Two research teams have now independently concluded that in 2011 the Hubble Space Telescope found the trail of the first solitary black hole in the Milky Way. However, both groups come to different values for the object, which could therefore also be a neutron star. The group led by Casey Lam and Jessica Lu from the University of California, Berkeley actually assumes that they have discovered a black hole that is 1.6 to 4.4 times as massive as our sun. It could be the first known of 200 million suspected to be in our home galaxy. Another research group had come up with a mass of 7.1 suns.
First of hundreds of millions?
Both teams examined a star that had become significantly brighter for a remarkably long period of time in 2011. This was the result of the so-called microlensing effect, in which an object in front of a star ensures that its light rays are bent on the way to us in such a way that the star appears brighter or enlarged to us. The 2011 brightening was one of five that another team had said was not caused by a black hole. This has piqued Lam and Lu’s curiosity, because if it were true, existing models of the number of such lonely black holes would have needed to be adjusted, they write. It is simply unlikely that none of the five events could actually be attributed to a black hole.
In fact, close analysis of the data suggested that at least one of the observations was due to a black hole – MOA-2011-BLG-191/OGLE-2011-BLG-0462. Most telling was the length of the brightening, as models suggest that such lonely black holes should trigger many of the more than 120-day brightenings – OB110462 lasted 270 days. Using additional astrometric data from the Hubble telescope, Lam and Lu then found that the star’s apparent change in position due to the microlensing is still measurable ten years later. Then they would have confirmed that a stellar black hole is responsible. Since such an object must have a mass of at least 2.2 suns, they cannot rule out a neutron star either.
The brightened star
(Image: NASA, ESA, K. Sahu (STScI), J. DePasquale (STScI))
This makes it more likely that OB110462 really does go back to a black hole that travels lonely and therefore particularly difficult to detect through the Milky Way. This was first determined by a team from the Space Telescope Science Institute, and their previously published work has now been accepted for publication in the Astrophysical Journal. Lam and Lu publish their analysis in The Astrophysical Journal Letters. The fact that their results for the properties of the object differ from one another is due to the different interpretation of the data. This also results in different information about the distance of the black hole and its speed. Further analysis should show who is right.
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