Searching for the Third Wheel: High-Contrast Imaging Constraints on Tertiaries to Black Hole and Neutron Star Binaries
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| Format: | Preprint |
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2026
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| _version_ | 1866910225114595328 |
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| author | Nagarajan, Pranav El-Badry, Kareem Sanghi, Aniket |
| author_facet | Nagarajan, Pranav El-Badry, Kareem Sanghi, Aniket |
| contents | 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-Loéve 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$σ$ detections are likely artifacts. We derive $5σ$ 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2605_16498 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Searching for the Third Wheel: High-Contrast Imaging Constraints on Tertiaries to Black Hole and Neutron Star Binaries Nagarajan, Pranav El-Badry, Kareem Sanghi, Aniket Solar and Stellar Astrophysics High Energy Astrophysical Phenomena 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-Loéve 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$σ$ detections are likely artifacts. We derive $5σ$ 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. |
| title | Searching for the Third Wheel: High-Contrast Imaging Constraints on Tertiaries to Black Hole and Neutron Star Binaries |
| topic | Solar and Stellar Astrophysics High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2605.16498 |