BlackGEM observations of compact pulsating 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.