Gravitational Waves and Gamma-ray Bursts

There are (at least) three paths that lead to excitement in the LIGO data, so understanding why all of the scientists all over the world are so excited can be a little confusing. I’m going to try to walk through them (briefly) at the request of a friend.

Just to put this in perspective, nearly everyone who works in any kind of astronomy on the planet is working on this right now, and the nuances of what’s interesting and/or important won’t become clear for another few years.

That being said, no one has ever accused me of not liking to talk (at length) about something that’s interesting (and that I know a little about), so here we go.

Didn’t we already see gravitational waves?

Yes, LIGO has seen gravitational waves before. A couple of them. But now that we know that gravitational waves exist, the hard part is trying to actually use them to study what’s happening in the Universe.

What’s exciting about this event (just the gravitational wave side of the ledger) is that this is the first time that we’ve seen a signal coming from the merger of two neutron stars rather than from two black holes. Here’s a simulation of what that looks like over the last few seconds of the life of the binary neutron star system (fair warning, “epic” music going on here, you may want to turn off the sound on your speakers):

And this is not a subtle difference in the gravitational wave signal that we see. Unlike the previous mergers that lasted a second or two, LIGO was able to track the signal from this black hole for almost a minute. If you take the signal that LIGO measured in gravitational waves and plug it into a speaker, you get something that sounds like this (turn your speakers back on, and wait for it):

Movie Credit: LIGO/University of Oregon/Ben Farr

Okay, so if that’s all that had happened, there would have been a press conference, we all would have said “Oh cool, that’s totally amazing!” and moved on. However, the exciting thing about neutron stars is that they are made of stuff that can interact.

Two black holes merging is a relatively “clean” environment. All of the gas and out (we think) has already been swallowed up by the black holes, so once they merge you don’t expect to see anything other than the gravitational wave signal.

Neutron stars as messy. As the name suggests, they’re made of neutrons (the kind that are in the nucleus of an atom) and when you slam them together you make all of kind of other cool stuff (heavy elements like gold, platinum, uranium, etc). But for now the important thing is just that there’s stuff there. When stuff slams into other stuff you can release a lot of energy in an explosion. Which leads us to…

The gamma-ray burst

Gamma-ray bursts were the gravitational waves of the 1980s and 1990s (well, gravitational waves were the gravitational waves of the 1980s and 1990s, but gamma-ray bursts were the new, unexplained thing on the block). These are massive events that outshine entire galaxies for a few seconds. And we don’t really know what they are.

However, there is one type of gamma-ray burst (so-called “short” gamma-ray bursts with durations lasting less than a second) where the best explanations that we had for them was that they are the result of two merging neutron stars. Even so, these things are hard to see (your space telescope has to see the right part of the sky and you have be to in the “beam” of the gamma-ray burst to see the explosion). So it was a little surprising when (at the same time as the gravitational wave signal was seen), the Fermi and INTEGRAL space telescopes reported a short gamma-ray burst.

And I want to be clear, it’s not like the two groups got together to see if anything had been seen; LIGO found a neutron star gravitational wave signal and completely independently, Fermi also found a short gamma-ray burst. So, we got really lucky, or we don’t quite understand the nature of gamma-ray bursts. Or both.

Okay, if that is all that happened, we all would have said “Oh that’s really, really cool, we definitely know that LIGO works and the short gamma-ray bursts are neutron star mergers!” But that’s not all.

The follow-up campaign

Astronomers have been waiting decades for this moment. One of the ways that we able to study gamma-ray bursts in the 1990s was to get a ground-based optical telescope to try to find the explosion hours or minutes after the gamma-rays were detected. This has led to a full industry of people who build robotic telescopes that slew all over the night sky, ready to pounce if something interesting goes boom in the night.

Because of the physics of how the LIGO gravitational wave detectors and the Fermi gamma-ray detectors work, neither can give particularly good localizations for a source. This means that astronomers with more conventional telescopes have to be either clever, or lucky, to find the optical and radio emission (which we call the “counterpart” to the gravitational wave signal).

In this case, LIGO and Fermi were able to pinpoint a set of 49 galaxies as likely candidates. Everyone in the world who had a telescope raced off to try to find the counterpart. The short version is that we found it. We saw the counterpart in optical light and in radio waves. This has allowed astronomers to follow the event over time and will let us try to figure out what’s going on in the explosion.

Okay, so now what?

For now, we wait. LIGO is offline for the next year as it’s undergoing upgrades. When it turns back on (next Fall), we hope that we’ll be getting many, many more events like this. In the meantime, we’ll keep trying to learn as much as we can from this source from the data that we have in hand. Over 3000 astronomers were involved enough to be authors on the discovery paper (there aren’t that many of us), and bringing together all of this information is going to take a bit of time. There are also new insights into the nature of the explosion that we’ll learn from watching the remnant (the leftover stuff after the explosion occurred) evolve with time. Radio astronomers are doing this right now.

Neutron star mergers are (hopefully) going to be laboratories where we can study neutron-rich matter in some of the densest environments in the Universe, which will in turn leads to new insights in nuclear physics and understanding the composition of the Universe.