2025ApJ...979L..37L

Recently, an identified noninteracting black hole (BH) binary, Gaia ID 3425577610762832384 (hereafter G3425), contains a BH (∼3.6 M_⊙) falling within the mass gap and has a nearly circular orbit, challenging the classical binary evolution and supernova theory. Here, we propose that G3425 originates from a triple through a triple common envelope (TCE) evolution. The G3425 progenitor originally may consist of three stars with masses of 1.49, 1.05, and 21.81 M_⊙ and inner and outer orbital periods of 4.22 and 1961.78 days, respectively. As evolution proceeds, the tertiary fills its Roche lobe, leading to a TCE. We find that the orbital energy generated by the inspiral of the inner binary serves as additional energy imparted for ejecting the common envelope (CE), accounting for ∼97% of the binding energy in our calculations. This means that the outer orbit needs to expend only a small amount of the orbital energy to successfully eject the CE. The outcome of the TCE is a binary consisting of a 2.54 M_⊙ merger produced by the inner binary merger and a 7.67 M_⊙ helium star whose CE successfully ejected, with an orbital period of 547.53 days. The resulting post-TCE binary (PTB) has an orbital period that is 1-2 orders of magnitude greater than the orbital period of a successfully ejected classical binary CE. In subsequent simulations, we find that the successfully ejected helium star has a 44.2% probability of forming a BH. In the case of a noncomplete fallback forming a BH, with an ejected mass of 2.6 M_⊙ and a relatively low natal kick ($1{1}_{-5}^{+16}$ km s^–1 to $4{9}_{-39}^{+39}$ km s^–1), this PTB can form G3425 in the Milky Way.