Newly discovered 'stealth' black hole suggests hidden population

Quiet black hole likely among tens of thousands to millions lurking undetected in the Milky Way

News Staff - 27 June 2016

An international group of astronomers led by UAlberta physicists have combined data from NASA's Chandra X-ray Observatory, the Hubble Space Telescope, and the National Science Foundation's Karl G. Jansky Very Large Array to conclude that a peculiar source of radio waves is a binary star system containing a low-mass star and a black hole. The findings suggest that there may be a vast number of black holes in our galaxy that have gone unnoticed until now.

For about two decades, astronomers have known about an object called VLA J213002.08+120904 (VLA J2130+12 for short), close to the line of sight to the globular cluster M15 and thought to be a distant galaxy because of its bright radio waves.

"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."-Arash Bahramian

Thanks to recent distance measurements with an international network of radio telescopes including the European VLBI (Very Long Baseline Interferometry) Network telescopes, the National Science Foundation's Green Bank Telescope, and Arecibo Observatory, astronomers realized that VLA J2130+12 is at a distance of just 7,200 light years-well within our own Milky Way galaxy and about five times closer than M15. A deep image from the Chandra observatory reveals it can only be giving off a very small amount of X-rays, while recent data from the Very Large Array indicates the source remains bright in radio waves.

This new study indicates that VLA J2130+12 is a black hole a few times the mass of our Sun that is very slowly pulling in material from a companion star. At this paltry feeding rate, VLA J2130+12 was not previously flagged as a black hole since it lacks some of the telltale signs that black holes in binaries typically display.

"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," says the study's lead author, UAlberta PhD candidate Bailey Tetarenko. "This one is so quiet that it's practically a stealth black hole."

This is the first time a black hole binary system outside of a globular cluster has been initially discovered while it is in such a quiet state.

m15 stealth black hole

By combining data from Chandra and several other telescopes, astronomers have identified the true nature of an unusual source in the Milky Way galaxy. This discovery implies that there could be a much larger number of black holes in the galaxy that have previously been unaccounted for. The main panel shows X-rays from Chandra (purple) that have been overlaid on an optical image from Hubble. The insets show the source is bright in radio waves, but can only be giving off a very small amount of X-rays. These pieces of information indicate the source contains a black hole with a few times the mass of the Sun. Credits: X-ray: NASA/CXC/Univ. of Alberta/B.Tetarenko et al; Optical: NASA/STScI; Radio: NSF/NRAO/VLA/Curtin Univ./J.C.A.Miller-Jones et al

Hubble observations identified that the star in VLA J2130+12 has only about one-tenth to one-fifth the mass of the Sun. The observed radio brightness and the limit on the X-ray brightness from Chandra allowed the researchers to rule out other possible interpretations, such as an ultra-cool dwarf star, a neutron star, or a white dwarf pulling material away from a companion star.

Because this study only covered a very small patch of sky, the implication is that there should be many of these quiet black holes around the Milky Way. Estimates suggest that tens of thousands to millions of these black holes could exist within our galaxy-anywhere from three to thousands of times as many as previous studies have suggested.

"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," says co-author Arash Bahramian, another graduate student at the University of Alberta.

Sensitive radio and X-ray surveys covering large regions of the sky will need to be performed to uncover more of this missing population.

If, like many others, this black hole was formed in the plane of the Milky Way's disk, it would have needed a large kick at birth to launch it to its current position about 3,000 light years above the plane of the galaxy.

"From start to finish, University of Alberta graduate students Bailey, Arash, and Robin Arnason [now at Western University] played the key roles in discovering the evidence of this missing population and working with our international team of collaborators to understand this source," says Tetarenko's PhD supervisor, Assistant Professor of Physics and co-author Gregory Sivakoff. "Future graduate students here will continue to be at the forefront of the research to reveal this population of black holes in our Milky Way."

The results of the study have been published in The Astrophysical Journal, and included researchers from University of Alberta, Western University, International Centre for Radio Astronomy Research, Curtin University, Technion-Israel Institute of Technology, Texas Tech University, Michigan State University, and the Chalmers University of Technology. NASA's Marshall Space Flight Center in Huntsville, Alabama manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations. The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.