Conny Aerts

Context. Supermassive black hole binaries (SMBHBs) are expected to produce periodic modulations in active galactic nuclei (AGN) light curves, but distinguishing such signals from stochastic red-noise variability remains a major challenge. Aims. We present the first systematic search for statistically significant AGN periodicities using the optical photometry from the Gaia space mission Data Release 3 (DR3), with the goal of identifying SMBHB candidates and establishing a methodological data analysis framework that can be scaled to the forthcoming Data Release 4 (DR4). Methods. We analysed GaiaG -band light curves of 384 152 sources from the Gaia celestial reference frame (CRF3). Stochastic variability was modelled as a damped random walk Gaussian process, and empirical false alarm probabilities were derived by comparing observed Lomb–Scargle periodogram peaks against 100 000 synthetic red-noise realisations. Candidates from this first stage were then re-evaluated using full Markov chain Monte Carlo inference under both exponential and powered-exponential kernels. Results. We find 13 sources satisfying our statistical criterion ( p < α = 10 ⁻⁵ ) after both stages of filtering, all of which cover fewer than 2.5 cycles of the candidate period and are systematically concentrated in a region of the parameter space indicative of model misspecification. Conclusions. No reliable periodic SMBHB candidates are retained. The ∼950-day baseline of Gaia DR3 confines all detections to the few-cycle regime where red noise most convincingly mimics periodicity, a limitation that photometric precision alone cannot overcome. The longer baseline of Gaia DR4 will be essential to push beyond this regime. We offer our data analysis software pipeline in open access to the community.

Context. The search for supermassive black hole (SMBH) binaries has, in recent years, seen the dawn of exploration with several hundred candidates claimed from photometric and spectroscopic surveys monitoring active galactic nuclei (AGNs). While only a select number persist to date, the advent of upcoming high-precision wide-field photometric missions motivates continuing the pursuit of confirming SMBH binaries (SMBHBs) in the optical. Aims. We explore the possibility of using the Plato space mission of ESA to detect photometric signatures of SMBH binarity. Since many accretion-related signatures are not uniquely related to binaries, nor easily disentangled, the distinct photometric signature, strict occurrence in phase, and duration of a gravitational self-lensing flare (SLF) can strongly reduce such signal confusion. Motivated by the Kepler observation of Spikey, the best known SLF candidate to date, this work aims to benchmark the scientific outcome if Plato were to observe Spikey-like objects via its Guest Observer programme. Methods. Starting from the Gaia database, we assembled a catalogue of 12 226 bright ( G < 19) high-probability quasars (or QSOs) for the two pointing fields of Plato ’s nominal mission. This Plato quasar catalogue will be pivotal for future follow-up observations of larger photometric searches such as the Vera Rubin LSST survey. We used the Plato camera simulator, PlatoSim , to realistically explore the noise budget in Plato ’s faint limit, while generating mock light curves to benchmark Plato ’s ability to recover signatures of SMBH binarity. These signatures were secured using nested sampling for robust probabilistic modelling. Results. We show that, although not originally designed for this purpose, Plato is capable of detecting Spikey-like SMBHB candidates through their relativistic photometric signatures using Bayesian inference and evidence. Plato will, in particular, be able to confirm or rule out Spikey and Spikey-like objects with a limiting magnitude of G ≤ 18. Conclusions. With a minimum 2 yr baseline per pointing field, we show that Plato could not only play an essential role in future SMBHB research, but it may be an integral part of the observational fleet of continuous high-precision facilities monitoring SMBHB candidates in the near future.

Gravity modes in main-sequence stars are traditionally studied using a pre-whitening approach, which iteratively identifies modes in the Fourier domain and subsequently tunes their frequencies, amplitudes, and phases through time-domain regression. While effective, this method becomes inefficient when analysing large volumes of long time-series data and often relies on subjective stopping criteria to determine the number of iterations. We aim to perform frequency extraction of gravity modes in slowly pulsating B (SPB) stars using a statistically robust, data-driven approach based on advanced power spectrum and harmonic analysis techniques. Our approach employs the multitaper non-uniform fast Fourier transform, mtNUFFT , a power spectrum estimator that addresses the statistical issues of bias and inconsistency that traditional methods such as the Lomb-Scargle periodogram face. We applied its extension, the multitaper F test, to extract coherent gravity modes from 4-year Kepler light curves of SPB stars and to search for period spacing patterns among the extracted modes. The multitaper F test enables fast and accurate extraction of the properties of gravity modes with quasiinfinite lifetimes, preferentially selecting modes that exhibit purely periodic behaviour. Although the method typically extracts fewer frequencies than conventional pre-whitening, it recovers most known modes and, in some cases, reveals new ones. We also find evidence for gravity modes with long but finite lifetimes, and detect more than one period spacing pattern in some of the studied SPB stars. Overall, the multitaper F test offers a more objective and statistically sound alternative to pre-whitening. It scales efficiently to large datasets containing thousands of pulsators, and has the potential to facilitate mode identification and to distinguish between the different excitation mechanisms operating in SPB stars.

The PLanetary Transits and Oscillations of stars (PLATO) mission will begin its four-year nominal mission in early 2027 by monitoring its Long-duration Observation Phase field at South (LOPS2) for at least two years continuously. The primary aim of PLATO is a very ambitious and challenging one: the discovery of Earth-like planets in the habitable zone of nearby and bright solar analogs. To this purpose, the PLATO Mission Consortium, through its Ground-based Observing Program, will perform the follow-up needed to confirm part of the candidate planets photometrically detected by PLATO and measure their masses through radial velocity curves. For the LOPS2, the Ground-based Observing Program is committed (as part of the PLATO mission) to follow up the candidate exoplanets discovered orbiting the 15 000 high-quality target subset of the PLATO Input Catalog (PIC) known as the Prime Sample. The Prime Sample will be made public nine months before launch in the context of the first Guest Observer call for proposals to be issued by the European Space Agency. Here, we present the quantitative metrics and thresholds defined to select and prioritize the Prime Sample. Our method is perfectly general and suitable to rank any list of stars surveyed for transiting planets. We also describe the astrophysical properties of the LOPS2 Prime Sample, both in a statistical sense and for some specific targets of interest.

Context. Isochrone fitting is a classical method used to determine the ages of open clusters. However, the derived ages depend strongly on the chosen input physics of the stellar models used to deduce the isochrones. Additionally, it remains an open question whether the isochronal ages based on stellar surface parameters are consistent with the asteroseismic ages derived from stellar interior properties. Aims. Our aim was to investigate how isochrones based on different input physics and computed for a variety of initial conditions affect the age dating of the open cluster NGC 6866. Using the four-year light curves of its members assembled by the Kepler mission, we further compared these results with asteroseismically derived ages. Methods. We extracted 180 cluster members by applying a clustering algorithm to Gaia DR3 data, and subsequently developed an isochrone-cloud fitting method that simultaneously accounts for the range of free parameters occurring in the input physics. Synthetic colour–magnitude diagrams (CMDs) with various initial rotation rates were generated and compared with the observations to determine the best-fitting rotation distribution. Variable stars were then identified among the cluster members. For the 19 gravity-mode (g-mode) pulsators, we performed modelling using a dedicated grid of rotating stellar models. This modelling was constrained by stellar surface parameters, the measured asymptotic gravity-mode (g-mode) period spacing Π ₀ , and the near-core rotation rate. Two approaches were considered: modelling the pulsators individually and under the assumption that they share a common age. Results. We found discrepant age estimates from PARSEC and MIST isochrones, which yielded 690 ⁺¹⁴⁰ ₋₃₀ Myr and 467 ⁺⁷⁰ ₋₅₀ Myr, respectively. The isochrone-cloud fitting indicates that NGC 6866 has an initial rotation distribution peaking at v / v _crit = 0.6, which is a discrepancy with a factor of about two compared to the asteroseismic values. The asteroseismic modelling of the g-mode pulsators revealed agreement between seismic and isochronal masses, whereas the derived ages differ substantially due to the difference in internal mixing. When the g-mode pulsators were modelled under the assumption of one shared cluster age, we obtained a value of 579 ⁺⁵⁴ ₋₈₂ Myr, in agreement with the PARSEC-based isochronal age. Conclusions. We conclude that using different input physics and various initial conditions impacts the age-dating results of open clusters. Our findings point to the need for more carefully calibrated evolutionary models. The seismic content of NGC 6866 makes it a suitable middle-aged cluster to achieve such calibrations.

Context. Convection and rotation are both key processes in stellar evolution modelling. While standard mixing-length theory (MLT) provides a widely used modelling of convection, it neglects the effects of rotation on convective transport. Aims. We investigate how rotating mixing-length theory (R-MLT), which accounts for the influence of rotation on convection, affects the internal structure, convective mixing, and angular momentum transport in stellar models in comparison to the standard non-rotating MLT. Methods. Using the MESA stellar structure and evolution software, we model the main-sequence evolution of a 5 M _⊙ star, for three cases: non-rotating, rotating with standard MLT for modelling convection, and rotating with R-MLT in convection zones, with the initial rotation rate set to 20 percent of the critical (Keplerian) value at the surface for the rotating models. Results. We find that R-MLT reduces both the convective velocity and mixing length in the stellar core, leading to a smaller convective diffusion coefficient and a ∼20 percent reduction in the extent of the convective overshooting region. While the overall size of the convective core remains nearly unchanged, R-MLT changes the resulting chemical gradient at the core–envelope boundary, shifting the peak of the Brunt–Väisälä frequency and modifying the angular momentum transport in that region. Conclusions. Including the effects of rotation in the treatment of convection through R-MLT introduces measurable structural and transport differences, underscoring the importance of incorporating rotation–convection coupling in models of stars.

Context. While space-based telescopes offer unparalleled precision for asteroseismology, ground-based observations remain crucial for identifying compact pulsator candidates and enabling their pulsational study through multi-colour photometry. The BlackGEM telescope array, with its high-cadence multi-colour photometry survey, significantly enhances the detection and characterisation possibilities for compact pulsators that tend to be much fainter than dwarfs or giant pulsators. Aims. Using BlackGEM multi-colour photometry of the hot pre-white dwarf PG 1159–035, we demonstrate its capability to detect short, multi-periodic pulsations with amplitudes down to a few milli-magnitudes. The primary aim of this study is to establish the feasibility of pulsation mode identification in hot subdwarfs and white dwarfs via mode amplitude-ratio analysis derived from BlackGEM multi-colour observations. Methods. Pulsation frequencies were extracted from our target using iterative pre-whitening analysis. To validate our data-driven mode identification concept using multi-colour photometry, we used the well-studied hot pre-white dwarf PG 1159–035, with previously identified pulsation modes, as a prototypical object that served for validation. Results. The pre-whitening analysis using BlackGEM’s standard q -, u -, and i -band light curves of PG 1159–035 revealed pulsation frequencies of ℓ = 1 and ℓ = 2 modes, consistent with values obtained from the literature. Using the frequencies identified from the q band, amplitudes in the i and u bands could be estimated. Subsequent amplitude ratio calculations resulted in discernible distributions for the ℓ = 1 and ℓ = 2 modes. The future assembly of more BlackGEM amplitude ratios for well-known white dwarfs with already identified modes will lead to density estimators suitable for identifying newly detected modes in known or as-yet-undiscovered pulsators. Conclusions. Our proof-of-concept study paves the way for large-scale asteroseismic analyses of optically faint compact pulsating stars using ground-based facilities, such as BlackGEM. As BlackGEM continues its observations, a substantial number of these objects will be observed as part of a regular survey, enabling a robust characterisation of their pulsation modes in the context of population studies.

Context . Due to their proximity, the Pleiades are an important benchmark open cluster. Despite its status, asteroseismic analyses of its members are rare. In particular, the gravity-mode (g-mode) pulsators, which allow near-core stellar properties to be inferred, have not been analysed yet. Aims . We aim to identify and analyse the population of g-mode pulsators in the Pleiades. Our focus lies on the internal rotation as measured from asteroseismology to obtain a well-defined sample of stellar rotation on the early main sequence. Methods . Based on full-frame images from the Transiting Exoplanet Survey Satellite (TESS), we constructed light curves for intermediate-mass Pleiades members and searched for g-mode pulsators among them. For pulsators exhibiting period spacing patterns, we determined their near-core rotation rate and buoyancy periods. For all other g-mode pulsators, we estimated the near-core rotation rate based on the dominant mode frequency to obtain a comprehensive rotation rate distribution. Results . Among our 105 target stars, we find 28 g-mode pulsators distributed across the entire upper main sequence, 19 of which are hybrid pulsators, but only three stars exhibit period spacing patterns in the current TESS data. The near-core rotation rates in A- and early F-type members are distributed between 1 and 3 d ⁻¹ without any clear mass-dependence. This distribution is much broader than the one in the similar open cluster NGC 2516. A comparison of the buoyancy periods shows that the Pleiades and NGC 2516 are of a similar asteroseismic age. Conclusions . With the large population of g-mode and hybrid pulsators, the Pleiades constitute a valuable asteroseismic benchmark cluster, reaffirming its important role in stellar astrophysics.

The Sloan Digital Sky Survey V (SDSS-V) is pioneering panoptic spectroscopy: it is the first all-sky, multiepoch, optical-to-infrared spectroscopic survey. SDSS-V is mapping the sky with multiobject spectroscopy (MOS) at telescopes in both hemispheres (the 2.5 m Sloan Foundation Telescope at Apache Point Observatory and the 100-inch du Pont Telescope at Las Campanas Observatory), where 500 zonal robotic fiber positioners feed light from a wide-field focal plane to an optical ( R ∼ 2000, 500 fibers) and a near-infrared ( R ∼ 22,000, 300 fibers) spectrograph. In addition to these MOS capabilities, the survey is pioneering ultra–wide-field (∼4000 deg ² ) integral field spectroscopy enabled by a new dedicated facility (LVM-I) at Las Campanas Observatory, where an integral field spectrograph (IFS) with 1801 lenslet-coupled fibers arranged in a 0 . ° 5-diameter hexagon feeds multiple R ∼ 4000 optical spectrographs that cover 3600–9800 Å. SDSS-V’s hardware and multiyear survey strategy are designed to decode the chemodynamical history of the Milky Way and tackle fundamental open issues in stellar physics in its Milky Way Mapper program, trace the growth physics of supermassive black holes in its Black Hole Mapper program, and understand the self-regulation mechanisms and the chemical enrichment of galactic ecosystems at the energy injection scale in its Local Volume Mapper program. The survey is well timed to multiply the scientific output from major all-sky space missions. The SDSS-V MOS programs began robotic operations in 2021; IFS observations began in 2023 with the completion of the LVM-I facility. SDSS-V builds on decades of heritage of SDSS’s pioneering advances in data analysis, collaboration spirit, infrastructure, and product deliverables in astronomy.

Context. Asymmetries in the observed rotational splittings of a multiplet contain information about the star’s rotation profile and internal magnetic field. Moreover, the frequency regularities of multiplets can be used for mode identification. However, to exploit this information, highly accurate theoretical predictions are needed. Aims. We aim to quantify the difference in the predicted mode asymmetries between a 1D perturbative method and a 2D method that includes a 2D stellar structure model, which takes rotation into account. We then place these differences between 1D and 2D methods in the context of asteroseismic measurements of internal magnetic fields. We only focus on the asymmetries and not on possible additional frequency peaks that can arise when the magnetic and rotation axis are misaligned. Methods. We coupled the 1D pulsation codes GYRE and StORM to the 2D stellar structure code ESTER and compared the oscillation predictions with the results from the 2D TOP pulsation code. We focused on zero-age main-sequence models representative of rotating β Cephei pulsators spinning at up to 20 per cent of the critical Keplerian rotation rate. Specifically, we investigated low-radial-order gravity and pressure modes. Results. We find a generally good agreement between the oscillation frequencies resulting from the 1D and 2D pulsation codes. We report differences in predicted mode multiplet asymmetries of mostly below 0.06 d ⁻¹ . Since the magnetic asymmetries are small compared to the differences in the rotational asymmetries resulting from the 1D and 2D predictions, accurate measurements of the magnetic field are in most cases challenging. Conclusions. Differences in the predicted mode asymmetries of a rotating star between 1D perturbative methods and 2D non-perturbative methods can greatly hinder accurate measurements of internal magnetic fields in main-sequence pulsators with low-order modes. Nevertheless, reasonably accurate measurements could be possible with n _pg ≥ 2 modes if the internal rotation is roughly below 10 per cent of the Keplerian critical rotation frequency for (aligned) magnetic fields of the order of a few hundred kilogauss. While the differences between the 1D and 2D frequency predictions are mostly too large for internal magnetic field detections, the rotational asymmetries predicted by StORM are in general accurate enough for asteroseismic modelling of the stellar rotation in main-sequence stars with identified low-order modes.