Hydrodynamic Simulations Show Natal Kicks Up to 1000 km/s Rarely Reorient Black Hole Binary Orbits

Researchers ran 3D smooth particle hydrodynamics simulations of asymmetric mild explosions in four pre-supernova stellar models ranging from 13 to 34 solar masses. They tracked how hydrodynamic natal kicks imparted during black hole formation affect the alignment between individual spins and the binary orbital angular momentum. The setups tested kick velocities reaching approximately 1000 km/s, among the highest values recorded for neutron stars and inferred for some black holes.

Results indicate that random or misaligned kicks leave effective spin parameters positive, preserving the signature of isolated binary evolution. Only narrowly directed kicks combined with elevated explosion energies produced orbital reorientation sufficient to yield negative effective spins. Most configurations required conditions unlikely to occur across a realistic population of progenitors.

This constrains formation-channel interpretations of LIGO-Virgo-KAGRA events. Isolated binaries are expected to dominate the positive effective-spin distribution unless extreme kick physics operates more frequently than these models allow. Dynamic cluster formation remains the primary route for negative effective spins.

Next steps include expanding the model grid to lower-mass progenitors and incorporating full neutrino-driven explosion physics to test whether rarer channels can produce the observed spin distribution.