Separating White Dwarfs, Neutron Stars and Black Holes by Microlensing Mass Inference Alone

This paper computes the mass precision a microlensing survey must reach before the three compact-remnant families, whose mass ranges nearly touch, can be assigned from lensing data without follow-up imaging. For a representative lens of 1.4 solar masses at 3.0 kiloparsecs, in front of a source at 8.0 kiloparsecs, the angular Einstein radius is 1.54 milliarcseconds, the event timescale is 141 days at a relative proper motion of 4.0 milliarcseconds per year, the microlensing parallax is 0.135, and the physical Einstein radius projected at the lens is 4.62 astronomical units. The central finding is that a twenty per cent mass error blurs the white dwarf and neutron star boundary, while a ten per cent error resolves most of it; the black hole boundary is easier because the mass gap is wider. 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.