2019MNRAS.486.5400S

In microlensing of a Galactic star by a brown dwarf or other compact object, the amplified image really consists of two unresolved images with slightly different light traveltimes. The difference (of the order of a microsecond) is GM/c³ times a dimensionless factor depending on the total magnification. Since magnification is well measured in microlensing events, a single time-delay measurement would provide the mass of the lens, without degeneracies. The challenge is to find an observable that varies on submicrosecond time-scales. This paper notes that the narrow-band intensity of the unresolved image pair will show photon bunching (the Hanbury Brown and Twiss effect), and argues that the lensed intensity will have an autocorrelation peak at the lensing time delay. The ultrafast photon-counting technology needed for this type of measurement exists, but the photon numbers required to give sufficient signal-to-noise appear infeasible at present. Preliminary estimates suggest time-delayed photon bunching may be measurable for lensed early-type main-sequence stars at ∼10 kpc, with the help of 30 m-class telescopes.