Stealth black hole found lurking in the Milky Way

Our galaxy may harbour a plethora of quietly feeding black holes, with the discovery of a black hole gently grazing from a tiny star only 7,200 light-years from Earth.

Astronomers examined what was thought to be a distant galaxy but realised it was an object well within the Milky Way. They suggested it’s a black hole, a few times the mass of the sun, slowly pulling in material from companion star less than a fifth the size of the sun.

The work, published in The Astrophysical Journal, calculates there may be up to 170 million such quiet pairings in the galaxy – some 1,000 times more than previous estimates.

“If this is true, we’d like to know, because we might have to rethink some of the assumptions in our theoretical models of how these things form,” says James Miller-Jones from the International Centre for Radio Astronomy Research at Curtin University in Australia and co-author of the paper.

Discovered around 20 years ago, VLA J2130+12 was pegged as a galaxy hundreds of thousands of light-years beyond Messier 15, a group of stars in the Milky Way around 33,000 light-years from Earth.

But two years ago, while taking a closer look at Messier 15, Franz Kirsten – also from the International Centre for Radio Astronomy Research – found something odd about VLA J2130+12.

As Earth moves around the sun, heavenly objects appear to shift side to side, with those closer to us “wobbling” more than those distant. Miller-Jones likens this to holding a finger up at arm’s length and looking at it while winking one eye then the other. The finger appears to shift left and right, relative to the background.

And so the network of telescopes on Earth saw VLA J2130+12 was far wobblier than a distant galaxy should be. It had to be an object much closer to us – nearer, even, than Messier 15.

So Kirsten, Miller-Jones and colleagues – led by University of Alberta’s Bailey Tetarenko – decided to delve further. VLA J2130+12 clearly wasn’t a galaxy. So what was it?

They had a few ideas. Could it be a pulsar, a rotating neutron star? Or a planetary nebula, the expanding shell of gas around an ageing star? Or even a magnetar, a neutron star with an extremely strong magnetic field?

Armed with observations from radio, optical and X-ray space and ground-based telescopes, they started crossing options off the list. 

VLA J2130+12 was exceptionally radio-bright – meaning it emitted lots of radio waves, the longest wavelengths in the electromagnetic spectrum – but very little in the way of the shorter X-rays. This ruled out pulsars and white dwarf stars, which tend to pump out a different ratio of radio and X-rays.

Its radio signal was a dense, compact spot, so it couldn’t have been the fluffy ring of ejecta of a planetary nebula.

And because it’s situated 3,000 light-years from the plane of the Milky Way – the dense streak of stars you see in the night sky – it couldn’t have been a magnetar. Magnetars form from the collapse of massive stars, and these big guys tend to grow in the gas-rich galactic plane. But there’s no way a magnetar, which dies quickly, could have travelled that huge distance from its nursery without first fizzling away.

The most likely option, Tetarenko and colleagues decided, was somewhat less explosive or dynamic than their discounted options: a small black hole, quietly feeding on a dying little star. 

This was supported by archival optical Hubble Space Telescope images which also showed a faint, red star at the site.

When black holes feed voraciously from companion stars, they blast out jets of leftover matter and radiation which is normally picked up by telescopes as X-rays. But a nibbling black hole, or one that’s not feeding at all, dribbles out a few X-rays – too few to be detected by the telescopes that scan the sky daily for big explosive X-ray blasts.

While VLA J2130+12 “was a completely serendipitous discovery”, Miller-Jones says, astronomers may be treated to plenty more in the next few years.

The Very Large Array in New Mexico, 10-20 times more sensitive than it was before its upgrade a few years ago, will perform an all-sky survey for radio-bright objects.

“Some of these undiscovered black holes could be closer to the Earth than we previously thought,” says co-author Robin Arnason from the University of Alberta. 

But there’s no need to worry, he adds: “Even these black holes would still be many light years away from Earth.”