Inner density profile of PBH-seeded axion minihalos

Characterize the innermost density profile of a primordial-black-hole-dominated QCD-axion ultracompact minihalo, including the transition produced by angular-momentum support and its dependence on the primordial black-hole mass, axion properties, and kinetic history.

Background

The secondary-infall calculation robustly produces an outer density profile proportional to r^{-9/4}, but that radial-infall description fails near the central primordial black hole when angular momentum becomes important. The paper notes that existing treatments yield different inner behaviors, including a constant-density core or broken cusps proportional to r^{-3/4} and r^{-3/2}. Because the inner density controls both dynamical-friction dephasing and axion–photon conversion, resolving this profile is important for reliable multimessenger predictions.