Astronomers baffled by ‘stealth black hole’ discovery; Potent radio waves may represent new black hole population

Astronomers have found out that a mysterious source of radio waves thought to be from a different galaxy is actually a binary star system containing a black hole and a low mass star. The discovery suggests that a huge number of black holes in our Galaxy Milky Way have gone unnoticed.

The scientists combined data from National Science Foundation's Karl G. Jansky Very Large Array (VLA), Hubble Space Telescope and NASA’s Chandra X-ray Observatory to come to the conclusion that the object VLA J2130+12 is well within our galaxy and about five times closer than the globular cluster M15. The results of the study appear in a paper in The Astrophysical Journal.

For a long time, astronomers thought that this source of radio waves was a distant galaxy. However, recent distance measurements with an international network of radio telescopes, which included the Arecibo Observatory, European Very Long Baseline Interferometry Network (EVN) and NSF's Green Bank Telescope, have shown that the object is at a distance of 7,200 light years.

A Chandra image has showed that VLA J2130+12 is giving only a little amount of X-rays though VLA data shows the source is bright in radio waves.

“Usually, we find black holes when they are pulling in lots of material. Before falling into the black hole this material gets very hot and emits brightly in X-rays. This one is so quiet that it's practically a stealth black hole,” Bailey Tetarenko of the University of Alberta, Canada, who led the study, said in a statement.

According to the scientists, this is a first-of-its-kind find where a black hole binary system has been initially discovered outside of a globular cluster while it is in such a quiet state. The object also has a star that has one-tenth to one-fifth the mass of our sun.

“Unless we were incredibly lucky to find one source like this in a small patch of the sky, there must be many more of these black hole binaries in our Galaxy than we used to think,” said University of Alberta’s Arash Bahramian, who is also the co-author of the study.

In order to detect more such stealth black holes, X-ray and sensitive radio surveys that will cover large regions of the sky have to be performed.