A hot star in its death throes trembles like a plucked string. Its surface oscillates with amplitudes of thousandths of a magnitude, like noticing a light bulb in a room flicker 0.3% brighter. For a space telescope, such precision is routine, but from Earth, the atmosphere gets in the way. Astronomers decided to test whether the ground-based BlackGEM array, six telescopes in Chile that usually hunt for transients and gravitational waves, could handle it.
The target was PG 1159-035, a hot pre-white dwarf star with known pulsation types. The telescopes observed it in three colors: q (ultraviolet), u (blue), and i (infrared). Then, like stripping layers of paint, they removed the dominant frequencies from the light curves and revealed faint secondary oscillations. Their amplitudes were only a few millimagnitudes — a hundred times weaker than those of typical pulsating giant stars.
Comparing amplitudes in the q, u, and i filters gave a clear separation: the l=1 and l=2 modes fell into distinct regions on the diagram. It's like telling a violin from a viola, even when both play softly. So, from ground-based observations alone, you can determine the pulsation type of any compact star — even one never studied before.
Now astronomers have a working tool: build statistics on known white dwarfs, then apply them to new, faint objects. Thus, a trembling string becomes a tuning fork for the entire Galaxy.
as written by the authors arXiv:2603.07765
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 significantly enhances the detection and characterisation possibilities for compact pulsators that tend to be much fainter than dwarf or giant pulsators. 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. 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. 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. 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. 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.
The pulsation amplitude of PG 1159-035 is only a few millimagnitudes — a brightness change of 0.3%, like a 100-watt bulb brightening by 0.3 watts, noticed from hundreds of light-years away.