Caption: Artist impression of an isolated black hole drifting through the Milky Way. The black hole distorts the space around it, which warps the light from background objects.
Credit: FECYT, IAC.

Since the majority of black hole systems in the Milky Way are expected to be isolated (Belczynski et al. 2004, Wiktorowicz et al. 2019), the only way to find and weigh these objects is through gravitational microlensing. In particular, massive lenses may cause a measurable astrometric deflection of the background source star (called astrometric microlensing).

 
Recently, the first ever isolated black hole was confirmed through a measurement of astrometric microlensing with high-resolution data from the Hubble Space Telescope (Lam et al. 2022, Lam & Lu 2023 Sahu et al. 2022). The total astrometric deflection measured was approximately 1 milliarcsecond. With the exquisite astrometric precision that Roman will deliver, upwards of ~100 isolated compact objects including black holes are expected to be detected and characterized with GBTDS data.

Caption: A simulated black hole microlensing event observed with the Roman F146W filter. The first three seasons of the GBTDS capture the photometric microlensing signal (outer panel), while the astrometric signal changes across all six seasons (inset panel) and is well characterized by a 8.5 solar-mass black hole lensing a background bulge star. The 72-second exposures are taken every ~15 minutes and are combined in 1-day bins (e.g. ~90 frames stacked each day).
Credit: S. Terry (UMD).

In order to optimize the number of isolated compact objects that can be characterized by the GBTDS, the survey is expected to conduct lower cadence ‘gap-filling’ observations during the off-seasons that will not have high-cadence monitoring (e.g. between seasons three and four, see above figure).