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

TL;DR

This study used multi-epoch spectroscopy to investigate Gaia DR3 ellipsoidal variables, finding that their variability is more consistent with binary star models than with hosting black holes or neutron stars.

Contribution

It provides the first spectroscopic follow-up of Gaia DR3 ellipsoidal variables, challenging previous candidate identifications for black hole or neutron star companions.

Findings

All candidates have companion masses unlikely to be BHs or NSs.

Variability is better explained by contact binaries with starspots.

Some objects are likely detached binaries or pulsating stars.

Abstract

We present multi-epoch spectroscopic follow-up of a sample of ellipsoidal variables selected from Gaia 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 $M_{2,\min} \leq 0.5 ~ M_{\odot}$ in all cases. Assuming random inclinations, the typical inferred companion mass is $M_2 \sim 0.15 ~ M_{\odot}$. This makes it unlikely 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 starspots. Some of the objects in our sample may also be detached main-sequence binaries, or even single stars with pulsations or starspot 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.