Cause of the slope transition in final mass versus normalized angle

Identify the physical causes of the observed slope change in the final black hole ADM mass Mf as a function of the normalized incidence angle θ/θ0 near the critical threshold in dynamical capture of two equal‑mass non‑spinning black holes, and quantify the relative contributions of gravitational‑wave emission, spin induction, and mass increase mechanisms to this transition.

Background

The paper reports that the remnant ADM mass Mf generally increases with θ/θ0 for higher initial momenta, whereas for smaller p/M the trend flattens or becomes slightly negative near the critical angle. This non‑monotonic behavior suggests competing physical effects during the capture and merger phases.

The authors highlight that the origin of the slope transition is not fully understood and likely involves an interplay among gravitational‑wave losses, spin induction during the close encounter, and mass growth via horizon absorption. Pinpointing the dominant mechanisms is important for predictive modeling of remnants from dynamical captures.