An isolated stellar-mass black hole has been detected floating through interstellar space for the first time.
Astronomers typically spot black holes by measuring their interactions with nearby stars, which can produce vast plumes of gas or radiation. But isolated stars, which astronomers have observed in their millions, imply that isolated black holes should also fill the sky, as dying stars can birth black holes once they explode in a supernova.
鈥淸Isolated stellar-mass black holes] aren鈥檛 rare, but they鈥檝e never been found,鈥� says at the Space Telescope Science Institute (STSI) in Maryland.
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Now, Sahu and his colleagues have spotted one of these untethered black holes about 5000 light years away, in the constellation Sagittarius. They used the Hubble Space Telescope to detect it through a phenomenon called microlensing, in which the gravity from massive objects, such as black holes, can bend and magnify the light of stars that they pass in front of.,
The brightening and bending of light is miniscule when viewed from Earth 鈥� the star that Sahu and his team looked at moves the distance of about 25 millimetres when viewed from 2500 kilometres away. 鈥淚t鈥檚 so astonishing that we can measure an angle that small, but with the Hubble Space Telescope, it鈥檚 possible,鈥� says at STSI.
The first hints of this black hole鈥檚 existence were found in 2011, when a star appeared to be growing much brighter than normal.
The star was then observed over a period of almost seven years using the Hubble Space Telescope, which took more accurate, detailed readings to disentangle its motion. 鈥淵ou have to observe this event long enough to separate the ordinary straight-line motion of the background star from the extra deflection due to the foreground black hole,鈥� says Bond.
Sahu and his team calculated the mass of whatever was causing the light to bend and measured if it had contributed any light to the star鈥檚 apparent brightness. They found a lens that gave off no light and had a mass around seven times as massive as our sun, which had only one plausible explanation: an isolated black hole.
This isn鈥檛 the first time that astronomers have thought they had found an isolated stellar-mass black hole. Previous brightening events could be possible candidates, but they were less concrete observations.
鈥淵ou couldn鈥檛 really tell whether they were black holes or whether they were just very, very slow-moving, low-mass stars,鈥� says at the University of Cambridge. 鈥淏ut using this astrometry technique has broken that [uncertainty] in the modelling and it鈥檚 really quite exciting.鈥�
Rogue supermassive black holes have been found before, but the only way to detect them was by the light of the matter they consumed, which suggests they were surrounded by other cosmic objects. 鈥淭he very fact that we were able to 鈥榮ee鈥� those rogue supermassive black holes meant that they were surrounded by an accretion disk or star cluster. The case in point here is truly alone, and we鈥檙e only seeing it due to its gravitational effect on background light,鈥� says Reynolds.
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