Seppo Mattila

Hydrogen-rich superluminous supernovae without narrow emission lines (SLSNe-II) are rare transients whose powering mechanisms remain debated, particularly the role of circumstellar medium (CSM) interaction. We present photometric and spectroscopic observations of SN 2019cqc to investigate its power source and progenitor mass loss. We model the lightcurve using MOSFiT with the csm and csmni models to constrain the explosion and CSM parameters. SN 2019cqc peaked at M _g = −20.21 ± 0.07 and emitted ∼1.7 × 10 ⁵⁰ erg of energy in the form of radiation. Photometric and spectroscopic modeling indicates that CSM interaction dominates in both scenarios, with best-fit CSM and ejecta masses of ∼4.5 M _⊙ and ∼32 M _⊙ , respectively. The inferred CSM properties imply an extreme eruptive mass loss at a rate of ∼0.2–0.3 M _⊙ , yr ⁻¹ in the years preceding the core collapse, consistent with a variable progenitor of luminous blue variable progenitor. We observe a persistent blueshifted asymmetry in the H α emission line. At early times (≲+110 days), this is attributed to electron scattering with the bulk velocity of ejecta, while the profile at late epochs (≳ +402 days onward) suggests the subsequent formation of dust in the ejecta. Additionally, we identify a distinct, short-lived feature at ∼4600 Å, likely a blend of ionized C iii /N iii lines powered by the interaction of the SN-shock with the extended atmosphere.

The discovery of stripped-envelope core-collapse supernovae (SNe) interacting with dense circumstellar medium (CSM) challenges our current understanding of massive star evolution. We present late-time observations of the interacting Type Ic SN 2017dio and investigate its mass-loss mechanism and progenitor channel. We analysed late-time spectra and light curves that are dominated by a strong ejecta–CSM interaction and reveal a slow photometric evolution primarily powered by this interaction. We examined the CSM and the source of the infrared (IR) excess by modelling the radiation produced by the ejecta–CSM interaction and the IR echo from circumstellar dust. In addition, we studied the temporal evolution of spectral features, with a particular emphasis on the Hα emission line. From the combined analysis of the light curves and spectral properties, we infer that the peak mass-loss rate for the CSM reaches sim0.2 mathrm M_⊙ yr^ -1 and that the typical value over most epochs is sim0.06 mathrm M_⊙ yr^ -1 . The nearby CSM was formed over a period of 4 to 65 years before the explosion. The CSM radius begins at ∼ 1.3 cm. The IR excess identified in the light curves is consistent with the radiation from dust with a mass increasing from sim0.001 to sim0.005 in the case of carbon dust or sim0.005 to sim0.02 in the case of silicate dust. 15 M_⊙ M_⊙ From IR echo modelling, we estimate an upper limit on the dust mass of 4 -5 M_⊙ , which implies an SN progenitor mass-loss rate of 2.4 -5 M_⊙ yr^ -1 at the dust evaporation radius determined by the SN peak luminosity (0.017 pc for carbon dust, corresponding to mass loss sim170 years before the explosion). This implies a very rapid increase in the mass-loss rate ahead of the explosion. Although the progenitor of SN 2017dio has lost its helium envelope, it interacted with a hydrogen-rich CSM formed shortly before the explosion, strongly suggesting that this material originated from a companion star rather than the progenitor itself.

We present optical photometric and spectroscopic observations of the Type IIb supernova (SN) 2017ati. It reached maximum light about 27 d after explosion, and the light curve shows a broad, luminous peak with an absolute r -band magnitude of M _r = −18.48 ± 0.16 mag. At about 50 d after maximum light, SN 2017ati exhibited a decline rate close to that expected from the ⁵⁶ Co → ⁵⁶ Fe radioactive decay, at 0.98 mag per 100 days, as is usually observed in SNe IIb. However, it remains systematically brighter at late times by about 1–2 mag, exceeding the usual upper luminosity range of this class. As a result, modelling the light curve of SN 2017ati with a standard ⁵⁶ Ni decay scenario requires a large nickel mass of up to ∼0.37 M _⊙ and still fails to reproduce the early-time light curve adequately. In contrast, incorporating additional energy input from a magnetar yields a significantly improved fit to the light curve of SN 2017ati, which would reduce the nickel mass to ∼0.21 M _⊙ , which is still close to the upper end of the range typically inferred for SNe IIb. Comparing the fitted results of SN 2017ati with the known sample of SNe IIb indicates that its luminosity evolution is best explained by a combination of neutron star spin-down energy and radioactive nickel deposition. From late-time nebular spectra of SN 2017ati, the luminosity of the [O I ] λλ 6300, 6364 doublet implies an oxygen mass of ∼1.82 − 3.34 M _⊙ , and the combination of a [Ca II ]/[O I ] flux ratio of ∼0.5 with nebular spectral model comparisons favours a progenitor zero-age main-sequence mass of ≥17 M _⊙ .

Context. Core-collapse supernovae (CCSNe) in luminous infrared galaxies (LIRGs) can have extreme line-of-sight host galaxy dust extinctions, which leads to a large fraction of the events remaining undetected by optical and infrared surveys. This population of undetected CCSNe is important to constrain in order to determine the cosmic CCSN rates, which can be used to estimate the cosmic star formation history independently from methods based on galaxy luminosities. Aims. Our aim is to confirm and refine our estimates for the undetectable fraction of CCSNe in LIRGs in the local Universe. Our study is based on the near-infrared K -band multi-epoch SUNBIRD survey monitoring dataset of a sample of nine LIRGs using the Gemini-South telescope with the multi-conjugate GSAOI/GeMS laser guide star adaptive optics system. Methods. We determined the limiting magnitudes for CCSN detection for each epoch in our dataset with artificial supernova injection and image subtraction methods. Subsequently, we used a Monte Carlo method to determine the combined effects of limiting magnitudes, survey cadence, CCSN subtype distribution, and their light curve evolution diversity. The intrinsic CCSN rates of the sample galaxies were estimated based on detailed modelling of their spectral energy distribution. Finally, we combined the resulting CCSN detection probabilities with the intrinsic CCSN rates for the dataset, and compared that against the real CCSN detections over the survey period. Results. Based on our GSAOI/GeMS dataset, assuming optical or near-infrared example surveys with capabilities to detect CCSNe in local LIRGs with host extinctions of A _V = 3 or 16 mag, respectively, the resulting total undetectable fractions are 86.0 ^+4.7 _−5.9 % and 53.6 ^+15.6 _−19.6 %. When folding in the results from our previous near-infrared adaptive optics assisted LIRG monitoring dataset, the corresponding total undetectable fractions are 88.3 ^+2.6 _−3.2 % and 61.4 ^+8.5 _−10.6 %, respectively.

Observations of core-collapse supernovae (CCSNe) suggest that some massive stars undergo intense mass loss shortly before explosion, but the underlying mechanisms remain unknown. Here, we report evidence of giant outbursts of clumpy material from a massive star in the final decades before explosion. Photometric, spectroscopic, and polarimetric data of SN 2024qiw reveal a bumpy light curve, a broad H α profile, and variable polarization, all consistent with interactions taking place between SN ejecta and clumpy circumstellar material, implying a mass-loss rate of ≳10 ⁻² M _⊙ yr ⁻¹ . Taken together, the most likely explanation is multiple major eruptions, similar to those of luminous blue variables (LBVs), but occurring shortly before explosion. This challenges standard stellar evolution theory by requiring either that LBVs explode terminally or that other evolutionary phases produce eruptive episodes. In spite of very high pre-SN mass loss, the resulting SN is classed as Type II, rather than Type IIn, highlighting diverse and previously unrecognized late-stage mass-loss processes.

Context. Tidal disruption events (TDEs) are expected to release much of their energy in the far-ultraviolet (UV), which we do not observe directly. However, infrared (IR) observations can observe re-radiation of the optical/UV emission from dust, and if this dust is observed in the process of sublimation, we can infer the un-observed UV radiated energy. Tidal disruption events have also been predicted to show spectra shallower than a blackbody in the IR, but this has not yet been observed. Aims. We present IR observations of the TDE AT 2019azh that span from −3 d before the peak until > 1750 d after. We evaluate these observations for consistency with dust emission or direct emission from the TDE. Methods. We fitted the IR data with a modified blackbody associated with dust emission. We compared the UV+optical+IR data with simulated spectra produced from general relativistic radiation magnetohydrodynamics simulations of super-Eddington accretion. We modelled the data at later times (> 200 d) as an IR echo. Results. The IR data at the maximum light cannot be self-consistently fitted with dust emission. Instead, the data can be better fitted with a reprocessing model, with the IR excess arising due to the absorption opacity being dominated by free-free processes in the dense reprocessing envelope. We infer a large viewing angle of ∼60°, which is consistent with previously reported X-ray observations, and a tidally disrupted star with a mass > 2 M _⊙ . The IR emission at later times is consistent with cool dust emission. We modelled these data as an IR echo and found that the dust is distant (0.65 pc) and clumpy, with a low covering factor. We show that TDEs can have an IR excess that does not arise from dust and that IR observations at early times can constrain the viewing angle for the TDE in the unified model. Near-IR observations are therefore essential to distinguish between hot dust and a non-thermal IR excess.

We present archival photometry from the Gravitational-wave Optical Transient Observer (GOTO) for four Galactic novae discovered between 2017 and 2024, spanning some of the faintest ZTF24aaomlxy and PGIR22akgylf (at a marginal near-limit level consistent with the practical limiting magnitude of calibrated L to the brightest V1405 Cas and V1674 Her recent eruptions. For each object, we extract GOTO measurements obtained at or near the pre-eruption state, excluding data points with observational uncertainties exceeding 0.5 mag (except for the faintest PGIR22akgylf). The resulting light curves show that GOTO can detect nova progenitors close to its observable limiting depth at calibrated L magnitudes approaching the survey’s practical limiting magnitude, providing meaningful constraints on quiescent brightness, possibly for systems that were only sparsely monitored using surveys such as ZTF and PGIR. These detections demonstrate that wide-field imaging originally designed for gravitational-wave follow-up can yield meaningful limits on both faint and fast-evolving nova progenitors. Simultaneously, we improve the sky positions of five Galactic novae—ZTF24aaomlxy, V3732 Oph, V2000 Aql, V3666 Oph, and V659 Sct—whose published coordinates are affected by crowding or limited precision. Using high-cadence photometry from ZTF and AAVSO, we identify the actual eruption source in each field and obtain revised coordinates that differ by several arcseconds. These findings highlight the importance of time-domain archives for identifying faint nova progenitors and improving astrometric accuracy across the Galactic nova population.

Context. Type IIb supernovae (SNe IIb) are stellar explosions whose spectra reveal transitional features between hydrogen-rich (Type II) and helium-rich (Type Ib) SNe. Their progenitors are massive stars that were mostly stripped of their hydrogen envelope, likely through binary interaction and/or strong stellar winds. This makes such stars key tools in studies of the late stages of the evolution of massive stars. Aims. We present an extensive photometric and spectroscopic follow-up campaign of the Type IIb SN 2022ngb. Through the detailed modeling of this dataset, we aim to constrain the key physical parameters of the explosion, infer the nature of the progenitor star and its environment, and probe the dynamical properties of the ejecta. Methods. We analyzed photometric and spectroscopic data of SN 2022ngb. By constructing and modeling the bolometric light curve with semi-analytic models, we were able to estimate the primary explosion parameters. The spectroscopic data were compared with those of well-studied SNe IIb and NLTE models to constrain the properties of the progenitor and the structure of the resulting ejecta. Results. SN 2022ngb is a low-luminosity SN IIb with a peak bolometric luminosity of L _Bol = 7.76 ^+1.15 _−1.00 × 10 ⁴¹ erg s ⁻¹ and a V -band rising time of 24.32 ± 0.50 days. The light curve modeling indicates an ejecta mass of ∼2.9 − 3.2 M _⊙ , an explosion energy of ∼1.4 × 10 ⁵¹ erg, and a low synthesized ⁵⁶ Ni mass of ∼0.045 M _⊙ . The nebular phase spectra exhibit asymmetric line profiles, pointing to a nonspherical explosion and an anisotropic distribution of radioactive material. Our analysis reveals a relatively compact stripped-envelope progenitor with a pre-SN mass of approximately 4.7 M _⊙ (corresponding to a 15–16 M _⊙ ZAMS star). Conclusions. Our analysis suggests that SN 2022ngb originated from the explosion of a moderate-mass relatively compact, stripped-envelope star in a binary system. The asymmetries inferred from the nebular phase spectral line features indicate the occurrence of a nonspherical explosion.

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 ⁵⁶ Ni mass are 2.2 M _⊙ , 9.1 × 10 ⁵⁰ 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.

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 × 10 ⁵⁰ 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.