Professor Stephen Smartt CBE FRS MRIA

Abstract:

We present the Virtual Research Assistant (VRA) of the ATLAS sky survey, which performs preliminary eyeballing on our clean transient data stream. The VRA uses histogram-based gradient-boosted decision tree classifiers trained on real data to score incoming alerts on two axes: “Real” and “Galactic.” The alerts are then ranked using a geometric distance such that the most “real” and “extragalactic” receive high scores; the scores are updated when new lightcurve data is obtained on subsequent visits. To assess the quality of the training we use the recall at rank K, which is more informative to our science goal than general metrics (e.g., accuracy, F1-scores). We also establish benchmarks for our metric based on the pre-VRA eyeballing strategy, to ensure our models provide notable improvements before being added to the ATLAS pipeline. Then, policies are defined on the ranked list to select the most promising alerts for humans to eyeball and to automatically remove bogus alerts. In production the VRA method has resulted in a reduction in eyeballing workload by 85% with a loss of follow-up opportunity <0.08%. It also allows us to automatically trigger follow-up observations with the Lesedi telescope, paving the way toward automated methods that will be required in the era of LSST. Finally, this is a demonstration that feature-based methods remain extremely relevant in our field, being trainable on only a few thousand samples and highly interpretable; they also offer a direct way to inject expertise into models through feature engineering.

Abstract:

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 to SN 2009ip , we call this brightening ‘Event A’. During Event A, SN 2023ldh reached a maximum absolute magnitude of M 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 of M r = −18.53 ± 0.23 mag, replicating the evolution of SN 2009ip and similar to that of type IIn SNe. The three spectra of SN 2023ldh obtained 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 H α line profile. Metal lines with P Cygni profiles and velocities of about 2000 km s −1 are clearly visible. Beginning around three months past maximum and until very late phases, the Ca II lines become among the most prominent features, while H α is dominated by an intermediate-width component with a boxy profile. Although SN 2023ldh mimics the evolution of other SN 2009ip -like transients, it is slightly more luminous and has a slower photometric evolution. The surprisingly homogeneous observational properties of SN 2009ip -like events may indicate similar explosion scenarios and similar progenitor parameters.

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.

Abstract:

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 low r -band absolute magnitude of −16.4 mag. The pseudo-bolometric light curves peak at (1 − 10)×10 42 erg s −1 , with total radiated energies on the order of (1 − 10)×10 48 erg. Spectroscopically, these SNe display a relatively slow spectral evolution. The early spectra are characterised by a hot blue continuum and prominent He  I emission lines. The early spectra also show blackbody temperatures exceeding 10 000 K, with a subsequent decline in temperature during later phases. Narrow He  I lines, 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)×10 50 erg. 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.

Abstract:

We present results from the Pan-STARRS optical search for kilonovae without the aid of gravitational wave and gamma-ray burst triggers. The search was conducted from 2019 October 26 to 2022 December 15. During this time, we reported 29 740 transients observed by Pan-STARRS to the IAU Transient Name Server. Of these, 175 were Pan-STARRS credited discoveries that had a host galaxy within 200 Mpc and had discovery absolute magnitudes . A subset of 11 transients was plausibly identified as kilonova candidates by our kilonova prediction algorithm. Through a combination of historical forced photometry, extensive follow-up, and aggregating observations from multiple sky surveys, we eliminated all as kilonova candidates. Rapidly evolving outbursts from massive stars (likely to be Luminous Blue Variable eruptions) accounted for 55 per cent of the subset’s contaminating sources. We estimate the rate of such eruptions using the Asteroid Terrestrial-impact Last Alert System 100 Mpc volume-limited survey data. As these outbursts appear to be significant contaminants in kilonova searches, we estimate contaminating numbers when searching gravitational wave skymaps produced by the LIGO-Virgo-Kagra science collaboration during the Rubin era. The Legacy Survey of Space and time, reaching limiting magnitudes of , could detect 2–6 massive stellar outbursts per 500 deg within a 4-d observing window, within the skymaps and volumes typical for binary neutron star mergers projected for Ligo-Virgo-Kagra Observing run 5. We conclude that while they may be a contaminant, they can be photometrically identified.