Program: GN-2016B-Q-70

Only 18 BHs (i.e. ~30% of the total discovered by transient activity) have been dynamically confirmed after 50 years of X-ray surveys. The remaining are lost because they become too faint (optically) in quiescence for current instruments. We have recently presented a new avenue to deepen BH and neutron star searches with the discovery of a correlation between the width (FWHM) of the disc Halpha emission in quiescence and the projected velocity of the donor star K2. This describes a fundamental velocity scaling, which allows measuring mass functions from single epoch low-resolution spectroscopy. Here we propose to calibrate this relation in the near-infrared using the prominent Br-gamma line. This will open a new window to undertake dynamical studies in highly reddened quiescent BHs, a very significant population that have so far eluded follow-up monitoring. We estimate that the proposed calibration will double the current number of dynamical BHs in the near future.