Using radial velocities to reveal black holes in binaries: a test case

TL;DR

This study demonstrates that radial velocity variations alone can lead to false positives in black hole candidate identification, emphasizing the importance of additional observational data.

Contribution

The paper provides a case study showing that radial velocity signals can be misleading and that optical and X-ray observations are crucial for accurate classification.

Findings

The candidate is an eclipsing stellar binary, not a black hole system.

The binary system is an RS CVn type with mass transfer.

Radial velocity alone can produce false positives in black hole searches.

Abstract

Aims. Large radial velocity variations in the LAMOST spectra of giant stars have been used to infer the presence of unseen companions. Some of them have been proposed as possible black hole candidates. We test this selection by investigating the classification of the one candidate having a known X-ray counterpart (UCAC4 721-037069). Methods. We obtained time-resolved spectra from the Liverpool Telescope and a 5ks observation from the Chandra observatory to fully constrain the orbital parameters and the X-ray emission of this system. Results. We find the source to be an eclipsing stellar binary that can be classified as a RS CVn. The giant star fills its Roche Lobe and the binary mass ratio is greater than one. The system may be an example of stable mass transfer from an intermediate-mass star with a convective envelope. Conclusions. Using only radial velocity to identify black hole candidates can lead to many false positives. The presence of an optical orbital modulation, such as observed for all LAMOST candidates, will in most cases indicate that this is a stellar binary.