earlier this yearAstronomers used microlensing and the Hubble Space Telescope to detect for the first time a rogue black hole, some 5,000 light-years away from Earth. Now, with more precise measurements, they are 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 may not actually be a black hole.
The newly discovered wandering object lies about 5,000 light-years away in the Carina-Sagittarius spiral arm of our galaxy. Two large international teams used Hubble data to learn more about the object, OGLE-2011-BLG-0462/MOA-2011-BLG-191 or OB110462 for short). One team was led by Kailash Sahu of the Space Telescope Science Institute in Baltimore, who led the team in the original discovery of black holes. The second team was led by Casey Lam of the University of California, Berkeley. And while the two teams’ results differ slightly, both suggest the presence of a relatively compact object.
The amount of deflection from the object’s intense warping of space allowed Sahu’s team to estimate that it weighs seven solar masses. Lamm’s team reports a slightly lower mass limit, which means the object could be either a neutron star or a black hole. They estimate that the invisible compact object has a mass between 1.6 and 4.4 times that of the Sun. At the high end of this range, the object would be a black hole; At the lower end, it would be a neutron star.
“As much as we would like to say that this is definitely a black hole, we should report all accepted solutions,” said Jessica Lu of the Berkeley team. “It contains both a lower-mass black hole and possibly a neutron star. Whatever it is, the object is the first dark stellar remnant discovered to be swirling through the Milky Way with no other stars.”
However, there are other clues and features of this object that skew Data towards it being a black hole.
The story of this object begins in 2011 when Hubble data indicated a star brightening. It was determined that this was caused by a foreground black hole drifting in front of the star along our line of sight. The star brightened as the black hole passed and then later faded to its normal brightness over several months. Since a black hole does not emit or reflect light, it cannot be directly observed. But its unique thumbprint on the fabric of space can be measured through these microlensing events.
Dozens of astronomers in Sahu’s team have now spent more than six years studying the object. And while astronomers have used gravitational microlensing for about 30,000 events so far — the study of objects like stars and exoplanets — the signature of a black hole is unique among other microlensing events.
The very intense gravitational lensing of the black hole would extend the duration of the event to more than 200 days, the team said. Furthermore, if the object in the middle were instead a foreground star, it would cause a momentary color change in the star’s illumination as measured because the light from the foreground and background stars would be momentarily mixed together. But no color change was observed in viewing this object. So Sahu’s team published their paper earlier this year claiming to have found a rogue black hole.
The existence of stellar-mass black holes has been known since the early 1970s. And so far, all black hole masses have been estimated either statistically or through interactions in binary systems or in the cores of galaxies. Since stellar-mass black holes are commonly found with companion stars, this new object is very unusual.
It is estimated that as many as 100 million black holes roam among the stars in our Milky Way galaxy, and this is probably the first time that an isolated black hole has actually been discovered. If this is in the form of the discovery of a wandering black hole, astronomers will be able to estimate that the closest isolated stellar-mass black hole to Earth may be 80 light-years away. For context, the closest star to our Solar System, Proxima Centauri, is a little over 4 light years away.
“The detection of isolated black holes will provide new insights into the population of these objects in the Milky Way,” Sahu said. He hopes that what astronomers learn in these observations will allow them to uncover more free-roaming black holes inside our galaxy.
But even with the use of a wonderful tool called microlensing, it will be a needle-in-a-hystack discovery. Astronomers also predict that only one in a few hundred microlensing events is caused by isolated black holes.
“Astrometric microlensing is conceptually simple, but very difficult to observe,” Sahu said. “Additionally, microlensing is the only technique 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.
This article is originally from . was published on universe today by Nancy Atkinson. Read the original article here.
