Elizabeth R. Stanway

The study of young and fast astronomical transients is often limited by the difficulty of acquiring data, particularly spectra, within hours to days of explosion. Historically, follow-up programmes have rarely obtained spectra until many days after discovery; coupled with lower-cadence discovery observations, this has meant that the early evolution of transients has not been systematically probed. The Fast Analysis and Spectroscopy of Transients (FAST) programme aims to build a rich data set on the early evolution of a range of transients by linking a high-cadence discovery survey on the Gravitational-wave Optical Transient Observer (GOTO) with immediate visitor-mode spectroscopic follow-up. We report results from two prototype runs, FAST 23 and 24, carried out in August 2023 and February 2024 using the 2.54 m Isaac Newton Telescope. Between these, 51 transient classifications were reported, 19 of which were discovered by GOTO with a median time between discovery and first spectrum of < 2 d. We highlight the discovery and characterization of SN 2024cld, enabled by spectroscopic observations beginning within 30 min of discovery, which reveals the complex mass-loss history of an evolved supergiant star in its final years prior to supernova. Further GOTO–FAST runs are ongoing at the Nordic Optical Telescope, and we hope this paper will be informative for those considering the creation of similar programmes.

Wide-field and high-cadence sky surveys are the first step in the chain of discovery and characterisation of astrophysical transients such as supernovae, kilonovae, and tidal disruption events, each linked to the varied demise of stellar systems. The Gravitational-wave Optical Transient Observer (GOTO) is a telescope array of thirty-two 40 cm unit telescopes split over two almost antipodal sites. It performs a regular time-domain sky-survey in the optical to ~20 mag in addition to immediate scheduling of follow-up observations at the locations of external multi-wavelength and -messenger triggers. To facilitate the timely recovery of optical counterparts to these triggers, as well as the presence of serendipitous discoveries of astrophysical transients in the regular sky-survey, a low-latency data pipeline and workflow was developed. The implementation of this workflow is described herein and the quality of GOTO data delivered by it assessed, alongside its performance for prompt transient recovery. Utilising difference image analysis to identify candidate discoveries, the process is typically complete ~7 minutes after shutter close on the telescope. We further describe later processing of these candidates – both automated and human-in-the-loop – including reporting to the wider community and the triggering of more detailed observations, with a focus on immediate, intra-night characterisation. The workflow is meeting the needs of GOTO to promptly discover, report and characterise infant transients. Nevertheless, areas for further development and improvements are also highlighted.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

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.

Stellar population synthesis models are an essential tool with which galaxy physical parameters are extracted from observations. However, they are built on assumptions designed for use in the local Universe, and not always appropriate to high redshift galaxies. Here, we consider the impact of including the hitherto-neglected stellar pre-main sequence delay time-scale on the interpretation of composite stellar populations at ages of <1 Gyr. We find that doing so has an impact on the optical luminosity of very young stellar populations of up to ∼10 per cent, although smaller changes in observed light (<5 per cent) are expected in most use cases. However, the impact on the inferred stellar mass and mass-to-light ratios is significant (a factor of 2 or more), depending on how those properties are defined. We find that the short time-scales for star formation in the distant Universe require a clearer definition for the stellar mass in a population, and will impact assumptions about the inferred shape of the stellar initial mass function from observations.