Free Astronomy Magazine January-February 2023

42 JANUARY-FEBRUARY 2023 ASTRO PUBLISHING ous attempts — as well as those of others — turned up a menagerie of binary systems that masquerade as black holes, but this is the first time the search has borne fruit.” The team originally identified the system as potentially hosting a black hole by analyzing data from the Eu- ropean Space Agency’s Gaia space- craft. Gaia captured the minute irregularities in the star’s motion caused by the gravity of an unseen massive object. To ex- plore the system in more detail, El-Badry and his team turned to the Gemini Multi-Ob- ject Spectrograph in- strument on Gemini North, which meas- ured the velocity of the companion star as it orbited the black hole and provided pre- cise measurement of its orbital period. The Gemini follow-up ob- servations were crucial to constraining the or- bital motion and hence masses of the two components in the binary system, allow- ing the team to iden- tify the central body as a black hole roughly 10 times as massive as our Sun. “Our Gemini fol- low-up observations confirmed be- yond reasonable doubt that the binary contains a normal star and at least one dormant black hole,” elab- orated El-Badry. “We could find no plausible astrophysical scenario that can explain the observed orbit of the system that doesn’t involve at least one black hole.” The team re- lied not only on Gemini North’s su- perb observational capabilities but also on Gemini’s ability to provide data on a tight deadline, as the team had only a short window in which to perform their follow-up observations. “When we had the first indications that the system con- tained a black hole, we only had one week before the two objects were at the closest separation in their orbits. Measurements at this point are es- sential to make accurate mass esti- mates in a binary system,” said El-Badry. “Gemini’s ability to provide observations on a short timescale was critical to the project’s success. If we’d missed that narrow window, we would have had to wait another year.” Astronomers’ current models of the evolution of binary systems are hard-pressed to explain how the peculiar configuration of Gaia BH1 system could have arisen. Specifi- cally, the progenitor star that later turned into the newly detected black hole would have been at least 20 times as massive as our Sun. This means it would have lived only a few million years. If both stars formed at the same time, this mas- sive star would have quickly turned into a supergiant, puffing up and engulfing the other star before it had time to become a proper, hydro- gen-burning, main-sequence star like our Sun. It is not at all clear how the solar- mass star could have survived that episode, ending up as an apparently normal star, as the observations of the black hole binary indicate. The- oretical models that do allow for survival all predict that the solar- mass star should have ended up on a much tighter orbit than what is ac- tually observed. This could indicate that there are important gaps in our under- standing of how black holes form and evolve in binary systems, and also suggests the exis- tence of an as-yet-un- explored population of dormant black holes in binaries. “It is interest- ing that this system is not easily accommo- dated by standard bi- nary evolution models,” concluded El-Badry. “It poses many questions about how this binary system was formed, as well as how many of these dormant black holes there are out there.” “As part of a network of space- and ground-based observatories, Gemini North has not only provided strong evidence for the nearest black hole to date but also the first pristine black hole system, uncluttered by the usual hot gas interacting with the black hole,” said NSF Gemini Program Officer Martin Still. “While this potentially augurs future dis- coveries of the predicted dormant black hole population in our Galaxy, the observations also leave a mys- tery to be solved — despite a shared history with its exotic neighbor, why is the companion star in this binary system so normal?” T his video summarizes the discovery of the closest black hole to Earth. [nternational Gemini Observatory/NOIRLab/NSF/AURA/ J.Pollard/J. da Silva/Spaceengine/ M. Zamani/ T. Müller (MPIA), Pan- STARRS DR1 (K. C. Chambers et al. 2016), ESA/Gaia/DPAC (CC BY-SA 3.0 IGO)/ NASA’s Scientific Visualization Studio/ Walt Feimer (HTSI)/ Michael McClare (HTSI)/Dana Berry (Skyworks Digital)/ Michael Mc- Clare (HTSI), N. Bartmann] !