Astronomers have long suspected a strange object known as VLA J213002.08+120904 (kindly rounded to VLA J2130+12) to be a far away galaxy, well beyond the M15 globular cluster that appears next to it in the night sky.
But by using an international network of radio telescopes to measure its distance from Earth, astronomers have found that VLA J2130+12 is actually five times closer to Earth than M15. At 7,200 light years away, it falls squarely within our very own Milky Way.
What’s more, it turns out that VLA J2130+12 is not a galaxy at all but actually a small black hole, only a few times the mass of our Sun, gradually slurping up its companion star. Images from NASA’s Chandra X-ray Observatory reveal that while VLA J2130+12 is a strong source of radio waves, it emits very weak X-rays – an important clue that rules out other possibilities such as a neutron star or white dwarf.
Black hole binary systems like this aren’t uncommon, but they tend to be much easier for astronomers to find. This is the first time such a “quiet” black hole has been observed.
“Usually, we find black holes when they are pulling in lots of material” says Bailey Tetarenko from the University of Alberta, Canada, who led the study. “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.”
Given that the study only looked at a sliver of the night sky, the odds are that many similar black holes are also going about their stealthy business within our galaxy – up to thousands of times more than we previously thought. And not only are there more of them, but they’re much closer to us too.
“However there’s no need to worry,” explains Robin Arnason, a co-author from Western University in Canada. “Even these black holes would still be many light years away from Earth.” That’s certainly good to know.
Next steps will be for astronomers to expand radio and x-ray surveys to sweep larger portions of the sky in order to get a better picture of exactly how many of these black hole binaries there are out there, and where.
In the meantime, contemplate the awesome possibility that every time you gaze into the black spaces between stars, a horde of black holes stare back at you.