2025ApJ...988L...7R

A binary system (G3425) consisting of a massive unseen component and a red giant star on a nearly circular orbit was recently discovered. The formation of such a system is puzzling because orbital stability generally breaks down due to the large mass loss from the system caused by the supernova (SN) explosion while forming the unseen component. Analytical solutions of the variable-mass two-body problem suggest that the explosion should have occurred when the component was close to its apocenter to explain the near-circular remnant system. This provides a strong constraint on the total mass and orbital configuration of the progenitor system. The nearly circular orbit of G3425 rules out Type II SN scenarios and allows only for a fine-tuned SN Ib/c explosion to occur when the secondary was close to its apocenter. Such a scenario, although possible, is highly unlikely. However, the most likely scenario is a failed SN that produced a black hole (BH), for which no additional constraints on the position of the secondary are needed. We propose that the unseen component of G3425 is a mass-gap BH with a mass constrained between the theoretical minimum for failed SN progenitors (4 M_⊙) and the observed upper limit (4.4 M_⊙). Our analysis can be applied to any wide binary system containing an unseen component on a nearly circular orbit.