but narrow forbidden [Ca II]λλ7291, 7324 doublet emission that has not been seen in SNe Ia except for a handful of Type Iax events. There is also a marginal detection of the [O I]λλ6300, 6364 doublet emission in nebular spectra, which is extremely rare. Both the [Ca II] and [O I] lines are redshifted by roughly 2000 km s⁻¹. ASASSN-20jq also exhibits a strong [Fe II]λ7155 emission line with a tilted-top line profile, which is identical to the [Fe II]λ16433 line profile. The asymmetric [Fe II] line profiles, along with the redshifted [Ca II] and emission lines, suggest a high central density white dwarf progenitor that underwent an off-center delayed-detonation explosion mechanism, synthesizing roughly equal amounts of⁵⁶Ni during the deflagration and detonation burning phases. The equal production of⁵⁶Ni in both burning phases distinguishes ASASSN-20jq from normal bright and subluminous SNe Ia. Assuming this scenario, we simultaneously modeled the optical and near-infrared nebular spectra, achieving a good agreement with the observations. The light curve and spectroscopic features of ASASSN-20jq do not align with any single sub-class of SNe Ia. However, the significant deviation from the luminosity versus light-curve shape relations (along with several light-curve and spectroscopic features) exhibits similarities to some 2002es-like objects. Therefore, we have identified ASASSN-20jq as an extreme candidate within the broad and heterogeneous parameter space of 2002es-like SNe Ia. 

We discuss the results of the spectroscopic and photometric monitoring of the type IIn supernova (SN) 2023ldh. Survey archive data show that the SN progenitor experienced erratic variability in the years before exploding. Beginning May 2023, the source showed a general slow luminosity rise that lasted for over four months, with some superposed luminosity fluctuations. In analogy toSN 2009ip, we call this brightening ‘Event A’. During Event A,SN 2023ldhreached a maximum absolute magnitude ofM_r = −15.52 ± 0.24 mag. The light curves then decreased by about 1 mag in all filters for about two weeks reaching a relative minimum, which was followed by a steep brightening (Event B) to an absolute peak magnitude ofM_r = −18.53 ± 0.23 mag, replicating the evolution ofSN 2009ipand similar to that of type IIn SNe. The three spectra ofSN 2023ldhobtained during Event A show multi-component P Cygni profiles of H I and Fe II lines. During the rise to the Event B peak, the spectrum shows a blue continuum dominated by Balmer lines in emission with Lorentzian profiles, with a full width at half maximum velocity of about 650 km s⁻¹. Later, in the post-peak phase, the spectrum reddens, and broader wings appear in the Hαline profile. Metal lines with P Cygni profiles and velocities of about 2000 km s⁻¹are clearly visible. Beginning around three months past maximum and until very late phases, the Ca II lines become among the most prominent features, while Hαis dominated by an intermediate-width component with a boxy profile. AlthoughSN 2023ldhmimics the evolution of otherSN 2009ip-like transients, it is slightly more luminous and has a slower photometric evolution. The surprisingly homogeneous observational properties ofSN 2009ip-like events may indicate similar explosion scenarios and similar progenitor parameters. 

We present the photometric and spectroscopic analysis of five Type Ibn supernovae (SNe): SN 2020nxt, SN 2020taz, SN 2021bbv, SN 2023utc, and SN 2024aej. These events share key observational features and belong to a family of objects similar to the prototypical Type Ibn SN 2006jc. The SNe exhibit rise times of approximately 10 days and peak absolute magnitudes ranging from −16.5 to −19 mag. Notably, SN 2023utc is the faintest Type Ibn SN discovered to date, with an exceptionally lowr-band absolute magnitude of −16.4 mag. The pseudo-bolometric light curves peak at (1 − 10)×10⁴²erg s⁻¹, with total radiated energies on the order of (1 − 10)×10⁴⁸erg. Spectroscopically, these SNe display a relatively slow spectral evolution. The early spectra are characterised by a hot blue continuum and prominent He Iemission lines. The early spectra also show blackbody temperatures exceeding 10 000 K, with a subsequent decline in temperature during later phases. Narrow He Ilines, which are indicative of unshocked circumstellar material (CSM), show velocities of approximately 1000 km s⁻¹. The spectra suggest that the progenitors of these SNe underwent significant mass loss prior to the explosion, resulting in a He-rich CSM. Our light curve modelling yielded estimates for the ejecta mass (M_ej) in the range 1 − 3 M_⊙with kinetic energies (E_Kin) of (0.1 − 1)×10⁵⁰erg. The inferred CSM mass ranges from 0.2 to 1 M_⊙. These findings are consistent with expectations for core collapse events arising from relatively massive envelope-stripped progenitors. 

SN 2020zbf is a hydrogen-poor superluminous supernova (SLSN) atz = 0.1947 that shows conspicuous C IIfeatures at early times, in contrast to the majority of H-poor SLSNe. Its peak magnitude isM_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 IIlines 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 of⁵⁶Ni, 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 massM_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 IIemission 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 5M_⊙. Modeling the spectral energy distribution of the host galaxy reveals a host mass of 10^8.7M_⊙, a star formation rate of 0.24_−0.12^+0.41M_⊙yr⁻¹, and a metallicity of ∼0.4Z_⊙. 

We present photometric and spectroscopic observations of SN 2020xga and SN 2022xgc, two hydrogen-poor superluminous supernovae (SLSNe-I) atz = 0.4296 andz = 0.3103, respectively, which show an additional set of broad Mg IIabsorption lines, blueshifted by a few thousands kilometer second⁻¹with 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-frameg-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 IIline profiles and infer the properties of the CSM shells. We find that the CSM shell of SN 2020xga resides at ∼1.3 × 10¹⁶cm, moving with a maximum velocity of 4275 km s⁻¹, and the shell of SN 2022xgc is located at ∼0.8 × 10¹⁶cm, 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 − 9 M_⊙. The ejecta masses inferred from the magnetar modeling are not consistent with the PPI scenario pointing toward stars > 50 M_⊙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 10^7.8M_⊙, a star formation rate of 0.96_−0.26^+0.47M_⊙yr⁻¹, and a metallicity of ∼0.2 Z_⊙. 

ABSTRACT Hydrogen-rich Type II supernovae (SNe II) are the most frequently observed class of core-collapse SNe (CCSNe). However, most studies that analyse large samples of SNe II lack events with absolute peak magnitudes brighter than −18.5 mag at rest-frame optical wavelengths. Thanks to modern surveys, the detected number of such luminous SNe II (LSNe II) is growing. There exist several mechanisms that could produce luminous SNe II. The most popular propose either the presence of a central engine (a magnetar gradually spinning down or a black hole accreting fallback material) or the interaction of supernova ejecta with circumstellar material (CSM) that turns kinetic energy into radiation energy. In this work, we study the light curves and spectral series of a small sample of six LSNe II that show peculiarities in their H α profile, to attempt to understand the underlying powering mechanism. We favour an interaction scenario with CSM that is not dense enough to be optically thick to electron scattering on large scales – thus, no narrow emission lines are observed. This conclusion is based on the observed light curve (higher luminosity, fast decline, blue colours) and spectral features (lack of persistent narrow lines, broad H α emission, lack of H α absorption, weak, or non-existent metal lines) together with comparison to other luminous events available in the literature. We add to the growing evidence that transients powered by ejecta–CSM interaction do not necessarily display persistent narrow emission lines. 

Type II supernovae (SNe II) show great photometric and spectroscopic diversity which is attributed to the varied physical characteristics of their progenitor and explosion properties. In this study, the third of a series of papers where we analyse a large sample of SNe II observed by the Carnegie Supernova Project-I, we present correlations between their observed and physical properties. Our analysis shows that explosion energy is the physical property that correlates with the highest number of parameters. We recover previously suggested relationships between the hydrogen-rich envelope mass and the plateau duration, and find that more luminous SNe II with higher expansion velocities, faster declining light curves, and higher 56 Ni masses are consistent with higher energy explosions. In addition, faster declining SNe II (usually called SNe IIL) are also compatible with more concentrated 56 Ni in the inner regions of the ejecta. Positive trends are found between the initial mass, explosion energy, and 56 Ni mass. While the explosion energy spans the full range explored with our models, the initial mass generally arises from a relatively narrow range. Observable properties were measured from our grid of bolometric LC and photospheric velocity models to determine the effect of each physical parameter on the observed SN II diversity. We argue that explosion energy is the physical parameter causing the greatest impact on SN II diversity, that is, assuming the non-rotating solar-metallicity single-star evolution as in the models used in this study. The inclusion of pre-SN models assuming higher mass loss produces a significant increase in the strength of some correlations, particularly those between the progenitor hydrogen-rich envelope mass and the plateau and optically thick phase durations. These differences clearly show the impact of having different treatments of stellar evolution, implying that changes in the assumption of standard single-star evolution are necessary for a complete understanding of SN II diversity. 

Aims. The modelling of spectroscopic observations of tidal disruption events (TDEs) to date suggests that the newly formed accretion disks are mostly quasi-circular. In this work we study the transient event AT 2020zso, hosted by an active galactic nucleus (AGN; as inferred from narrow emission line diagnostics), with the aim of characterising the properties of its newly formed accretion flow. Methods. We classify AT 2020zso as a TDE based on the blackbody evolution inferred from UV/optical photometric observations and spectral line content and evolution. We identify transient, double-peaked Bowen (N  III ), He  I , He  II, and H α emission lines. We model medium-resolution optical spectroscopy of the He  II (after careful de-blending of the N  III contribution) and H α lines during the rise, peak, and early decline of the light curve using relativistic, elliptical accretion disk models. Results. We find that the spectral evolution before the peak can be explained by optical depth effects consistent with an outflowing, optically thick Eddington envelope. Around the peak, the envelope reaches its maximum extent (approximately 10 15 cm, or ∼3000–6000 gravitational radii for an inferred black hole mass of 5−10 × 10 5 M ⊙ ) and becomes optically thin. The H α and He  II emission lines at and after the peak can be reproduced with a highly inclined ( i  = 85 ± 5 degrees), highly elliptical ( e  = 0.97 ± 0.01), and relatively compact ( R in = several 100 R g and R out = several 1000 R g ) accretion disk. Conclusions. Overall, the line profiles suggest a highly elliptical geometry for the new accretion flow, consistent with theoretical expectations of newly formed TDE disks. We quantitatively confirm, for the first time, the high inclination nature of a Bowen (and X-ray dim) TDE, consistent with the unification picture of TDEs, where the inclination largely determines the observational appearance. Rapid line profile variations rule out the binary supermassive black hole hypothesis as the origin of the eccentricity; these results thus provide a direct link between a TDE in an AGN and the eccentric accretion disk. We illustrate for the first time how optical spectroscopy can be used to constrain the black hole spin, through (the lack of) disk precession signatures (changes in inferred inclination). We constrain the disk alignment timescale to > 15 days in AT2020zso, which rules out high black hole spin values ( a  < 0.8) for M BH  ∼ 10 6 M ⊙ and disk viscosity α  ≳ 0.1. 

ABSTRACT We present an in-depth study of the late-time near-infrared plateau in Type Ia supernovae (SNe Ia), which occurs between 70 and 500 d. We double the existing sample of SNe Ia observed during the late-time near-infrared plateau with new observations taken with the Hubble Space Telescope, Gemini, New Technology Telescope, the 3.5-m Calar Alto Telescope, and the Nordic Optical Telescope. Our sample consists of 24 nearby SNe Ia at redshift < 0.025. We are able to confirm that no plateau exists in the Ks band for most normal SNe Ia. SNe Ia with broader optical light curves at peak tend to have a higher average brightness on the plateau in J and H, most likely due to a shallower decline in the preceding 100 d. SNe Ia that are more luminous at peak also show a steeper decline during the plateau phase in H. We compare our data to state-of-the-art radiative transfer models of nebular SNe Ia in the near-infrared. We find good agreement with the sub-Mch model that has reduced non-thermal ionization rates, but no physical justification for reducing these rates has yet been proposed. An analysis of the spectral evolution during the plateau demonstrates that the ratio of [Fe ii] to [Fe iii] contribution in a near-infrared filter determines the light curve evolution in said filter. We find that overluminous SNe decline slower during the plateau than expected from the trend seen for normal SNe Ia.