We introduce here a program that utilizes the robotic observing capabilities of the South African Astronomical Observatory's 1-meter Lesedi telescope, equipped with the Mookodi instrument to observe newly discovered near-Earth asteroids (NEAs). Observations are automatically scheduled and robotically observed on Lesedi using scripts that continuously monitor new discoveries being reported by the community to The International Astronomical Union's Minor Planet Center (IAU MPC), enabling rapid follow-up of newly discovered NEAs, the majority within hours of discovery. The main goal of the program is to study the understudied smaller NEA population, and for that reason this rapid response is essential, as smaller asteroids (< 100 meters) quickly dim as they move away from Earth, making precise measurements with small-to-medium aperture telescopes challenging. Since the start of this project in February 2023, over 200 NEAs have been observed under this program with approximately 75% of all our observed asteroids having a diameter of less than 100 meters, thereby achieving the primary objective of the program. Although the program primarily targets smaller asteroids, it has also resulted in the observation of 15 NEAs that have subsequently been classified as potentially hazardous asteroids (PHAs). The astrometric data collected during this program have contributed to Minor Planet Electronic Circulars (MPECs) for similar to 68% of observed targets, thereby contributing to orbital refinement and supporting the official designation of these NEAs - ultimately contributing to planetary defense efforts.

Small near-Earth asteroids (NEAs), diameters < 150 m, represent the most numerous yet one of the least well-understood populations among near-Earth objects, despite their potential hazard. Their rapid fading after discovery makes it challenging to obtain sufficient follow-up observations for characterisation studies, leaving a critical gap in our knowledge of their taxonomic distribution. We present results from a robotic follow-up program using the South African Astronomical Observatory's Lesedi telescope. This system uses automated scripts to rapidly identify NEA discoveries reported to the Minor Planet Center and execute follow-up observations within hours of detection. Using multifilter photometry in the g, r, and i bands, we performed taxonomic classification of 59 small NEAs, with absolute magnitudes H ranging from 22 <= H < 29, using a trained machine learning algorithm. Our results reveal that the composition of the small NEA population slightly differs from the population of larger size, pointing to size-dependent taxonomic variations relevant to impact hazard assessments. Specifically, we find an approximately 1:1 ratio between stony types (S+V+Q) and carbonaceous/metallic types (C+X), broadly consistent with earlier studies of larger NEAs. However, we identify a significantly higher fraction of X-type asteroids (almost a third of the observed sample) compared to previous taxonomic surveys of larger NEAs. This study provides a compositional analysis of sub-150 m NEAs and suggests that the taxonomic distribution may vary with size, highlighting the importance of dedicated small-object characterization programs to better understand the most abundant, and thus most likely source of Earth impactors.

Ultracompact Galactic binaries with orbital periods below an hour are among the strongest persistent gravitational-wave (GW) sources in the mHz band and will constitute the dominant population detected by the Laser Interferometer Space Antenna (LISA). Tens of thousands are predicted to be individually resolved, with a substantial fraction bright enough for electromagnetic (EM) follow-up. This opens an unprecedented multi-messenger window on compact binary evolution, tidal interactions, mass transfer, and the progenitors of Type Ia supernovae. We highlight key science enabled by joint GW + EM constraints and emphasize the critical need for rapid, high-cadence spectroscopic capabilities in the 2040s. In particular, the most compact (<10 min) binaries detected by LISA will require read-noise-free, zero-dead-time spectroscopic facilities, potentially realized through coordinated arrays of telescopes with time-staggered exposures, to measure radial velocities, tidal heating signatures, and orbital evolution with the precision needed for transformative multi-messenger studies.

Relativistic transients such as gamma-ray bursts (GRBs), jetted tidal disruption events, luminous fast blue optical transients, and fast X-ray transients, represent the brightest explosions in the Universe and serve dual roles as laboratories for extreme physics and as cosmic lighthouses probing the earliest epochs of the Universe. The 2040s will bring transformative capabilities: wide-field optical surveys discovering tens of thousands of optical transients nightly, proposed high-energy missions like THESEUS providing 10-100x improved high-energy monitoring, and third-generation gravitational wave detectors identifying 𝒪(10^5) compact object mergers annually, many accompanied by relativistic jets. This industrial-scale discovery rate will enable population studies addressing fundamental questions such as jet launching mechanisms, nucleosynthesis, the first stars, and how progenitor environments shape these transients across cosmic time. However, realizing this science requires overcoming a critical bottleneck: these transients evolve on timescales of seconds to days, with their physics encoded in rapidly-changing multi-wavelength signatures demanding immediate spectroscopic characterization down to m 25. Current facilities, optimized for classical/queue scheduling, do not provide the rapid, flexible, multi-target response necessary for industrial-scale follow-up. This white paper demonstrates that without a dedicated large-aperture (10-30 m effective collecting area) time-domain facility with robotic scheduling and optical-NIR spectroscopic capabilities, the transformative potential of relativistic transient science in the 2040s will be considerably limited.

Multi-messenger astronomy will be transformed in the 2040s by an unprecedented volume of detections from next-generation gravitational wave, high-energy, and ultra-high energy neutrino, cosmic ray, and time domain observatories. This white paper, prepared for the European Southern Observatory (ESO) Expanding Horizons call, outlines the key science questions enabled by this emerging multi-messenger ecosystem, ranging from nucleosynthesis and dense matter physics to cosmology, fundamental physics, and the growth of black holes across cosmic time. We demonstrate that fully exploiting these discoveries requires a step change in optical to near infrared spectroscopic capability, including low latency response, high throughput, and flexible time domain operations across both hemispheres. We argue that without a dedicated large-aperture time domain facility, the scientific return of multi-messenger astronomy in the 2040s will be considerably limited.

Modern astronomical surveys deliver immense volumes of transient detections, yet distinguishing between real astrophysical signals (e.g., explosive events, variable stars) and bogus imaging artifacts remains challenging. Convolutional neural networks (CNNs) are effective for such real-bogus classification in optical imaging data; however, their reliance on latent representations makes it difficult to discern the underlying physical reasoning behind each classification. Here, we show that large language models (LLMs) achieve accuracy comparable to CNNs on three major optical transient survey datasets (Pan-STARRS, MeerLICHT, and ATLAS) while simultaneously providing direct, human-readable descriptions for every transient. Using only 15 examples and a concise set of instructions, Google's LLM, Gemini, achieves a 93\% average accuracy across these datasets which have quite diverse resolution and pixel scales. This is the first demonstration of a successful application of an LLM to imaging data from optical transient surveys and it eliminates the need for extensive and complex labeled sets. Furthermore, we demonstrate that a second LLM can evaluate the coherence of the first LLM classifications, thus guiding iterative improvements by indicating problematic examples. This opens up new possibilities: rather than laboriously training a network from scratch, one can simply define the desired output characteristics and rely on the LLM to deliver them. Furthermore, by generating textual descriptions of observed features, LLMs enable users to query classifications as if navigating an annotated catalog, rather than deciphering abstract latent spaces. As next-generation telescopes and surveys further increase data streams, LLM-based classification could help bridge the gap between automated detection and transparent, human-level understanding.

Context. After the main sequence phase, stars more massive than 2.5 M-circle dot rapidly evolve through the Hertzsprung gap as yellow giants and yellow supergiants (YSGs) before settling into the red giant branch. Identifying Hertzsprung gap stars in nearby galaxies is crucial for pinpointing progenitors of luminous red novae (LRNe) - astrophysical transients attributed to stellar mergers. In the era of extensive transient surveys like the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST), this approach offers a new way to predict and select common envelope transients. Aims. This study investigates potential progenitors and precursors of LRNe by analysing Hubble Space Telescope (HST) photometry of stellar populations in galaxies within similar to 20 Mpc to identify YSG candidates. Additionally, we use the Zwicky Transient Facility and MeerLICHT/BlackGEM to identify possible precursors, preparing for future observations by the LSST. Methods. We compiled a sample of 369 galaxies with HST exposures in the F 475W, F 555W, F 606W, and F814W filters. We identified YSG candidates using MESA stellar evolution tracks and statistical analysis of colour-magnitude diagrams. Results. Our sample includes 154 494 YSG candidates with masses between 3 M-circle dot and 20 M-circle dot and is affected by various contaminants, notably foreground stars and extinguished main sequence stars. After excluding foreground stars using Gaia proper motions, contamination is estimated at 1% from foreground stars (based on TRILEGAL simulations) and similar to 20% from extinction affecting main sequence stars. Combining our YSG candidates with time-domain catalogues yielded several interesting candidates. In particular, we identified 12 LRN precursor candidates for which follow-up is encouraged. Conclusions. We highlight the importance of monitoring future transients that match YSG candidates to avoid missing potential LRNe and other rare transients. LSST will be a game changer in the search for LRN progenitors and precursors; it is predicted to discover over 300 000 new YSG candidates and 100 LRN precursors within 20 Mpc.

Novae are thermonuclear explosions on the surface of accreting white dwarfs and are key laboratories for studying explosive nucleosynthesis, particle acceleration, shock physics, and binary evolution. Despite major progress driven by wide-field time-domain surveys and multi-wavelength facilities, our understanding of nova explosions remains limited by incomplete temporal coverage, heterogeneous spectroscopic follow-up, and poorly constrained ejecta properties. In this white paper we outline the open scientific questions that will define nova research in the 2040s, focusing on the mass, composition, geometry, and dynamics of the ejecta, the role of the underlying binary system, and the connection between nuclear burning, shocks, and emission across the electromagnetic spectrum. We argue that decisive progress requires rapid-response, high-cadence, multi-wavelength observations, anchored by systematic high-resolution optical and near-infrared spectroscopy from eruption to quiescence. Finally, we identify key technological requirements needed to enable transformative advances in the physics of nova explosions over the coming decades.

Understanding how mass and angular momentum flow through accretion discs remains a fundamental unsolved problem in astrophysics. Accreting white dwarfs offer an ideal laboratory for addressing this question: their variability occurs on accessible timescales of seconds to minutes, and their optical spectra contain continuum and emission-line components that trace distinct disc regions. Broad-band timing studies have revealed time-lags similar to those observed in X-ray binaries and active galactic nuclei, suggesting propagating fluctuations and possible coupling to an inner hot flow. However, the blending of line and continuum light in broad filters prevents a physical interpretation of these signals. The 2040s will bring an unprecedented number of disc-accreting systems discovered by Rubin-LSST, space-based gravitational-wave observatories, and third-generation ground and space-based detectors. To extract disc physics from these sources, high-cadence optical spectral-timing, simultaneously resolving continuum and individual lines, is essential. Such measurements would directly map how variability propagates through discs, determine how the outer disc responds to changes in the inner flow, and test whether accretion physics is scale-invariant from white dwarfs to supermassive black holes. This white paper outlines the scientific motivation and observational capabilities required to realise this vision. It highlights the opportunity for ESO to enable a transformative new window on accretion physics in the coming decade.

Stargazing Live! aims to capture the imagination of learners with a combination of live and interactive planetarium lessons, real astronomical data, and lessons built around interactive knowledge representations. The lessons were created using a co-creation model and tackle concepts in the pre-university (astro)physics which students find difficult to grasp with traditional interventions. An evaluation study in 9 Dutch classrooms showed that learners are inspired and engaged by the planetarium lessons but are not always able to link the content to the classroom. Pre- and post-tests showed that the accompanying star properties activity significantly increased learners’ understanding of the causal relationships between mass and other properties (such as luminosity, gravity, and temperature) in a main sequence star.

Galactic compact binaries with orbital periods shorter than a few hours emit detectable gravitational waves at low frequencies. Their gravitational wave signals can be detected with the future Laser Interferometer Space Antenna (LISA). Crucially, they may be useful in the early months of the mission operation in helping to validate LISA's performance in comparison to pre-launch expectations. We present an updated list of 55 candidate LISA binaries with measured properties, for which we derive distances based on Gaia Data release 3 astrometry. Based on the known properties from electromagnetic observations, we predict the LISA detectability after 1, 3, 6, and 48 months with state-of-the-art Bayesian analysis methods. We distinguish between verification and detectable binaries as being detectable after 3 and 48 months respectively. We find 18 verification binaries and 22 detectable sources, which triples the number of known LISA binaries over the last few years. These include detached double white dwarfs, AM CVn binaries, one ultracompact X-ray binary and two hot subdwarf binaries. We find that across this sample the gravitational wave amplitude is expected to be measured to $\approx10\%$ on average, while the inclination is expected to be determined with $\approx15^\circ$ precision. For detectable binaries these average errors increase to $\approx50\%$ and to $\approx40^\circ$ respectively.

We present X-ray to radio frequency observations of the bright long gamma-ray burst GRB 210702A. Our Atacama Large Millimeter/submillimeter Array 97.5 GHz observations show a significant rebrightening by a factor of approximate to 2 beginning at 8.2 days post-burst and rising to peak brightness at 18.1 days before declining again. This is the first such rebrightening seen in a millimeter afterglow light curve. A standard forward shock model in a stellar wind circumburst medium can explain most of our X-ray, optical, and millimeter observations prior to the rebrightening, but significantly overpredicts the self-absorbed radio emission, and cannot explain the millimeter rebrightening. We investigate possible explanations for the millimeter rebrightening, and find that energy injection or a reverse shock from a late-time shell collision are plausible causes. Similar to other bursts, our radio data may require alternative scenarios such as a thermal electron population or a structured jet to explain the data. Our observations demonstrate that millimeter light curves can exhibit some of the rich features more commonly seen in optical and X-ray afterglow light curves, motivating further millimeter wavelength studies of GRB afterglows.

The discovery of gravitational wave (GW) events and the detection of electromagnetic counterparts from GW170817 has started the era of multimessenger GW astronomy.The field has been developing rapidly and in this paper,we discuss the preparation for detecting these events with the ESA Gaia satellite,during the 4th observing run of the LIGO-Virgo-KAGRA (LVK) collaboration that has started on May 24,2023. Gaia is contributing to the search for GW counterparts by a new transient detection pipeline called GaiaX. In GaiaX, a new source appearing in the field of view of only one of the two telescopes on-board Gaia is sufficient to send out an alert on the possible detection of a new transient. Ahead of O4, an experiment was conducted over a period of about two months. During the two weeks around New Moon in this period of time, the MeerLICHT (ML) telescope located in South Africa tried (weather permitting) to observe the same region of the sky as Gaia within 10 minutes. Any GaiaX detected transient was published publicly. ML and Gaia have similar limiting magnitudes for typical seeing conditions at ML. At the end of the experiment, we had 11861 GaiaX candidate transients and 15806 ML candidate transients, which we further analysed and the results of which are presented in this paper. Finally, we discuss the possibility and capabilities of Gaia contributing to the search for electromagnetic counterparts of gravitational wave events during O4 through the GaiaX detection and alert procedure.

ABSTRACTBlue Large-Amplitude Pulsators (BLAPs) are a relatively new class of blue variable stars showing periodic variations in their light curves with periods shorter than a few tens of minutes and amplitudes of more than 10 per cent. We report nine blue variable stars identified in the OmegaWhite survey conducted using ESO’s VST, which shows a periodic modulation in the range 7–37 min and an amplitude in the range 0.11–0.28 mag. We have obtained a series of followup photometric and spectroscopic observations made primarily using SALT and telescopes at SAAO. We find four stars which we identify as BLAPs, one of which was previously known. One star, OW J0820–3301, appears to be a member of the V361 Hya class of pulsating stars and is spatially close to an extended nebula. One further star, OW J1819–2729, has characteristics similar to the sdAV pulsators. In contrast, OW J0815–3421 is a binary star containing an sdB and a white dwarf with an orbital period of 73.7 min, making it only one of six white dwarf-sdB binaries with an orbital period shorter than 80 min. Finally, high cadence photometry of four of the candidate BLAPs show features that we compare with notch-like features seen in the much longer period Cepheid pulsators.

Luminous red novae (LRNe) are astrophysical transients believed to be caused by the partial ejection of a binary star’s common envelope (CE) and the merger of its components. The formation of the CE is likely to occur during unstable mass transfer, initiated by a primary star which is evolving off the main sequence (a Hertzsprung gap star) and a lower mass companion. In agreement with observations, theoretical studies have shown that outflows from the pre-CE phase produce a detectable brightening of the progenitor system a few years before the ejection event. Based on these assumptions, we present a method to identify Galactic LRNe precursors, the resulting precursor candidates, and our follow-up analysis to uncover their nature. We begin by constructing a sample of progenitor systems, i.e. Hertzsprung gap stars, by statistically modelling the density of a colour magnitude diagram formed from “well behaved’ Gaia DR2 sources. Their time-domain evolution from the Zwicky Transient Facility (ZTF) survey is used to search for slowly brightening events, as pre-CE precursor candidates. The nature of the resulting candidates is further investigated using archival data and our own spectroscopic follow-up. Overall, we constructed a sample of ∼5.4 × 104 progenitor sources, from which 21 were identified as candidate LRNe precursors. Further analysis revealed 16 of our candidates to be Hα emitters, with their spectra often suggesting hotter (albeit moderately extincted) A-type or B-type stars. Because of their long-term variability in optical and mid-infrared wavelengths, we propose that many of our candidates are mass-transferring binaries with compact companions surrounded by dusty circumstellar disks or alternatively magnetically active stellar merger remnants.

The BlackGEM array Phase I consists of three wide field, optical telescopes, located at the ESO La Silla Observatory, Chile. Each telescope is of a modified Dall-Kirkham design, using an 0.6m primary mirror and a 110 Mpix STA1600 CCD to give a 2.7 square degrees field-of-view sampled at 0.56 ''/pixel. Preliminary commissioning data shows performance on-par with design specifications. Data obtained with the BlackGEM prototype MeerLICHT highlights the capabilities of the design with a 5-sigma limiting magnitude of m(AB)=22.2 in 300s of integration under dark-sky conditions. Extrapolation to the 1 '' seeing-conditions expected at La Silla shows that the main goal of BlackGEM to probe down to m(AB)=23 in 300s can be met. The project suffered a 2-year COVID-19 delay. Commissioning of the array has currently been resumed and science operations are expected to start in Q3/Q4 of 2022. The science programs include the follow-up of gravitational wave alerts from LIGO/Virgo/KAGRA, a six-filter Southern Sky Survey, a Fast Synoptic Survey on selected fields, a Local Universe intra-night monitoring program and a inter-night single-band monitoring for slower transients.

Aims. The impact of mega-constellations of satellites in low-Earth orbit during nighttime optical observations is assessed. Methods. Orbital geometry is used to calculate the impact of stellar occultations by satellites on the photometry of individual stars as well as the effect on the photometric calibration of wide-field observations. Results. Starlink-type satellites will have occultation disks several arcseconds across. Together with occultation crossing times of 0.1−100 ms, this will lead to photometric ‘jitter’ on the flux determination of stars. The level of impact for a given star depends on the ratio of the integration time of the frame over the occultation crossing time. In current-day CCD-based synoptic surveys, this impact is negligible (≪1%), but with future CMOS-based wide-field surveys obtaining data at frequencies >1 Hz, the impact will grow towards complete dropouts. At integration times similar to the occultation crossing time, the orbit of a satellite can be traced using the occultation method. At even shorter integration times, the shape of the occulting satellite can be deduced. Conclusions. Stellar occultations by passing satellites, enabled by high-speed CMOS technology, will be a new method for studying orbiting satellites. Large-scale monitoring programs will be needed to independently determine and update the orbits of satellites.

We report on the detection of MKT J174641.0−321404, a new radio transient found in untargeted searches of wide-field MeerKAT radio images centred on the black hole X-ray binary H1743−322. MKT J174641.0−321404 is highly variable at 1.3 GHz and was detected three times during 11 observations of the field in late 2018, reaching a maximum flux density of 590 ± 60 μJy. We associate this radio transient with a high proper motion, M dwarf star SCR 1746−3214 12 pc away from the Sun. Multiwavelength observations of this M dwarf indicate flaring activity across the electromagnetic spectrum, consistent with emission expected from dMe stars, and providing upper limits on quiescent brightness in both the radio and X-ray regimes. TESS photometry reveals a rotational period for SCR 1746−3214 of 0.2292 ± 0.0025 days, which at its estimated radius makes the star a rapid rotator, comparable to other low mass systems. Dedicated spectroscopic follow up confirms the star as a mid-late spectral M dwarf with clear magnetic activity indicated by strong Hα emission. This transient’s serendipitous discovery by MeerKAT, along with multiwavelength characterisation, make it a prime demonstration of both the capabilities of the current generation of radio interferometers and the value of simultaneous observations by optical facilities such as MeerLICHT. Our results build upon the literature of of M dwarfs’ flaring behaviour, particularly relevant to the habitability of their planetary systems.

The wide-field optical telescopes MeerLICHT and BlackGEM feature an atmospheric dispersion corrector (ADC) which is fully integrated in its three-lens field corrector. Atmospheric dispersion is compensated for through lateral displacement of one of the three lenses. In this contribution, we discuss the on-sky calibration and performance of the ADC of the MeerLICHT telescope. We use a novel technique where we determine the optimal position of the ADC lens by minimising colour-dependent offsets on the astrometry of thousands of stars in an image.

ABSTRACT ESO 149-G003 is a close-by, isolated dwarf irregular galaxy. Previous observations with the ATCA indicated the presence of anomalous neutral hydrogen ($\rm{H{\small I}}$) deviating from the kinematics of a regularly rotating disc. We conducted follow-up observations with the MeerKAT radio telescope during the 16-dish Early Science programme as well as with the MeerLICHT optical telescope. Our more sensitive radio observations confirm the presence of anomalous gas in ESO 149-G003, and further confirm the formerly tentative detection of an extraplanar $\rm{H{\small I}}$ component in the galaxy. Employing a simple tilted-ring model, in which the kinematics is determined with only four parameters but including morphological asymmetries, we reproduce the galaxy’s morphology, which shows a high degree of asymmetry. By comparing our model with the observed $\rm{H{\small I}}$, we find that in our model, we cannot account for a significant (but not dominant) fraction of the gas. From the differences between our model and the observed data cube, we estimate that at least 7–8 per cent of the $\rm{H{\small I}}$ in the galaxy exhibits anomalous kinematics, while we estimate a minimum mass fraction of less than 1 per cent for the morphologically confirmed extraplanar component. We investigate a number of global scaling relations and find that, besides being gas-dominated with a neutral gas-to-stellar mass ratio of 1.7, the galaxy does not show any obvious global peculiarities. Given its isolation, as confirmed by optical observations, we conclude that the galaxy is likely currently acquiring neutral gas. It is either re-accreting gas expelled from the galaxy or accreting pristine intergalactic material.