Long-Duration Events and Isolated Black Holes: A Mass-Inference Budget for a Ten-Solar-Mass Lens

This paper computes whether the long, parallax-rich events attributed to isolated stellar-mass black holes yield masses precise enough to separate them cleanly from neutron stars and heavy white dwarfs. For a representative lens of 10.0 solar masses at 2.0 kiloparsecs, in front of a source at 8.0 kiloparsecs, the angular Einstein radius is 5.53 milliarcseconds, the event timescale is 404 days at a relative proper motion of 5.0 milliarcseconds per year, the microlensing parallax is 0.0679, and the physical Einstein radius projected at the lens is 11.1 astronomical units. The central finding is that duration alone cannot separate a black hole from a slower, lighter lens; the separation comes from combining parallax with an astrometric or finite-source measure of the Einstein radius. All quantities are computed from stated assumptions and published survey values used as comparison points; the study is simulated and computed, not measured. Where the computation shows that an inference commonly drawn from such events is not supported at the stated precision, that negative result is reported in full.