A potential mass-gap black hole in a wide binary with a circular orbit

The mass distribution of black holes identified through X-ray emission suggests a paucity of black holes in the mass range of 3 to 5 solar masses. Modified theories have been devised to explain this mass gap, and it is suggested that natal kicks during a supernova explosion can more easily disrupt binaries with lower-mass black holes. Although recent Laser Interferometer Gravitational-Wave Observatory observations reveal the existence of compact remnants within this mass gap, the question of whether low-mass black holes can exist in binaries remains a matter of debate. Such a system is expected to be non-interacting and without X-ray emission, and can be searched for using radial-velocity and astrometric methods. Here we report on Gaia Data Release 3 (DR3) 3425577610762832384, which is a wide binary system that includes a red giant star and an unseen object, exhibiting an orbital period of approximately 880 days and a near-zero eccentricity. Through the combination of radial-velocity measurements from the Large Aperture Multi-Object Spectroscopic Telescope and astrometric data from Gaia DR2 and DR3 catalogues, we determine a mass of 3.6-0.5+0.8M circle dot\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$3.{6}_{-0.5}<^>{+0.8}\,{M}_{\odot }$$\end{document} of the unseen component. If the unseen companion is a black hole, its mass would fall within the gap and it would strongly suggest the existence of binary systems containing low-mass black holes. More notably, the formation of its surprisingly wide circular orbit challenges current binary evolution and supernova explosion theories. Through a joint analysis of radial velocity and astrometry, Wang et al. have discovered a putative mass-gap black hole candidate in a wide binary system. Its wide circular orbit challenges current binary evolution and supernova explosion theories.