Join the search for hidden black holes

Welcome to Three Alpha! In this edition of the newsletter we’re focusing on my day job, the Black Hole Hunters project. This is an article I wrote for the latest edition of E&O, the newsletter of the British Astronomical Association's Education and Outreach section. Read on for more…

The Milky Way is teeming with stellar black holes. These ultra-dense, ultra-mysterious objects form at the ends of the lives of the largest stars. We know how old the Galaxy is, and we know how long stars live before they die. From this, we know there should be millions, or maybe tens of millions, of stellar black holes in the Milky Way.

Should be.

In reality, we have only found about twenty stellar black holes in the Milky Way, with another 50 or so unconfirmed candidates. For each black hole we know about, there could be another million lurking, unseen and waiting to be discovered. Actually finding them will not be easy (they are black, after all), but recent advances in large-scale astronomical surveys have given us a fighting chance. Even with the best technology, we’re going to need your help. You might not think it, but you already have all the skills you need to uncover the Galaxy’s hidden black holes. The search is happening now on my Black Hole Hunters project.

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So why are black holes so tricky to find? Why have we managed to find 70 of them, while millions go undetected? The key is that the ones we’ve found are mostly in a special class of black holes which happen to be in very close orbits around stars. These are known as X-ray binaries: “binaries” because there are two objects orbiting each other, and “X-ray” because these black holes are emitting very bright X-ray radiation. That’s what makes them so easy to spot. The X-rays come from stellar matter which is being siphoned off the black hole’s companion star, due to their proximity, and heated by the extreme forces as it falls into the black hole. Most black holes are not in such close orbits with stars and therefore do not emit X-rays. Those black holes are essentially invisible.

Lucky for us, even invisible black holes affect their surroundings, due to the thing black holes are most famous for: their gravity. There are a few ways we can use black holes’ gravity to find them, but they mostly come down to looking for the way their gravity influences the stars near them. A star whose position wobbles a little might be orbiting an unseen companion. A star whose brightness changes in a certain way might be getting stretched and distorted by a companion. And so on.

There is also one technique that can let us discover black holes not by how they influence stars, but by what they do to the stars’ light. You might have heard of gravitational lensing. It’s the phenomenon where the immense gravity of large galaxy clusters can focus and magnify the light from much more distant galaxies. It’s not just galaxies and clusters of galaxies which can cause lensing. Individual objects, such as stars, planets, and black holes, can do the same thing in a phenomenon called microlensing. With microlensing, the focused images are too small for us to see, but we do see a brief increase in the brightness of the background star. Studying this magnification can tell us about the foreground object, the “lens”. The lens and the background object can be very far apart, even in different parts of the galaxy. The microlensing effect happens by chance when a distant object and a closer one happen to line up from our perspective as they move though space.

In Black Hole Hunters, we’re looking for a special kind of microlensing called self-lensing. This is where the lens and the background source are in orbit around each other, rather than being separated by a great distance. Unlike normal microlensing, self-lensing causes a repeating magnification (once per orbit) which makes it quite distinctive. Because of the way the geometry works out, self-lensing can only be caused by black holes, neutron stars, and white dwarfs, not normal stars or planets. That makes it a potentially great way of finding black holes. So far, a few self-lensing white dwarf systems are known, but we’re hoping to be the first to find self-lensing black holes and neutron stars.

By taking part in the project, maybe you could be the first one to spot a self-lensing black hole! All you need to do is look at graphs showing the brightness of stars over time, and tell us if you see anything that looks like microlensing. We’ll show you some examples so you know what to look for, and there is an active community of other volunteers who are always happy to help if you’re unsure of anything. Myself and other members of the research team also try to be as active as we can, and we’re always happy to answer questions about how to do the project and about the scientific background.

Check it out at black-hole-hunters.org and let’s see what we can find!

What is Three Alpha? Other than being the name of the newsletter you’re reading now, the name “three alpha” comes from the triple-alpha process, a nuclear chain reaction in stars which turns helium into carbon. Read more here.

Who writes this? My name is Dr. Adam McMaster. I’m an astronomer in the UK, where I mainly work on finding black holes. You can find me on BlueSky, @adammc.space.

Let me know what you think! You can send comments and feedback by hitting reply or by emailing [email protected].