Professor Stephen Smartt CBE FRS MRIA

Abstract:

Context. Type IIb supernovae (SNe IIb) often exhibit an early light curve excess (EE) preceding the main peak powered by 56 Ni decay. The physical origin of this early emission remains an open question. Among the proposed scenarios, shock cooling (SC) emission, resulting from the interaction of the shock wave with extended envelopes, is considered the most plausible mechanism. However, the occurrence rate of such events has yet to be reliably constrained. Aims. This study aims to quantify the frequency of EE in SNe IIb and investigate its physical origin by analysing optical light curves from the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey, ultimately providing qualitative constraints on their progenitor systems. Methods. We identified 74 potential SNe IIb from 153 spectroscopically classified events reported in the Transient Name Server (TNS), observed by ATLAS with peak fluxes exceeding 150 μJy (18.46 mag) and explosion epoch uncertainties below six days. Using a spectral reclassification method, we selected a sample of 66 SNe IIb and a cleaned sample of 59 SNe IIb for analysis. We then applied light curve model fitting and outlier analysis to identify objects exhibiting EE emission and studied their photometric properties. Results. We identify 20 SNe IIb with EE, corresponding to a frequency of approximately 30.5% to 50%, the higher value being obtained under the most stringent observational data-quality cuts. The duration and colour evolution of the early excess support its interpretation as shock cooling in extended envelopes. We also find that EE SNe IIb exhibit faster post-peak declines than non-EE events, while both groups show similar peak absolute magnitudes and rise-time distributions. Conclusions. Our findings suggest that EE and non-EE SNe IIb likely share similar initial progenitor masses but differ in their ejecta properties, potentially due to varying degrees of binary interaction. This study constrains EE SNe frequency and photometric properties, paving the way for future theoretical work, such as hydrodynamical modelling of EE SNe light curves, which could corroborate these results and contribute to constraining the evolutionary pathways of SNe IIb progenitor systems.

Abstract:

With the 4-metre Multi-Object Spectroscopic Telescope (4MOST) expected to provide an influx of transient spectra when it begins observations in early 2026 we consider the potential for real-time classification of these spectra. We investigate three extant spectroscopic transient classifiers: the Deep Automated Supernova and Host classifier (dash), Next Generation SuperFit (ngsf), and SuperNova IDentification (snid), with a focus on comparing the completeness and purity of the transient samples they produce. We manually simulate fibre losses critical for accurately determining host contamination and use the 4MOST Exposure Time Calculator to produce realistic, 4MOST-like, host-galaxy contaminated spectra. We investigate the three classifiers individually and in all possible combinations. We find that a combination of dash and ngsf can produce a supernova (SN) Ia sample with a purity of 99.9 per cent, while successfully classifying 70 per cent of SNe Ia. However, it struggles to classify non-SN Ia transients. We investigate photometric cuts to transient magnitude and the transient’s fraction of total fibre flux, finding that both can be used to improve non-SN Ia transient classification completeness by 8–44 per cent with SNe Ibc benefitting the most and superluminous (SL) SNe the least. Finally, we present an example classification plan for live classification and the predicted purities and completeness across five transient classes: Ia, Ibc, II, SL, and non-SN transients. We find that it is possible to classify 75 per cent of input spectra with 70 per cent purity in all classes except non-SN transients. Precise values can be varied using different classifiers and photometric cuts to suit the needs of a given study.

Abstract:

We present ultraviolet to infrared observations of the extraordinary Type IIn supernova 2023zkd (SN 2023zkd). Photometrically, it exhibits persistent and luminous precursor emission spanning ∼4 yr preceding discovery (Mr ≈ −15 mag, 1500 days in the observer frame), followed by a secondary stage of gradual brightening in its final year. Post-discovery, it exhibits two photometric peaks of comparable brightness (Mr ≲ −18.7 mag and Mr ≈ −18.4 mag, respectively) separated by 240 days. Spectroscopically, SN 2023zkd exhibits highly asymmetric and multicomponent Balmer and He I profiles that we attribute to ejecta interaction with fast-moving (1000–2000 km s−1) He-rich polar material and slow-moving (∼400 km s−1) equatorially distributed H-rich material. He II features also appear during the second light curve peak and evolve rapidly. Shock-driven models fit to the multiband photometry suggest that the event is powered by interaction with ∼5–6 M⊙ of CSM, with 2–3 M⊙ associated with each light curve peak, expelled during mass-loss episodes ∼3–4 yr and ∼1–2 yr prior to explosion. The observed precursor emission, combined with the extreme mass-loss rates required to power each light curve peak, favors either super-Eddington accretion onto a black hole or multiple long-lived eruptions from a massive star to luminosities that have not been previously observed. We consider multiple progenitor scenarios for SN 2023zkd, and find that the brightening optical precursor and inferred explosion properties are most consistent with a massive (MZAMS ≥ 30 M⊙) and partially stripped He star undergoing an instability-induced merger with a black hole companion.