High-Contrast Imaging Constraints on Tertiaries to BH and NS Binaries

Black holes (BHs) and neutron stars (NSs) with low-mass stellar companions challenge traditional isolated binary evolution models, motivating hierarchical triple evolution as a promising alternative. To search for tertiaries, I performed deep, adaptive optics-assisted, near-infrared imaging of five quiescent BH low-mass X-ray binaries (LMXBs), Gaia BH1, and twelve Gaia NSs. I detected several faint stars previously unresolved in survey imaging, but none were close enough to robustly rule out a chance alignment. To achieve high contrast sensitivity at close separations, I used the reference star differential imaging strategy with the KLIP algorithm to model and subtract the point-spread function of each target. I identified tertiary candidates in the speckle-dominated regime, but injection-recovery tests suggested most 5-sigma detections are likely artifacts. I derived 5-sigma contrast curves and converted 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. I ruled out plausible MS tertiaries and young, hot WD tertiaries at projected separations > 500 au for the Gaia compact object binaries and > 2000 au for the more distant BH LMXBs. While the recent discovery of a 1.2 solar mass tertiary to V404 Cygni supports triple formation scenarios for BH LMXBs, these results suggest such companions are relatively rare. The results 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.

Read more about this research project in the associated publication here.