Date: 21.03.2023

Tsevi Mazeh (Tel Aviv University)

Gaia BH1 and BH2 the first two dormant black holes discovered by space-mission astrometry and confirmed by ground-based spectroscopy

Abstract:
Most known stellar black holes are members of close binaries, where accretion-fueled X-ray emission enabled their detection. However, those are the tip of the iceberg of a much larger undetected population of Dormant, non-interacting black holes in wide binaries. A promising detection channel for these systems is the astrometric wobble induced on the luminous star by its dark companion. In preparation for the release of the astrometric orbits of Gaia DR3, Shahaf, Mazeh, Faigler, and Holl (2019) proposed a triage technique to identify the astrometric binaries with compact companions. We applied this technique to the recent Gaia release and identified dozens of systems with neutron star or black-hole candidates as their faint massive secondaries. A spectroscopic follow-up campaign led by El-Badry confirmed the nature of two candidates, Gaia BH1 and BH2, which are currently the only two systems of their kind. The talk will review the triage technique, the properties of Gaia BH1 and BH2 and the implications of their discovery.