Triple Evolution Pathways to Black Hole Low-mass X-Ray Binaries: Insights from V404 Cygni

A recent discovery shows that V404 Cygni, a prototypical black hole low-mass X-ray binary (BH-LMXB) is a hierarchical triple: the BH and donor star are orbited by a 1.2 M_⊙ tertiary at a distance of at least 3500 au. Motivated by this system, we evolve a grid of ∼50,000 triple star systems, spanning a broad range of initial orbits. Our calculations employ MESA stellar evolution models, using POSYDON, and self-consistently track the effects of eccentric Kozai–Lidov (EKL) oscillations, mass loss, tides, and BH natal kicks. In our simulations, the progenitors of V404 Cygni-like systems have initial outer separations of 1000–10,000 au and inner separations of ∼100 au, such that they avoid Roche lobe overflow most of the time. Later on, EKL oscillations drive the inner binary to high eccentricities until tides shrink the orbit and mass transfer begins. Notably, such systems only form in simulations with very weak black hole natal kicks (≲5 km s⁻¹) because stronger kicks unbind the tertiaries. Our simulations also predict a population of BH-LMXB triples that form via the classical common-envelope channel, when the BH progenitor does overflow its Roche lobe. The formation rate for this channel is also higher in triples than in isolated binaries because early EKL oscillations cause inner binaries with a wider range of initial separations to enter and survive a common envelope. Our calculations demonstrate that at least some stellar BHs form with extremely weak kicks, and that triple evolution is a significant formation channel for BH-LMXBs.