Microlensing Parallax and the Mass-Inference Error Budget for a Single Lens Event

This paper computes how the microlensing parallax vector converts an otherwise mass-degenerate light curve into a mass estimate, and which measurement error dominates that conversion. For a representative lens of 0.5 solar masses at 4.0 kiloparsecs, in front of a source at 8.0 kiloparsecs, the angular Einstein radius is 0.713 milliarcseconds, the event timescale is 52.1 days at a relative proper motion of 5.0 milliarcseconds per year, the microlensing parallax is 0.175, and the physical Einstein radius projected at the lens is 2.85 astronomical units. The central finding is that parallax uncertainty, not timescale uncertainty, dominates the mass budget for ordinary stellar lenses once the timescale is measured to a few per cent. 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.