Hidden in Plain Sight: Searching for Dark Companions to Bright Stars with the Large Binocular Telescope and SHARK-VIS

TL;DR

This study demonstrates the use of high-resolution direct imaging with the Large Binocular Telescope to search for dark companions like black holes in binary systems, combining imaging and radial velocity data to identify and characterize potential compact objects.

Contribution

It presents a pilot methodology for detecting dark companions in binary systems using direct imaging and radial velocity data, highlighting the potential to identify non-interacting compact objects.

Findings

Identified luminous companions in four binary systems.

Ruled out massive compact objects in three systems.

Indicated the need for follow-up observations to confirm candidates.

Abstract

We report the results from a pilot study to search for black holes and other dark companions in binary systems using direct imaging with SHARK-VIS and the iLocater pathfinder "Lili" on the Large Binocular Telescope. Starting from known single-lined spectroscopic binaries, we select systems with high mass functions that could host dark companions and whose spectroscopic orbits indicate a projected orbital separation $\geq 30$ mas. For this first exploration, we selected four systems (HD 137909, HD 104438, HD 117044, and HD 176695). In each case, we identify a luminous companion and measure the flux ratio and angular separation. However, two of the systems (HD 104438 and HD 176695) are not consistent with simple binary systems and are most likely hierarchical triples. The observed companions rule out a massive compact object for HD 137909, HD 117044, and HD 176695. HD 104438 requires further study because the identified star cannot be responsible for the RV orbit and is likely a dwarf tertiary companion. The SHARK-VIS observation was taken near pericenter, and a second image near apocenter is needed to discriminate between a closely separated luminous secondary and a compact object. We show how the combination of RVs and direct imaging can be used to constrain the orbital inclination and companion mass, and discuss the potential of high resolution direct imaging surveys to identify and confirm non-interacting compact object candidates.