†Nanowork NewsAstronomers estimate that 100 million black holes roam among the stars in our Milky Way galaxy, but they have never definitively identified an isolated black hole. After six years of meticulous observations, NASA’s Hubble Space Telescope has provided direct evidence for the first time ever of a lone black hole drifting through interstellar space through an accurate mass measurement of the phantom object. Until now, all black hole masses have been inferred statistically, either through interactions in binary systems or in the cores of galaxies. Stellar-mass black holes are usually found with companion stars, which makes them unusual.
The newly discovered wandering black hole is located about 5,000 light-years away, in our galaxy’s Carina-Sagittarius spiral arm. However, its discovery allows astronomers to estimate that the closest isolated stellar-mass black hole to Earth could be 80 light-years away. The closest star to our solar system, Proxima Centauri, is just over 4 light-years away.
Black holes that roam our galaxy are born from rare, monstrous stars (less than one-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 could be kicked and blasted through our galaxy like a blown cannonball.
Telescopes cannot photograph a wayward black hole because it emits no light. However, a black hole distorts space, which then bends and amplifies the starlight from whatever is temporarily just behind it.
Ground-based telescopes, which track the brightness of millions of stars in rich star fields toward our Milky Way’s central bulge, look for a telltale flash from one of them when a massive object passes between us and the star. Then Hubble follows up on the most interesting events.
Two teams used Hubble data in their study — one led by Kailash Sahu of the Space Telescope Science Institute in Baltimore, Maryland (“An isolated black hole with stellar mass detected by astrometric microlensing”† and the other by Casey Lam of the University of California, Berkeley (“An isolated black hole or neutron star detected with astrometric microlens”† The results of the teams differ slightly, but both indicate the presence of a compact object.
The curvature of space due to the gravity of a foreground object passing in front of a star far behind will temporarily bend and amplify the background star’s light as it passes in front of it. Astronomers use the phenomenon of gravitational microlensing to study stars and exoplanets in the approximately 30,000 events observed in our galaxy so far.
The signature of a foreground black hole stands out as unique among other microlens events. The black hole’s very intense gravity extends the duration of the lensing event for more than 200 days. If the intervening object were instead a foreground star, it would cause a temporary color change in the starlight, as measured, because the light from the foreground and background stars would temporarily merge. But no color change was seen during the black hole.
Hubble was then used to measure the deflection of the image of the background star by the black hole. Hubble is capable of the extraordinary precision required for such measurements. The star’s image was shifted about a milli arcsecond from where it would normally be. That’s equivalent to measuring the diameter of a 25 cent coin in Los Angeles when viewed from New York City.
This astrometric microlens technique provided information about the mass, distance and speed of the black hole. The amount of deflection from the intense curvature of the black hole’s space 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 top of this range, the object would be a black hole; at the bottom it would be a neutron star.
“As much as we’d like to say it’s definitely a black hole, we need to report all of the allowed solutions. 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 remnant discovered by 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 angularly to the source star. “So it’s like trying to measure the tiny movement of a firefly next to a bright light bulb”Said said. “We had to accurately subtract the light from the nearby bright star to accurately measure the deflection of the faint source.”
Sahu’s team estimates that the isolated black hole is traveling through the galaxy at 100,000 miles per hour or 160,000 kilometers per hour (fast enough to travel from Earth to the moon in less than three hours). That’s faster than most other neighboring stars in that part of our galaxy.
“Astrometric microlensing is conceptually simple, but very difficult from an observation point of view”says Sahu. “Microlensing is the only technique available to identify isolated black holes.” As the black hole passed in front of a background star 19,000 light-years away in the galactic bulge, the starlight coming to Earth was amplified for 270 days as the black hole passed. However, it took several years of Hubble observations to track how the background star’s position appeared to be deflected by the deflection of light by the foreground black hole.
The existence of stellar black holes has been known since the early 1970s, but all their mass measurements – until now – have taken place 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 measured their masses in X-ray binaries because of 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 been as large as 90 solar masses.
“Detections of isolated black holes will provide new insights into the population of these objects in our galaxy”Said said. But it’s like looking with a needle in a haystack. It is predicted that only one in every few hundred microlensing events is caused by isolated black holes.
NASA’s upcoming Nancy Grace Roman Space Telescope will discover thousands of microlens events, many of which are expected to be black holes, and the deflections will be measured with very high accuracy.
In a 1916 paper on general relativity, Albert Einstein predicted that his theory could be tested by observing the sun’s gravity offsetting 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 that was shifted 2 arcseconds, confirming Einstein’s theories. These scientists could hardly have imagined that more than a century later the same technique would be used – with an unimaginable precision a thousand times better – to search for black holes in the galaxy.
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