Black holes are inherently invisible unless they are part of a binary star system or surrounded by an accretion disk. Most star-sized black holes are not, but astronomers have looked for them through gravitational micro events, in which black holes brighten and twist light from stars into the center of the Milky Way. A UC Berkeley-led team may have discovered the first free-floating black hole, although more data are needed to rule out neutron stars. If the death of large stars leaves black holes behind, as astronomers believe, there should be hundreds of millions of them scattered throughout the galaxy. The problem is that isolated black holes are invisible. Now, a team led by astronomers at the University of California, Berkeley, has discovered for the first time what a free-floating black hole might be -- so -- called gravitational events, by watching the brightness of more distant stars distorted by the object's strong gravitational field.
The team, led by graduate student Casey Lam and UC Berkeley associate professor of astronomy Jessica Lu, estimated the invisible, dense object to be between 1.6 and 4.4 times the mass of the Sun. Because astronomers believe the remnants of a dead star must be heavier than 2.2 solar masses to collapse into a black hole, researchers at the University of California, Berkeley, warn that the object may be a neutron star rather than a black hole. Neutron stars are also dense, very dense objects, but their gravity is balanced by the pressure of neutrons inside, which prevents further collapse into black holes. Whether it's a black hole or a neutron star, the object is the first dark remnant of a star -- a stellar "ghost" found while wandering the Milky Way without pairing with another star. This is the first free-floating black hole or neutron star discovered using gravitational length, Lu said. Using length, we were able to inspect and weigh these solitary, compact objects.
The analysis by Lam, Lu and their international team has been accepted for publication in The Astrophysical Journal Letters. The analysis included four other micro length events that the team concluded were not caused by black holes, although two could have been caused by white dwarfs or neutron stars. The team also concluded that the number of black holes in the Milky Way could be 200 million — about as predicted by most theorists.
Both teams used the same data: a photometric measurement of the brightness of a distant star as its light is distorted, or "length," by an ultra-dense object, in the distant star's position in the sky due to gravitational distortions measured the object being length. Photometric data were obtained from two micro length investigations: the Optical Gravitational Length Experiment (OGLE) using the 1.3-meter telescope operated by the University of Warsaw in Chile, and the Micro length Observation (MOA) experiment in astrophysics.
Because both micro length measurements captured the same object, it has two names: MOA-2011-BLG-191 and OGLE-2011-BLG-0462, or OB110462 for short.
While surveys like this one each year find about 2,000 stars in the Milky Way illuminated by length, the addition of metric data allowed the two teams to determine the mass of the dense object and its distance from Earth. The UC Berkeley-led team estimates it to be between 2,280 and 6,260 light-years (700-1920 parsecs) from the center of the Milky Way and the vicinity of the large bulge surrounding the massive black hole at the center of the Milky Way.
You must be logged in to post a comment.