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

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.

6 pages, 3 figures, Accepted for publication in Astronomy & Astrophysics

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Cited by in corpus (3)

• Gaia Data Release 3: Ellipsoidal Variables with Possible Black-Hole or Neutron Star secondaries
• Minute-cadence Observations of the LAMOST Fields with the TMTS: I. Methodology of Detecting Short-period Variables and Results from the first-year Survey
• Spectroscopic follow-up of black hole and neutron star candidates in ellipsoidal variables from Gaia DR3