The neutron-star side of the empirical anchor: their extreme density (>2 x 10^14 g/cm^3, d_0 < 0.01 cm stopping scale) makes them stop even the black holes that white dwarfs cannot (D >= 8), and their known ages bound how long a captured stable black hole can have been accreting. The strong magnetic fields are a complication (synchrotron screening of charged cosmic rays), which is why the binary-companion beam-dump geometry and the cosmogenic-neutrino flux carry the production side of the neutron-star bound.