Hintze Centre for Astrophysical Surveys

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:

The vertical evolution of galactic discs is governed by the sub-structures within them. Several of these features, including bulges and kinematically distinct discs, are best studied in edge-on galaxies, as the viewing angle allows the easier separation of component light. For this work, we examined the diversity of kinematic sub-structure present in the first 12 galaxies observed from the GECKOS survey, a VLT/MUSE large programme providing a systematic study of 36 edge-on Milky Way-mass disc galaxies. Employing the N GIST analysis pipeline, we derived the mean luminosity-weighted line-of-sight stellar velocity ( V ⋆ ), velocity dispersion ( σ ⋆ ), skew ( h 3 ), and kurtosis ( h 4 ) for the sample, and examined 2D maps and 1D line profiles. Common clear kinematic signatures were observed: all galaxies display h 3 – V ⋆ sign mismatches in the outer disc regions consistent with a (quasi-)axisymmetric, rotating disc of stars. After scrutinising visual morphologies, we found that the majority of this sample (8/12) possess boxy-peanut bulges and host the corresponding kinematic structure predicted for stellar bars viewed in projection. Inferences were made on the bar viewing angle with respect to the line of sight from the strength of these kinematic indicators; we found one galaxy whose bar is close to side-on with respect to the observer, and two that are close to end-on. Four galaxies exhibit strong evidence for the presence of nuclear discs, including central h 3 – V ⋆ profile anti-correlations, croissant-shaped central depressions in σ ⋆ maps, strong gradients in h 3 , and positive h 4 plateaus over the expected nuclear disc extent. The strength of the h 3 feature corresponds to the size of the nuclear disc, measured from the h 3 turnover radius, taking into account geometric effects. We can explain the features within the kinematic maps of the four unbarred galaxies via disc structure(s) alone. We do not find any need to invoke the existence of dispersion-dominated bulges in any of the sample galaxies. Obtaining the specialised data products for this paper and the broader GECKOS survey required significant development of existing integral field spectroscopic (IFS) analysis tools. Therefore, we also present the N GIST pipeline: a modern, sophisticated, and easy-to-use pipeline for the analysis of galaxy IFS data, and the key tool employed by the GECKOS survey for producing value-added data products. We conclude that the variety of kinematic sub-structures seen in GECKOS galaxies requires a contemporary view of galaxy morphology, expanding on the traditional view of galaxy structure, and uniting the kinematic complexity observed in the Milky Way with the extragalactic.