A team of astronomers has examined a potentially new source of gravitational waves, and discovered it’s possible — maybe — it could be detected with currently working instruments. The source would be the lumpy disk of material swirling madly around a black hole right after it forms^*.

First things first: Gravitational waves were the last prediction made by Einstein’s theory of relativity that remained unproven, at least until 2015 (and announced a year later after a lot of analysis). The idea is that what we think of as space (or spacetime) can be warped, distorted, by masses in it. That distortion is what we perceive as gravity. So it’s not so much that Earth pulls us down toward it so much as it makes a dimple in space and we tend to want to slide down it. Weird, I know. But it works.

And it gets weirder. If you accelerate a massive object, it not only dents space but also creates ripples in spacetime, called gravitational waves. It’s similar to waves created when you toss a rock in a pond; gravitational waves are ripples that move outward, away from the source at the speed of light. Since they actually distort space, they can be measured as they pass through a region of space. Space shrinks and expands as the waves pass by, and if you had a very accurate ruler, for example, you could measure that oscillation.

Astronomers have built just such a detector, called LIGO (Laser Interferometer Gravitational-wave Observatory). I’ve written about it many times; it detected the first gravitational waves in 2015 (there are other observatories that are part of a global collaboration with LIGO, too, and ESA is building a space-based version called LISA that will be freaking amazing… and astronomers can even use pulsars in the galaxy to look for these waves, which is pretty metal). Now here’s an important thing: Any accelerating mass makes GWs (please accept that abbreviation so I don’t have to type it out every dang time), but they tend to be mushy, spread out and weak. The waves get much sharper and stronger a) the more massive the objects are, and 2) the harder they’re accelerated. That’s why almost all the GWs detected have been from merging black holes: they’re very massive indeed, and as they merge they are whipped around each other at nearly the speed of light. 

We’ve also detected less massive but still ridiculously dense neutron stars merging as well. But could there be still lower mass objects that generate detectable GWs?

That’s what the new study looks at [link to journal paper]. But this time, instead of looking at how black holes merge, they look at how they form.

When a very massive star dies, the core collapses, forming a neutron star or, if it’s massive enough, a black hole (the outer layers explode outward, generating the brilliant supernova display). As it happens there are a lot of variations on how this can play out. If the star is rotating very rapidly, the outer parts of the core can escape the immediate collapse. As the black hole forms beneath them, the material from the outer core forms a thick, incredibly dense disk of matter that orbits the black hole. It likely doesn’t last long before it falls in, minutes or hours, but during that time it’s whirling at very high speed around the black hole. Such an object is (confusingly) called a collapsar.

If the disk is perfectly symmetric, an exact ring, then it won’t emit GWs. But if there’s a lump in it, some sort of asymmetry, then it can. As it happens, natural instabilities in the disk tend to form overdense regions, lumps in the disk, where some matter is clumped up a bit, and these grow rapidly (they’re called Rossby instabilities, if you want to know more). As this off-kilter lump orbits around the black hole, it does emit GWs.

[An actual physics-based model was used to make this simulation of the formation and behavior of a collapsar around a newly born black hole. Credit: Ore Gottlieb]

The new work looks into the (very, very complex) physics of the situation to see what kind of GWs are emitted. For some circumstances, they find the GWs emitted are powerful enough and at the right frequency that LIGO could detect them! They’d have to be within about 50 million light-years of Earth, but a lot of galaxies are in that volume. Given the known supernova rate, they predict that very roughly one such detectable GW-producing event should happen every year. LIGO’s been in operation for about a decade, which means it may have already seen a few of these events!

It’s not that easy, though. The signals aren’t nearly as strong as merging black holes, so they’re harder to find. Also, it’s not like you can just scan the data and look for a blip. They data are noisy, so the way scientists currently find GWs is to use known physics to create a zillion templates of GWs made from different kinds of black holes merging: ones of all different masses, distances, orbital shapes, and so on. They then use those to go through the data and look for a pattern match. It’s done by computer, of course, but it’s still a bit tedious.

The problem here is that the models the scientists used to simulate these collapsed-star disks is incredibly intensive, and they can’t just run them a million times to make all the different flavors of GW shapes that can be made. That makes finding them difficult. But they do have some, and it’s possible in theory to look through LIGO data to search for similar waves. In reality though it may have to wait until more models are made.

If a signal like this can be found, it will likely be the first ever seen from a non-merger event, which in itself is cool. But more than that it can reveal details about what happens when a massive star collapses that otherwise might be hidden from us. That’s important for several reasons. One is that it’s awesome. Another is that we don’t understand all the details of how black holes form (an understatement) and this can help. Also, these events create heavy elements, like the kinds we are literally made of (iron, calcium, and so on), as well as help make new stars and planets. If we want to understand — in a very literal sense — where we came from and how we got to be here, then looking at core collapse supernovae is the way to go. 

And now we may have yet another way to do just that.

^* This is something we know already exists, as opposed to, say, warp drives.

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