Searching for the Third Wheel: High-Contrast Imaging Constraints on Tertiaries to Black Hole and Neutron Star Binaries

TL;DR

This study uses high-contrast adaptive optics imaging to search for tertiary companions to black hole and neutron star binaries, setting limits on their presence and characteristics, and finding such companions are likely rare.

Contribution

It provides the first high-contrast imaging constraints on tertiaries to BH and NS binaries, improving understanding of hierarchical triple evolution.

Findings

No confirmed tertiaries within 500-2000 au, ruling out many main sequence and hot white dwarf companions.

Detected candidates are likely artifacts, with no definitive tertiary detections.

Results suggest tertiaries are rare or evolved into cool white dwarfs, consistent with triple formation scenarios.

Abstract

Hierarchical triple evolution provides a promising alternative to isolated binary formation models for black holes (BHs) and neutron stars (NSs) with low-mass stellar companions. To search for tertiaries, we perform deep, adaptive optics-assisted, near-infrared imaging of five quiescent BH low-mass X-ray binaries (LMXBs), Gaia BH1, and twelve Gaia NSs. We detect several faint stars previously unresolved in survey imaging, but none are close enough to robustly rule out a chance alignment. To achieve high contrast sensitivity at close separations, we use the reference star differential imaging strategy with the Karhunen-Lo\'eve Image Processing algorithm to model and subtract the point-spread function of each target. We identify tertiary candidates in the speckle-dominated regime, but injection-recovery tests suggest most 5$\sigma$ detections are likely artifacts. We derive $5\sigma$ contrast curves and convert these to limits on the mass of main sequence (MS) tertiaries and the effective temperature of white dwarf (WD) tertiaries consistent with a non-detection. We rule out plausible MS tertiaries and young, hot WD tertiaries at projected separations $\gtrsim 500$ au for the Gaia compact object binaries and $\gtrsim 2000$ au for the more distant BH LMXBs. While the recent discovery of a $1.2\,M_{\odot}$ tertiary to V404 Cygni supports triple formation scenarios for BH LMXBs, our results suggest such companions are relatively rare. Our observations remain consistent with intermediate-mass tertiaries that have since evolved into cool WDs, detectable with deeper JWST imaging. Follow-up observations are required to measure proper motions and confirm or rule out physical association of tertiary candidates.