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29 JULY-AUGUST 2022 ASTRO PUBLISHING This astrometric microlensing tech- nique provided information on the mass, distance, and velocity of the black hole. The amount of deflec- tion by the black hole’s intense warping of space allowed Sahu’s team to estimate that it weighs seven solar masses. Lam’s team reports a slightly lower mass range, meaning that the object may be either a neutron star or a black hole. They estimate that the mass of the invisible compact object is between 1.6 and 4.4 times that of the Sun. At the high end of this range the object would be a black hole; at the low end, it would be a neutron star. “As much as we would like to say it is definitively a black hole, we must report all allowed so- lutions. This includes both lower mass black holes and possibly even a neutron star,” said Jessica Lu of the Berkeley team. “ Whatever it is, the object is the first dark stellar rem- nant discovered wandering through the galaxy, unacompanied by an- other star” Lam added. This was a particularly difficult meas- urement for the team because there is another bright star that is ex- tremely close in angular separation to the source star. “So it’s like trying to measure the tiny motion of a fire- fly next to a bright light bulb,” said Sahu. “We had to meticulously sub- tract the light of the nearby bright star to precisely measure the deflec- tion of the faint source.” Sahu’s team estimates the isolated black hole is traveling across the galaxy at 160,000 kilometres per hour (fast enough to travel from Earth to the Moon in less than three hours). That’s faster than most of the other neighbouring stars in that region of our galaxy. “Astrometric microlensing is concep- tually simple but observationally very tough,” said Sahu. “Microlens- ing is the only technique available for identifying isolated black holes.” F ollowing six years of meticulous observations, the NASA/ESA Hubble Space Telescope has provided, for the first time ever, clear evidence for a lone black hole drifting through interstellar space. This is the first time the mass of an iso- lated black hole has been measured. This video summarizes the discovery. [ESA/ Hubble, ESA, NASA, STScI, Digitized Sky Survey, Nick Risinger (skysurvey.org )] When the black hole passed in front of a background star located 19,000 light-years away in the ga- lactic bulge, the starlight coming toward Earth was amplified for a duration of 270 days as the black hole passed by. However, it took sev- eral years of Hubble observations to follow how the background star’s position appeared to be deflected by the bending of light by the fore- ground black hole. The existence of stellar-mass black holes has been known since the early 1970s, but all of their mass measurements — until now — have been in binary star systems. Gas from the companion star falls into the black hole and is heated to such high temperatures that it emits X- rays. About two dozen black holes have had their masses measured in X-ray binaries through their gravita- tional effect on their companions. Mass estimates range from 5 to 20 solar masses. Black holes detected in other galaxies by gravitational waves from mergers between black holes and companion objects have been as high as 90 solar masses. “Detections of isolated black holes will provide new insights into the population of these objects in the Milky Way,” said Sahu. He expects that his programme will uncover more free-roaming black holes in- side our galaxy. But it is a needle-in- a-haystack search. The prediction is that only one in a few hundred mi- crolensing events are caused by iso- lated black holes. In his 1916 paper on general relativ- ity, Albert Einstein predicted that his theory could be tested by observing the offset in the apparent position of a background star caused by the Sun’s gravity. This was tested by a collaboration led by astronomers Arthur Eddington and Frank Dyson during a solar eclipse on 29 May 1919. Eddington and his colleagues meas- ured a background star being offset by 2 arc seconds, validating Einstein’s theories. These scientists could hardly have imagined that over a century later this same technique would be used — with a then-unimaginable thousandfold improvement in preci- sion — to look for black holes across our galaxy. !