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 \(3.{6}_{-0.5}^{+0.8}\,{M}_{\odot }\) 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.

通过 X 射线发射识别的黑洞质量分布表明，3 至 5 个太阳质量范围内的黑洞很少。人们设计了改进的理论来解释这种质量差距，并且有人认为超新星爆炸期间的出生踢可以更容易地破坏具有较低质量黑洞的双星。尽管最近的激光干涉仪引力波天文台观测揭示了该质量间隙内存在致密残余物，但双星中是否存在低质量黑洞的问题仍然存在争议。这样的系统预计是非相互作用的并且没有 X 射线发射，并且可以使用径向速度和天体测量方法来搜索。在这里，我们报告盖亚数据版本 3 (DR3) 3425577610762832384，这是一个广泛的双星系统，包括一颗红巨星和一个看不见的物体，其轨道周期约为 880 天，偏心率接近于零。通过结合大口径多目标光谱望远镜的径向速度测量和盖亚 DR2 和 DR3 目录的天体测量数据，我们确定了质量\(3.{6}_{-0.5}^{+0.8}\ ,{M}_{\odot }\)的未见组件。如果看不见的伴星是黑洞，那么它的质量将落在间隙内，这将强烈表明包含低质量黑洞的双星系统的存在。更值得注意的是，其令人惊讶的宽圆形轨道的形成挑战了当前的双星演化和超新星爆炸理论。