BlackGEM observations of compact pulsating stars

Mode identification for the DOV PG 1159–035 using multi-colour photometry

¹ Department of Astrophysics/IMAPP, Radboud University, P.O. Box 9010, 6500 GL, Nijmegen, The Netherlands
² Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
³ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85741 Garching bei München, Germany
⁴ Department of Astronomy, University of Cape Town, Private Bag X3 Rondebosch 7701, South Africa
⁵ South African Astronomical Observatory, P.O. Box 9 Observatory 7935, South Africa
⁶ The Inter-University Institute for Data Intensive Astronomy, University of Cape Town, Private Bag X3 Rondebosch 7701, South Africa
⁷ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁸ Texas Tech University, Department of Physics & Astronomy, Box 41051, 79409 Lubbock, TX, USA
⁹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1090 GE, Amsterdam, The Netherlands
¹⁰ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
¹¹ Department of Physics and Astronomy, Texas Tech University, 2500 Broadway, Lubbock, TX 79409, USA
¹² Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
¹³ Armagh Observatory & Planetarium, College Hill, Armagh BT61 9DG, UK
¹⁴ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 8 August 2025
Accepted: 27 February 2026

Abstract

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.