The young supernova remnant SNR 0540-69.3 in the Large Magellanic Cloud offers a detailed view of an energetic pulsar-wind nebula (PWN) interacting with the surrounding ejecta. We present infrared observations of the central region of SNR 0540-69.3 obtained with the JWST NIRSpec and Medium-Resolution Spectrometer integral field units. From the observations we reconstruct the 3D morphology of the strongest emission lines in the inner ejecta (≲1000 km s⁻¹), which reveal the distributions of H I, He I, [Ne ii], [Ne iii], [S iii], [S iv], [Fe ii], and [Ni II]. The 3D morphology of most lines is dominated by two highly fragmented lobes of approximately similar size. Based on the assumption that the lobes are symmetric around the pulsar, we infer a pulsar kick velocity of ∼300 km s⁻¹ away from the observer. There are differences in the 3D morphologies of individual emission lines due to a combination of varying physical conditions and abundances. The detection of H i 1.8756 μm in the inner ejecta confirms the classification of the supernova as a Type II and shows that hydrogen was mixed down to low velocities of <400 km s⁻¹ in the explosion. We compare the results to the Crab Nebula and conclude that asymmetries originating in the explosion most likely play a major role in shaping these PWNe.

We present imaging and spectroscopic observations of supernova SN 2025wny, associated with the lens candidate PS1 J0716+3821. Photometric monitoring from the Lulin and Maidanak observatories confirms multiple point-like images, consistent with SN 2025wny being strongly lensed by two foreground galaxies. Optical spectroscopy of the brightest image with the Nordic Optical Telescope and the University of Hawaii 88-inch Telescope allowed us to determine the redshift to be z_SN = 2.008 ± 0.001, based on narrow absorption lines originating in the interstellar medium of the supernova host galaxy. At this redshift, SN 2025wny shows a very high rest-frame UV flux and broad spectral features even weeks after the explosion, which is consistent with superluminous supernovae of Type I. We find a high ejecta temperature and depressed spectral lines compared to other similar objects. We also measured, for the first time, the redshift of the fainter of the two lens galaxies (the ‘perturber’) to be z_p = 0.375 ± 0.001, which is fully consistent with the DESI spectroscopic redshift of the main deflector at z_d = 0.3754. Thus, SN 2025wny represents the first confirmed galaxy-scale strongly lensed supernova with time delays likely in the range of days to weeks, as judged from the image separations. This makes SN 2025wny suitable for cosmography, offering a promising new system for independent measurements of the Hubble constant. Following a tradition in the field of strongly lensed supernovae, we give SN 2025wny the nickname SN Winny.

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)×10⁴² erg s⁻¹, with a total radiated energy of (5.0 ± 0.4)×10⁴⁸ 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 Hα 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 × 10⁵¹ erg, and a ⁵⁶Ni 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 R₀=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.

We present LOw Frequency ARray (LOFAR) studies of supernovae SN 1979C, SN 1986J, and SN 2006X, focusing on new observations from the LOFAR Two-metre Sky Survey (LoTSS) and the International LOFAR Telescope (ILT). For Type Ia supernovae (SNe Ia) SN 2006X, we derive a 3σ upper limit of 0.7 mJy at 0.146 GHz, and using radio emission models based on the CS15DD2 explosion model, we constrain the circumstellar density to n_H ≲ 10 cm⁻³ for the microphysical parameters _rel = _B = 0.01. SN 1979C is clearly detected in the LoTSS image with a flux density of 4.6 ± 0.36 mJy nearly 40 yr postexplosion. Modeling its radio evolution suggests a steep flux decay (F_ν ∝ t^−2.1) between 22 and 42 yr, a break in the spectrum near 1.5 GHz possibly due to synchrotron cooling, a progenitor mass of ∼13 M_⊙, and a progressive steepening with velocity for the density slope of the supernova ejecta. Our findings for SN 1979C contradict scenarios involving central compact object emission, and we obtain X-ray temperatures close to those derived from recent observations. For SN 1986J, we present the first ILT image showing a flux density of 6.77 ± 0.2 mJy at 0.146 GHz. The spectral index of the shell emission is found to be 0.66 ± 0.03, consistent with previous estimates, although variations at low frequencies warrant further investigation. Our results highlight the power of LOFAR for studying long-term radio evolution in supernovae.

We present a photometric and spectroscopic study of the superluminous Type Ia supernova SN 2022erq. Its early spectra, dominated by iron-group elements with weak intermediate-mass features, might indicate highly efficient nuclear burning, broadly similar to that inferred for some overluminous Type Ia supernovae. The rapid emergence and persistence of narrow Balmer emission lines superposed on this iron-rich spectrum provide clear evidence of long-lived interaction with a hydrogen-rich circumstellar medium (CSM), establishing SN 2022erq as a member of the rare Ia–CSM class. SN 2022erq reached a peak bolometric luminosity of ∼8 × 1043 erg s−1 and exhibited an exceptionally slow postpeak decline, indicating that its light curve is dominated by long duration ejecta–CSM interaction. By combining Hα diagnostics with bolometric light-curve modeling, we reconstruct the preexplosion mass-loss history of the progenitor. The mass-loss rate escalated by 1 order of magnitude over the final decades, rising from ∼0.04 to ∼0.6 M⊙ yr−1. This surge produced a massive, extended CSM shell of ∼3 M⊙ out to ∼3.5 × 1016 cm. The young stellar environment (∼100 Myr) together with this substantial, extensive CSM points to a progenitor system consisting of a white dwarf and an intermediate-mass companion that underwent increasing mass loss prior to explosion.

We present our observations and analysis of SN 2023gpw, a hydrogen-rich superluminous supernova (SLSN II) with broad emission lines in its post-peak spectra. Unlike previously observed SLSNe II, its light curve suggests an abrupt drop during a solar conjunction between ∼80 and ∼180 d after the light curve peak, which is possibly analogous to a normal hydrogen-rich supernova (SN). Spectra taken at and before the peak show hydrogen and helium “flash” emission lines attributed to early interaction with a dense confined circumstellar medium (CSM). A well-observed ultraviolet excess appears as these lines disappear, also as a result of CSM interaction. The blackbody photosphere expands roughly at the same velocity throughout the observations, indicating little or no bulk deceleration. This velocity is much higher than what is seen in spectral lines, suggesting asymmetry in the ejecta. The high total radiated energy (≳9 × 10⁵⁰ erg) and aforementioned lack of bulk deceleration in SN 2023gpw are difficult to reconcile with a neutrino-driven SN simply combined with efficient conversion from kinetic energy to emission through interaction. This suggests an additional energy source, such as a central engine. While magnetar-powered models qualitatively similar to SN 2023gpw exist, more modeling work is required to determine if they can reproduce the observed properties in combination with early interaction. The required energy might alternatively be provided by accretion onto a black hole created in the collapse of a massive progenitor star.

Supernovae (SNe) IIn are terminal explosions of massive stars that are surrounded by a dense circumstellar medium (CSM). Among SNe IIn, a notable subset is the SN 2009ip-like, which exhibits an initial, fainter peak attributed to stellar variability in the late evolutionary stages, followed by a brighter peak, interpreted as the SN explosion itself. In this context, we analysed the spectrophotometric evolution of SN 2024hpj, an object with a triple-peaked light curve and spectra typical of a SN IIn but with a complex line profile composed of broad P-Cygni features topped by narrow emissions. Comparing it with other SN 2009ip-like events in the literature, as well as with other unpublished objects (SNe 2019mry, 2022ytx, 2024uzf, and 2025csc), we identify star-forming regions as their preferred formation environment. On the other hand, the diversity of spectrophotometric features within the sample suggests that variations in CSM mass and distribution may influence the observed characteristics. We identify four sub-classes based on the luminosity and rapidity of the light curve evolution, which provides insights into possible differences in the progenitors, while a statistical analysis of their observed rate indicates progenitor masses around 25 − 31 M_⊙ or lower.

We present infant-phase observations of the broad-line Type Ic supernova (SN Ic-BL) 2020lao, including optical spectroscopy beginning within about 48 hours of the inferred explosion epoch and extending to nearly 100 days. The explosion time was constrained by power-law fits to the rising TESS and ZTF light curves, with the first ZTF detection occurring only ∼27 hours after explosion. The optical light curves show a rapid rise that lasted for ≈8.8 days and a peak luminosity typical of SNe Ic-BL (i.e., M_r ≃ −18.5 mag). Unlike some engine-driven SN Ic-BL events, the early light curve of SN 2020lao shows no evidence of an optical afterglow or excess emission, and the absence of any detectable shock–cooling component in the TESS and ZTF data constrains the progenitor to a compact Wolf-Rayet-like star whose R_★ is less than or equal to a few times the R_⊙, ruling out any extended envelope. The spectra resemble those of the X-ray-flash-associated SN 2006aj but with systematically higher expansion velocities. From Arnett-type fits to the bolometric light curve and measured Fe IIλ5169 line velocities, we infer a ⁵⁶Ni mass of 0.23 ± 0.03 M_⊙, an ejecta mass (M_ej) of 3.2 ± 0.8 M_⊙, and a kinetic energy (E_K) of ∼(23.1 ± 12.4)×10⁵¹ erg, corresponding to a specific kinetic energy (E_K/M_ej) of ≈(7.2 ± 3.5)×10⁵¹ erg M_⊙⁻¹. Spectral synthesis modeling broadly reproduces the photospheric-phase spectra of SN 2020lao and suggests E_K/M_ej ≈ 4.9 × 10⁵¹ erg M_⊙⁻¹. SN 2020lao and SN 2006aj synthesized comparable amounts of ⁵⁶Ni, yet SN 2020lao exhibits E_K/M_ej values on the order of 5–10 times larger. Published VLA and Swift/XRT non-detections reveal no afterglow emission, allowing us to place stringent limits on relativistic ejecta and dense circumstellar material. Given that SN 2020lao reaches a specific kinetic energy typical of engine-driven SNe Ic-BL, the lack of an early optical excess together with the non-detections in the radio and X-ray bands suggests that if a relativistic jet was launched, the explosion must have been viewed far off axis or the jet was choked before breakout. If there was no relativistic jet, SN 2020lao would therefore be an extreme nonrelativistic SN Ic-BL. This underscores the importance of continued infant-phase, multiwavelength monitoring of these explosions.

Context. Stripped-envelope supernovae (SESNe), including Type IIb, Ib, and Ic supernovae (SNe), originate from the explosions of massive stars whose outer envelopes have been largely removed during their lifetimes. The main stripping mechanism for the hydrogen (H) envelope in the progenitors of SESNe is often considered to be interaction with a binary companion, but the stripping mechanism for the helium (He) layer is unclear.

Aims. We study the process of the He-layer stripping in the progenitors of SESNe. This is closely related to the origin of their diverse observational properties.

Methods. We conducted photometric and spectroscopic observations of the Type Ib SN 2021efd, which shows signs of interaction with H-free circumstellar material (CSM). At early phases, its photometric and spectroscopic properties resemble those of typical Type Ib SNe. Around 30 days after the r-band light curve (LC) peak until at least ∼770 days, the luminosity of the multiband LCs is higher than that of regular SESNe and has at least three distinct peaks. The LC evolution is similar to that of SN 2019tsf, whose previously unpublished spectrum at 400 days is also presented here. The nebular spectrum of SN 2021efd shows narrow emission lines (∼1000 km s⁻¹) in various species, such as O I, Ca II, Mg II, He I, [O I], [Ca II], and [S II]. Based on the observations, we studied the properties of the ejecta and CSM of SN 2021efd.

Results. Our observations suggest that SN 2021efd is a Type Ib SN that interacts with the CSM with the following parameters: The estimated ejecta mass, explosion energy, and ⁵⁶Ni mass are 2.2 M_⊙, 9.1 × 10⁵⁰ erg, and 0.14 M_⊙, respectively, and the estimated CSM mass, composition, and distribution are at least a few times 0.1 M_⊙, H free, and clumpy, respectively. Based on the estimated ejecta properties, we conclude that this event is a transitional SN whose progenitor was experiencing He-layer stripping at the epoch of the explosion and was on the way to becoming a carbon-oxygen star (as the progenitors of Type Ic SNe) from a He star (as the progenitors of Type Ib SNe). The estimated CSM properties suggest that the progenitor had some episodic mass ejections at a rate of ∼5 × 10⁻³ − 10⁻² M_⊙ yr⁻¹ for the last decade and slightly lower before this final phase at least from ∼200 years before the explosion for the assumed CSM velocity of 100 km s⁻¹. For the case of ∼1000 km s⁻¹, the necessary mass-loss rate would be higher by a factor of ten, and the timescales would be shorter by a factor of ten.

Type Ia-CSM supernovae (SNe) are a rare and peculiar subclass of thermonuclear SNe characterized by emission lines of hydrogen or helium, indicative of high-density circumstellar medium (CSM). Their implied mass-loss rates of ∼10⁻⁴–10⁻¹ M_⊙ yr⁻¹ (assuming ∼100 km s⁻¹ winds) from optical observations are generally in excess of values observed in realistic SN Ia progenitors. In this paper, we present an independent study of CSM densities around a sample of 29 archival Ia-CSM SNe using radio observations with the Karl G. Jansky Very Large Array at 6 GHz. Motivated by the late (∼2 yr) radio detection of the Ia-CSM SN 2020eyj, we observed old (>1 yr) SNe, when we are more likely to see the emergent synchrotron emission that may have been suppressed earlier by free–free absorption by the CSM. We do not detect radio emission down to 3σ limits of ∼35 μJy in our sample. The only radio-detected candidate in our sample, SN 2022esa, was likely misclassified as a Ia-CSM with early spectra, and appears more consistent with a peculiar Ic based on later epochs. Assuming wind-like CSM with temperatures between 2 × 10⁴ K and 10⁵ K, and a magnetic field-to-shock energy fraction _B = 0.01 − 0.1, the radio upper limits rule out mass-loss rates between ∼10⁻⁴ and 10⁻² M_⊙ yr⁻¹ (100 km s⁻¹)⁻¹. This is somewhat in tension with the estimates from optical observations, and may indicate that more complex CSM geometries and/or lower values of _B may be present.