Earlier this yearastronomers used microlensing and the Hubble Space Telescope to discover, for the first time, a rogue black hole about 5,000 light-years from Earth. Now, with more accurate measurements, they have been able to determine the approximate mass of this difficult-to-detect object. However, the surprisingly low mass means that there is a chance that this object is not actually a black hole.
The newly discovered wandering object is about 5,000 light-years away, in our galaxy’s spiral arm Carina-Sagittarius. Two large international teams used Hubble data in their surveys to find out more about the object, OGLE-2011-BLG-0462 / MOA-2011-BLG-191 or abbreviated 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 other team was led by Casey Lam at the University of California, Berkeley. And although the results of the two teams differ slightly, both indicate the presence of a relatively compact object.
The amount of deflection of the object’s intense distortion of space made it possible for Sahu’s team to estimate that it weighs seven solar masses. Lam’s team reports a slightly lower mass range, which means that the object can be either one 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. In the upper part of this area, the object would be a black hole; in the lower part it would be a neutron star.
“As much as we would like to say that it is definitely a black hole, we must report all permissible solutions,” he said. Jessica Lu from the Berkeley team. “This includes both lower-mass black holes and possibly even a neutron star. Whatever it is, the object is the first dark star residue discovered to have migrated through the galaxy, unaccompanied by another star.”
However, there are other clues and properties of this object that cause the data to lean towards it being a black hole.
The story of this object begins in 2011 when Hubble data indicated a star that brightened. It was determined that this was caused by a black hole in the foreground that drifted in front of the star, along our line of sight. The star brightened and then faded over several months back to its normal brightness when the black hole passed. Because a black hole does not emit or reflect light, it cannot be directly observed. But its unique thumbprint on the structure of space can be measured by these microlensing events.
Dozens of astronomers in Sahu’s team have now been working for over six years studying this object. And while astronomers have used gravitational microlensing for about 30,000 events to date – studying objects such as stars and exoplanets – the signature of a black hole stands out as unique among other micro-lensing events.
The team said that the very intense gravity of the black hole will extend the duration of the lensing event to over 200 days. In addition, if the intermediate object were instead a foreground star, it would cause a transient color change in the starlight being measured because the light from the foreground and background stars would be temporarily mixed together. But no color change was seen when this object was observed. It is therefore Sahu’s team published their magazine earlier this year and claimed to have found a frivolous black hole.
The occurrence of black holes with star mass has been known since the early 1970s. And until now, all black masses of holes have been assumed statistically or through interactions in binary systems or in the nuclei of galaxies. Because black holes with star mass are usually found with companions, this new object is very unusual.
It has been estimated that 100 million black holes roam among the stars in our Milky Way galaxy, and this is potentially the first time an isolated black hole has actually been discovered. If it holds up as the discovery of a wandering black hole, astronomers would then be able to estimate that the nearest isolated black hole with stellar mass to Earth could be as close as 80 light-years away. For reference, the nearest star to our solar system, Proxima Centauri, is just over 4 light-years away.
“Detection 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 in these observations will allow them to reveal more freely roaming black holes inside our galaxy.
But even with the use of the amazing tool called microlensing, this would be a needle-in-a-stack search. Astronomers also predict that only one in a few hundred microlensing events is caused by isolated black holes.
“Astrometric microlensing is conceptually simple but observationally tough,” Sahu said. “In addition, microlensing is the only technology available to identify isolated black holes.”
That’s why the two teams – each with dozens of astronomers – will continue to study and monitor this object, hoping for more data and more microlensing events.
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