A Contact Binary Mis-Classified as an Ellipsoidal Variable: Complications for Detached Black Hole Searches

TL;DR

This study reveals that many candidate black hole systems identified via ellipsoidal variability are actually contact binaries, highlighting significant contamination issues in current detection methods.

Contribution

The paper demonstrates that contact binaries can be misclassified as ellipsoidal variables, complicating black hole candidate identification from photometric surveys.

Findings

Radial velocity follow-up shows true mass ratios are much smaller than initial estimates.

Many systems thought to be black hole candidates are likely contact binaries.

Contamination from contact binaries in black hole searches is higher than previously believed.

Abstract

Identifying sources exhibiting ellipsoidal variability in large photometric surveys is becoming a promising method to search for candidate detached black holes in binaries. This technique aims to exploit the orbital-phase dependent modulation in optical photometry caused by the black hole distorting the shape of the luminous star to constrain the mass ratio of the binary. Without understanding if, or how much, contamination is present in the candidate black hole samples produced by this new technique it is hard to leverage them for black hole discovery. Here, we follow up one of the best candidates identified from Gaia Data Release 3, Gaia DR3 4042390512917208960, with a radial velocity campaign. Combined photometric and radial velocity modelling, along with spectral disentangling, suggests that the true mass ratio (mass of the unseen object divided by the mass of the luminous star) is an order of magnitude smaller than that inferred assuming the modulations arise from ellipsoidal variability. We therefore infer that this system is likely a contact binary, or on the boundary of both stars nearly filling their Roche lobes, however, further observations are required to confidently detect the secondary. We find that the well-known problem of discriminating between ellipsoidal and contact binary light curves results in a larger contamination from contact binaries than previously suggested. Until ellipsoidal variables can be reliably distinguished from contact binaries, samples of black hole candidates selected based on ellipsoidal variability are likely to be highly contaminated by contact binaries or similar systems.