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

TL;DR

This paper reports the discovery of a wide binary system with a low-mass black hole in the mass gap, challenging existing theories of binary evolution and supernova explosions.

Contribution

It provides the first evidence of a low-mass black hole in a wide, circular orbit binary system, using combined radial velocity and astrometric data.

Findings

Unseen companion mass estimated at 3.6 solar masses within the mass gap.

The binary has a wide, nearly circular orbit of about 880 days.

The system challenges current models of binary formation and supernova mechanisms.

Abstract

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 supernova explosion can more easily disrupt binaries with lower mass black holes. Although recent LIGO 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 noninteracting without X-ray emission, and can be searched for using radial velocity and astrometric methods. Here we report Gaia DR3 3425577610762832384, a wide binary system including a red giant star and an unseen object, exhibiting an orbital period of approximately 880 days and near-zero eccentricity. Through the combination of radial velocity measurements from LAMOST and astrometric data from Gaia DR2 and DR3 catalogs, we determine a mass of $3.6^{+0.8}_{-0.5}$ $M_{\odot}$ of the unseen component. This places the unseen companion within the mass gap, strongly suggesting 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.