What happens when neutron stars collide?

Gamma ray bursts are intense flashes of radiation that appear in the night sky and often fade again within a few seconds. Even though they were first seen over 40 years ago, there's still a hot debate among astronomers as to what causes them. But, writing in Nature this week, Professor Nial Tanvir from the University of Leicester thinks he's found strong evidence that at least some bursts are triggered by collisions between neutron stars or black holes...

Nial - So, gamma ray bursts were discovered in the 1960s in fact by military satellites which has been put into orbit to look for Clandestine Nuclear Explosions which might be being conducted in space. Instead of seeing any of those, they detected flashes of gamma rays, high energy radiation coming from somewhere - as far as they knew - beyond the solar system. What we have subsequently found out about them is that they're coming in fact from other galaxies and it seems that there are a number of different kinds of very high energy powerful mechanisms which can give rise to these flashes of gamma rays.

Dominic - And how are we going about observing them? Is this basically geiger counters encounters in space?

Nial - That's pretty good description actually. So, the light in the form of these gamma rays, because they're high energy kinds of radiation, the photons of light, the little particles of light act really more or less like particles that you might get of a radioactive substance. And so, if you put a detector on a spacecraft to detect these things then indeed, they detected each individual photon as they arrive. Now, the tricky thing what that kind of technology of course, is trying to tell where the gamma rays are coming from because you don't tend to get very good directional information. You tend to just sort register that the photon has arrived. So, there has to be a whole sort of sequence of trying to use other telescopes to refine the position until eventually, we nail the exact location of each gamma ray burst that we're interested in.

Dominic - In Nature this week, you were talking about observations of one particular gamma ray burst that you saw back in June. What was surprising about that I gather was that it faded so very quickly after it initially flared up. What was the surprise there for you?

Nial - This was a so-called short duration gamma ray burst. One of the things we've learned after all these decades of research is that gamma ray bursts come in a number of different types, which we believe have really quite different origins even though they look rather similar to each other. And so, the short duration bursts, the initial flash of gamma rays only last probably less than a second and that was the case with this one that happened in June. After that, although it was quite a bright burst, it did fade very rapidly. So, following the initial flash of gamma rays, what we tend to see with all gamma ray bursts is a slowly declining sort of ember of light that we call the afterglow, and we see that actually in different kinds of light including optical and infrared, and radio and x-rays, or basically the whole electromagnetic spectrum.

In this case, the afterglow faded away apparently very quickly, and that's not too surprising. It's within the range of behaviour that we often see. But what was special was that we observed the location of the gamma ray burst again after about 9 days with the Hubble Space Telescope. Given how fast the afterglow had been fading, we, in a sense expected not to see anything at that point. But in fact, in the infrared pictures we took with the Hubble, we did see some light still there. Of course, we had anticipated this might be the case because people had speculated that the process that produces the short duration gamma ray bursts might also produce a sort of long lived radioactive afterglow as well, in addition to the normal afterglow, something that we call a kilonova.

Dominic - So, how must we know about what's actually causing these flashes in the sky?

Nial - The gamma ray bursts that we see have - as I say - a number of different subcategories and the ones that we see most often are what we call 'long duration gamma ray bursts' and they seem to be produced by some kind of core collapse supernova. Now, what that is, is a massive star at the end of its life. It runs out of fuel and so, gravity just starts to work on it and in a very short space and time, the whole thing collapses. And it seems that in the process of doing that in some cases, it produces a jet of material that as the star is collapsing, this jet thrusts its way out after extraordinary velocities, very close to the speed of light. If we happen to be looking down the axis of that jet then we see this flash and that is your long duration gamma ray burst.

Now, in the case of the short duration gamma ray bursts that we're talking about now, the mechanism we think is they were quite different and there, we think again, we do have a jet, but it's created by the coalescence - the merging of two so-called neutron stars. So, neutron stars are incredibly dense objects. In fact, they're formed as the remnants of other kind of supernova explosion. With a neutron star, you have essentially something like the mass of the sun, compressed into a ball about the size of a town, just a few miles across that is to say. So, you've got an incredibly dense object, the densest objects we know of in the universe apart from black holes and the idea is that if you have two neutron stars in orbit around each other then their orbits gradually decay until eventually the two things crash into each other. And that can release an enormous amount of energy. Again, by mechanisms that we really don't understand well at all, it seems that that can produce a super fast jet material, producing in this case, a short duration gamma ray burst.

Dominic - So, is the idea that this gamma ray burst has a very short duration because you've got two very compact objects, they're merging very quickly, perhaps forming a black hole which is very rapidly absorbing any material that might form an afterglow.

Nial - That's more or less exactly right. So, the natural timescales for that final, sort of merging event is really very short, much less than a second in fact. And so, the whole process, the energy release happens really quickly and so, that then seems plausible that that results in the short duration flash. The extra ingredient that really has come out of the new observations is this kilonova light and what that is thought to be produced by is that as these two neutron stars are undergoing their final death spiral as they come in towards each other, a certain amount of the material of them is thrown out into space. It's sort of ripped off each of the stars, thrown out into space, and because it's a super dense form of matter, once it's away from the neutron star itself, this fairly small percentage of the total mass of the star. But once it's ripped out of the neutron star, it expands very rapidly and forms this sort of radioactive ball of materials and it's the radioactivity from that material that was thrown out that gives rise we believe, to the later time emission after 9 days or so.

One of the really interesting possibilities that's opened up by the discovery of this kilonova is, there's been a long standing mystery as to the origin of certain elements in the universe, particularly what we call heavy elements, ones with big heavy atoms. And that includes certainly very well known familiar elements such as gold and platinum. Those are elements which we haven't really got an explanation for why they exist in the universe, and it's long been thought that they might be made in supernovae. On the other hand, that turns out to be quite difficult whereas merging neutron stars of the type that I've described, look exactly the sort of place where such elements should be made in abundance. So, it may be that all of the, say, the gold in your jewellery had its origin at an event like this, that pre-dated the formation of the solar system somewhere nearby in our galaxy where two new front stars merged, sprayed a quantity of these heavier elements out into gas clouds of the Milky Way, which subsequently collapsed and formed the solar system and the earth, and everything that we see today.