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Astronomers estimate that 100 million black holes roam among the stars in our galaxy, the Milky Way, but they have never conclusively identified a single black hole. After six years of meticulous observations, NASA’s Hubble Space Telescope has, for the first time, provided direct evidence of an isolated black hole drifting through interstellar space thanks to a precise measurement of the object’s mass. phantom. Until now, all the masses of black holes have been deduced statistically, or by interactions in binary systems or in the hearts of galaxies. Stellar-mass black holes are usually found with companion stars, which makes this one unusual.
The newly detected wandering black hole lies about 5,000 light-years away, in the Carina-Sagittarius spiral arm of our galaxy. However, its discovery allows astronomers to estimate that the closest isolated stellar-mass black hole to Earth could be as close as 80 light-years away. The closest star to our solar system, Proxima Centauri, is just over 4 light years away.
The black holes that roam our galaxy are born from rare, monstrous stars (less than a thousandth of the galaxy’s stellar population) that are at least 20 times more massive than our Sun. These stars explode as supernovae and the remaining core is crushed by gravity into a black hole. Because the self-detonation is not perfectly symmetrical, the black hole can be kicked and blast through our galaxy like a blasted cannonball.
Telescopes cannot photograph a wayward black hole because it emits no light. However, a black hole warps space, which then deflects and amplifies starlight from whatever momentarily lines up exactly behind it.
Ground-based telescopes, which monitor the brightness of millions of stars in the rich star fields toward our Milky Way’s central bulge, look for a telltale sudden brightening of one when a massive object passes between us and the star. Then Hubble tracks the most interesting such events.
Two teams have used Hubble data in their investigations — a led by Kailash Sahu of the Space Telescope Science Institute in Baltimore, Maryland; and The other by Casey Lam of the University of California at Berkeley. The teams’ results differ slightly, but both suggest the presence of a compact object.
The distortion of space due to gravity from a foreground object passing in front of a star far behind will momentarily bend and amplify the background star’s light as it passes in front of it. Astronomers use this phenomenon, called gravitational microlensing, to study stars and exoplanets in the approximately 30,000 events observed so far in our galaxy.
The signature of a prominent black hole stands out as unique among other microlensing events. The very intense gravity of the black hole will extend the duration of the lensing event over 200 days. Also, if the intervening object were a foreground star instead, this would cause a transient color change in the starlight as measured because the foreground and background starlight would be momentarily mixed together. . But no color change was observed in the black hole event.
Next, Hubble was used to measure the amount of deviation of the image of the background star by the black hole. Hubble is capable of the extraordinary precision needed for such measurements. The star’s image was offset from where it would normally be by about a millisecond of an arc. This is equivalent to measuring the diameter of a 25 cent piece in Los Angeles seen from New York.
This astrometric microlensing technique provided information about the mass, distance and speed of the black hole. The amount of deflection due to the black hole’s intense space warping allowed Sahu’s team to estimate that it weighs seven solar masses.
Lam’s team reports a slightly lower mass range, meaning the object could 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 upper end of this range, the object would be a black hole; at the lower end it would be a neutron star.
“While we’d like to say it’s definitely a black hole, we should point out all allowed solutions. This includes both lower-mass black holes and possibly even a neutron star,” said said Jessica Lu of the Berkeley team.
“Anyway, the object is the first dark stellar remnant discovered wandering the galaxy unaccompanied by another star,” Lam added.
This was a particularly difficult measurement because there is a bright, unrelated star that is extremely close in angular separation to the source star. “So it’s like trying to measure the little movement of a firefly next to a bright light bulb,” Sahu said. “We had to meticulously subtract the light from the nearby bright star to accurately measure the deviation from the faint source.”
Sahu’s team estimates that the isolated black hole is moving through the galaxy at 100,000 miles per hour, or 160,000 kilometers (fast enough to travel from Earth to the Moon in less than three hours). This is faster than most other nearby stars in this region of our galaxy.
“Astrometric microlensing is conceptually simple but observationally very challenging,” Sahu said. “Microlensing is the only technique available to identify isolated black holes.” When the black hole passed a background star 19,000 light-years away in the galactic bulge, starlight coming towards Earth was amplified for a duration of 270 days as the black hole passed. However, it took several years of Hubble observations to figure out how the position of the background star seemed to be deflected by the bending of light by the foreground black hole.
The existence of stellar-mass black holes has been known since the early 1970s, but all of their mass measurements – so far – have been made 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 gravitational 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 reached 90 solar masses.
“Detections of isolated black holes will provide new information about the population of these objects in our Milky Way,” Sahu said. But it’s a needle in a haystack search. The prediction is that only one microlensing event in a few hundred is caused by isolated black holes.
NASA’s upcoming Nancy Grace Roman Space Telescope will discover several thousand microlensing events, many of which are expected to be black holes, and the deviations will be measured with very high precision.
In a 1916 paper on general relativity, Albert Einstein predicted that his theory could be tested by observing the Sun’s gravity compensating for the apparent position of a background star. This was tested by a collaboration led by astronomers Arthur Eddington and Frank Dyson during a solar eclipse on May 29, 1919. Eddington and his colleagues measured a background star shifted by 2 arcseconds, validating the theories of Einstein. These scientists could hardly have imagined that more than a century later, this same technique would be used – with unimaginable accuracy a thousand times better – to search for black holes across the galaxy.
The Hubble Space Telescope is an international cooperation project between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland operates the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.