2025AJ....170..304H

The stellar companions in the binary black hole systems Gaia BH2 and BH3, both of which are α-enhanced red giant branch stars, are expected to show normal modes with the characteristic signature of convectively driven solar-like oscillations. We investigate this using photometry from the TESS mission and find such a signal for Gaia BH2. For Gaia BH2, we measure a power excess frequency of ${\nu }_{{\rm{\max }}}=60.15\pm 0.57$μHz and a large separation of Δν = 5.99 ± 0.03 μHz, yielding a mass of $1.1{9}_{-0.08}^{+0.08}$M_⊙, which is in agreement with spectroscopically derived parameters. Seismic modeling favors an age for the red giant of $5.0{3}_{-3.05}^{+2.58}$ Gyr, strongly suggesting that it is a young, α-enriched giant star, which are thought to arise from a binary accretion or merger scenario. Ground-based photometry of Gaia BH2 spanning 8 yr indicates a photometric period of 398 ± 5 days, which we tentatively attribute to rotation. If this rotation is physical, it cannot be explained solely by evolutionary spin-down or magnetic braking, and implies that the red giant underwent some tidal forcing mechanism. Suggestively, this period is close to the pseudosynchronous spin period of P_spin = 428 ± 1 days derived from the binary orbit. For Gaia BH3, we are unable to identify an asteroseismic signal in the TESS data despite predicting that the amplitude of the signal should lie well above the measured noise level. We discuss a number of scenarios for why this signal may not be visible.