Earlier this yearastronomers used micro lens and the Hubble Space Telescope to detect for the first time a rogue black hole about 5,000 light-years from Earth. With more accurate measurements, they have now been able to determine the estimated mass of this elusive object. However, due to its surprisingly low mass, there is a chance that this object is not really a black hole.
The newly detected wandering object is about 5,000 light-years away, in our galaxy’s Carina-Sagittarius spiral arm. Two major international teams used Hubble data in their investigations to learn more about the object, OGLE-2011-BLG-0462/MOA-2011-BLG-191 or OB110462. A team was led by Kailash Sahu from the Space Telescope Science Institute in Baltimore, who led the team in the original discovery of the black hole. The second team was led by Casey Lam from the University of California, Berkeley. And while the results of the two teams differ slightly, both suggest the presence of a relatively compact object.
The amount of deflection from the object’s intense curvature of 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 is 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 love to say it’s definitely a black hole, we need to report all allowed solutions,” he said Jessica Luc of the Berkeley team. “This includes lower-mass black holes and possibly even a neutron star. Whatever it is, the object is the first dark stellar remnant discovered by the galaxy, unaccompanied by another star.”
However, there are other clues and features of this object that make the data lean toward being a black hole.
The story of this object begins in 2011 when Hubble data indicated that a star was getting brighter. This was determined to be caused by a foreground black hole drifting in front of the star along our line of sight. The star brightened and then faded to its normal brightness for several months as the black hole passed. Because a black hole does not emit or reflect light, it cannot be observed directly. But its unique thumbprint on the fabric of space can be measured through these microlensing events.
Dozens of astronomers from Sahu’s team have spent more than six years studying this object. And although astronomers have used gravity microlensing for about 30,000 events to date — studying objects like stars and exoplanets — a black hole’s signature stands out as unique among other microlens events.
The team said the black hole’s very intense gravity will extend the duration of the lens event to 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 when observing this object, no color change was observed. That’s why Sahu’s team published their paper earlier this year, claiming to have found a rogue black hole.
The existence of stellar black holes has been known since the early 1970s. And so far, all black hole masses have been inferred statistically either through interactions in binary systems or in the cores of galaxies. Since stellar-mass black holes are usually found along with companion stars, this new object is highly unusual.
An estimated 100 million black holes roam among the stars in our Milky Way galaxy, and this may be the first time an isolated black hole has actually been discovered. If it holds up as the discovery of a wandering black hole, astronomers could estimate that the closest isolated stellar-mass black hole to Earth could be 80 light-years away. For reference, the closest star to our solar system, Proxima Centauri, is just over 4 light-years away.
“Detections of isolated black holes will provide new insights into the population of these objects in the Milky Way,” Sahu said. He expects that what astronomers have learned from these observations will allow them to discover more free-roaming black holes in our galaxy.
But even using the amazing tool called microlensing, this would be like a needle in a haystack. Astronomers also predict that only one in every few hundred microlensing events is caused by isolated black holes.
“Astrometric microlensing is conceptually simple, but very difficult from an observation point of view,” says Sahu. “In addition, microlensing is the only available technique to identify isolated black holes.”
Therefore, the two teams — each with dozens of astronomers — will continue to study and track this object, hoping for more data and more microlensing events.
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