Stealthy Black Hole Spotted by Accident

Using a fleet of telescopes both on the ground and orbiting the Earth, astronomers have discovered what is very likely to be a black hole only a few thousand light-years from Earth.

On its own that’s not terribly newsworthy; we know of lots of black holes in the Milky Way galaxy. What makes this one so very interesting is that it has been hiding in plain sight, located in the sky near a gorgeous globular cluster, masquerading as a distant galaxy.

And even that’s not the most interesting thing. The most interesting thing is what this implies for the total number of such black holes in the Milky Way. There could be a lot more than we first thought. A lot more.^*

The object has the handy nickname of VLA J213002.08+120904 (a combination of VLA for Very Large Array, the radio observatory that discovered it, and its coordinates on the sky). It’s also called M15 S2, which I find amusing. Let me explain, because this is fun.

Globular clusters are collections of hundreds of thousands of stars in close proximity to each other. M15 is a glorious example of such a beast; about 30,000 light-years away, quite bright and well-studied. In 1996, a group of astronomers scanned the cluster using the VLA, looking for gas flowing into it. By accident they found a point source of radio emission in the outskirts of the cluster, but they weren’t sure what it was. Some astronomers thought it might be a distant galaxy coincidentally aligned in the sky with the cluster.

In 2014, another team of astronomers observed this object (which they designated S2, for the second unidentified point source of radio emission in M15) using a collection of radio telescope across the planet, and using parallax (the object’s apparent motion in the sky which is actually a reflection of the Earth’s motion around the Sun) found it was located about 7,000 light-years away—one-fifth the distance to M15! Clearly this was neither a distant galaxy nor a weird star in the cluster. That distance puts it firmly inside our own galaxy. The plot thickens.

Finally, another team of astronomers looked at observations from a variety of telescopes, hoping to nail down the identification of this thing. They found it in more radio observations from VLA, and interestingly Hubble Space Telescope observations show what looks to be a faint red source at that position. The kicker is that they also looked in data of M15 taken by the orbiting Chandra X-Ray Observatory, and found nothing; whatever it is, it’s too faint to be seen in even a 30-hour observation.

All this put together points toward a very peculiar object: a black hole orbiting a tiny red dwarf star. If the black hole has a mass typical of such objects, about 10 times the mass of the Sun, the red dwarf is about 0.1 to 0.2 times the Sun’s mass (also typical). It’s likely the two orbit each other very closely, with an orbital period of roughly one to two hours. That’s close enough together that the star is grossly distorted by the black hole’s gravity, and material is flowing from it to the hole.

When material does this, it tends to form a disk around the black hole called an accretion disk. It piles up there before falling in to the black hole. The disk can be very hot, glowing brightly in X-rays, and also blast out a wind of subatomic particles. In this case though, the disk appears to be very weak and not as hot, which is why it’s not bright in X-rays even though its wind (or possibly other structures called jets) generates radio emission.

Other types of objects can be bright in radio and quiet in X-rays, but each one of these was systematically eliminated from consideration by the astronomers; for example a planetary nebula would show fuzziness in the Hubble images, but none is seen. In the end, the best candidate is indeed a low-mass binary black hole system.

Assuming this truly is as advertised, it’s the first accreting X-ray quiet black hole binary ever seen outside of a globular cluster. It’s also one of the lowest mass black hole binaries known. And that has some very cool implications.

This object was found essentially by accident in a radio observation of a globular cluster. But it’s not in the cluster; it’s much closer and just happens to lie near it in the sky—that’s fortunate, otherwise it may never have been discovered! Given that one of these was found in a very small region of the sky observed, that implies there are a lot of them scattered throughout the galaxy. Using some simple assumptions, the astronomers find that there may be anywhere from 25,000 to 150 million such low-mass black hole binaries in the Milky Way alone!

That’s a lot. It’s a wide range because of a lot of the uncertainties involved, but even on the low end, that’s three times more than predicted by looking at how stars are born and evolve over time (you need a low-mass star orbiting a high-mass one, so that the bigger one blows up and forms a black hole at the end of its life). And on the high end it’s thousands of times the number predicted.

How can the numbers be so far off? That’s a good question, and it’s not clear. Maybe this really was a very lucky observation, and just happened to spot one such object even though the odds were long. Maybe our stellar formation models are off. Either way, it shows us we need to search for more of these objects so we can figure out what’s going on. That’s not easy, as they’re usually found in X-ray surveys of the sky, and they’re X-ray quiet. Looking for the kind of radio emission these types of objects give off specifically will help, as will looking toward the galactic center for them, since they should be more common where there are more stars. A new generation of more sensitive X-ray telescopes coming soon will do well here, too.

I think this is all wonderful. It’s still possible to discover new objects by accident today and use them to learn important characteristics of the galaxy in which we live. And if it means more black holes to study, hooray! Getting a bigger sample of the weirdest objects in the Universe can only be a good thing for science and our understanding of the cosmos.

^*Before you freak out, not to worry: Space is big, and even if there are zillions of these things out there, over the entire lifetime of the Universe the chance of any getting near enough to Earth to hurt us is essentially zero.