Spectroscopic follow-up of black hole and neutron star candidates in ellipsoidal variables from Gaia DR3

We present multi-epoch spectroscopic follow-up of a sample of ellipsoidal variables selected from Gaia Data Release 3 (DR3) as candidates for hosting quiescent black holes (BHs) and neutron stars (NSs). Our targets were identified as BH/NS candidates because their optical light curves - when interpreted with models that attribute variability to tidal distortion of a star by a companion that contributes negligible light - suggest that the companions are compact objects. From the likely BH/NS candidates identified in recent work accompanying Gaia DR3, we select 14 of the most promising targets for follow-up. We obtained spectra for each object at 2-10 epochs, strategically observing near conjunction to best constrain the radial velocity semi-amplitude. From the measured semi-amplitudes of the radial velocity curves, we derive minimum companion masses of M- 2 , (min) = 0 . 5 M-? in all cases. Assuming random inclinations, the typical inferred companion mass is M 2 -0 . 15 M-?. This mak es it unlik ely that any of these systems contain a BH or NS, and we consider alternative explanations for the observed variability. We can best reproduce the observed light curves and radial velocities with models for unequal-mass contact binaries with star-spots. Some of the objects in our sample may also be detached main-sequence binaries, or even single stars with pulsations or star-spot variability masquerading as ellipsoidal variation. We provide recommendations for future spectroscopic efforts to further characterize this sample and more generally to search for compact object companions in close binaries.

Automated summary

We conducted multi-epoch spectroscopic follow-up of ellipsoidal variables identified as candidates for hosting quiescent black holes (BHs) and neutron stars (NSs) based on Gaia Data Release 3 (DR3). We obtained spectra for 14 of the most promising targets and derived minimum companion masses of 0.5 M-? in all cases. Our findings suggest that these systems likely do not contain BHs or NSs, and we propose alternative explanations for the observed variability.