Neutron stars collide, solve major astronomical mysteries

We've been extremely lucky. The LIGO and VIRGO detectors only operated simultaneously for a few weeks, but they were a remarkably busy few weeks. Today, those behind the joint collaboration announced that they've observed the merger of two neutron stars. And, because neutron stars don't swallow everything they encounter, the gravitational waves were accompanied by photons, including an extended afterglow. So dozens of telescopes, and many in space, had representatives involved in the announcement.

The number of major astrophysical issues cleared up by this collision is impressive. The collision was simultaneously detected with the Fermi space telescope, confirming that neutron star mergers produce a phenomenon known as a short gamma-ray burst. The gravitational waves were detected nearly simultaneously with the gamma ray burst, confirming that they move at the speed of light. And heavy elements like gold were detected in the debris, indicating that these mergers are a source of elements that would otherwise be difficult to produce in a supernova.

Finally, the gravitational waves from this event were detected over a period of roughly 100 seconds, which should allow a detailed analysis of their production.

Meet the neutrons

Neutron stars are the product of supernovae where the star doing the exploding doesn't have sufficient mass to form a black hole. The object that forms instead crushes one or two solar masses down to an object with a diameter of about 20km. At these densities, individual atoms are crushed out of existence, and the entire star becomes a single chunk of neutrons—and possibly other exotic particles (quark matter stars have been proposed but not yet confirmed to exist). In cases where two massive stars both go supernova, it's possible to form binary systems where two neutron stars orbit each other.

We've known about binary neutron star systems for years, including some that were inspiralling toward a collision. Theoreticians have been busy proposing what they would look like and how they would behave once the merger took place, but the simultaneous detection of the event in gravitational and electromagnetic waves has been essential to confirm a number of the theoreticians' ideas.

For that to happen, we needed to get lucky in two ways. Since neutron stars are substantially less massive than black holes, the events are weaker, and we'd only detect them if they were closer. In this case, the merger took place 130 million light years from Earth, something astronomers are calling a "relatively close distance." (For context, that "relatively close distance" means the event took place shortly after the ancestors of marsupials and placental mammals went their separate ways.)

We also needed LIGO and VIRGO in operation simultaneously. As shown by a diagram in the gallery above, having a third detector has radically shrunk the area of sky that contains a gravitational wave source. Thus, we have a high degree of certainty that the gamma ray burst was produced by the same source as the gravitational waves.

The two neutron stars that merged here have a relatively low mass: they were estimated at about 1.1 to 1.6 times the mass of the Sun, compared to black holes that have been greater than 20 solar masses. This means that the neutron stars spent more time orbiting at a close distance before merging. This allowed the detection of gravitational waves for nearly 100 seconds; black hole mergers have produced detectable waves for only a fraction of a second. This should provide a nice test of our understanding of gravitational wave production.

Let there be light

LIGO-VIRGO's analysis software is programmed to do a quick-and-dirty analysis of data for possible sources and send out an alert to telescopes to allow them to perform observations of the area of sky where an event may be taking place. In this case, however, the telescopes also got an alert from NASA's Fermi Space Telescope, which specializes in catching high-energy events. Fermi has a gamma-ray burst monitor, and it picked up an event about two seconds after the gravitational wave signal arrived. This increased the precision with which we could map the source of the event, and telescopes of every sort sprung into action. More than 70 have provided observations that went into today's announcement.

The rapid redirection of so much hardware caught people's attention, and people quickly figured out that this was likely to mean the detection of a neutron star merger. So today's announcement had been expected since shortly after the event took place in August.

Even before these telescopes got involved, however, the Fermi detection told us two things. One is that, as theoreticians had predicted, gravitational waves appear to travel at the speed of light. There's still some imprecision in the measurements that could allow them to travel close, but not quite, to the speed of light, but the new measurements mean that it would have to be very, very close.

The second thing is that it nails down neutron star collisions as the source of some gamma-ray bursts. Gamma rays are the highest energy photons we can detect, and so we knew some powerful event must be producing them. Theoreticians had pointed the finger at neutron star mergers, but that had been a difficult thing to confirm, since it was often impossible to identify a counterpart to the bursts that was detectable at lower-energy wavelengths, plus we'd only be looking at debris produced by the event, not its source. The gravitational waves, by contrast, leave no doubt that the source was a neutron star merger.

Then there were the observations of the debris. This picked up the presence of gold and other heavy elements in the debris, which again, clears up an outstanding mystery. Some heavy elements are readily formed in the environment created by a supernova, meaning it's easy to explain their abundance in the cosmos. But others can only form through pathways that involve the rapid ingestion of multiple neutrons—so fast that the atom doesn't have time to rearrange to accommodate the previous neutrons it had absorbed. Supernovae aren't thought to provide an environment that's sufficiently neutron-rich for this to occur.

Neutron stars obviously would, but they keep their matter gravitationally crushed into its exotic state. What would be needed is for some event to liberate some of the matter that was otherwise trapped in these stars. Again, theoreticians had pointed to neutron star collisions as providing that opportunity, as the collisions would blast some debris out into space in a phenomenon that has picked up the term "kilonova." But it hadn't been confirmed until this point.

It's an incredible wealth of information. As LIGO spokesman David Shoemaker put it in a statement, “From informing detailed models of the inner workings of neutron stars and the emissions they produce, to more fundamental physics such as general relativity, this event is just so rich. It is a gift that will keep on giving.” Plus it hints that further events of this sort will add to our understanding. Though they might not be as detailed, Laura Cadonati, another LIGO scientist, says that events of this proximity should only occur by chance once every 80,000 years. If we see more like this one, then the theoreticians will have yet another mystery to explain.

“It is tremendously exciting to experience a rare event that transforms our understanding of the workings of the Universe,” said France A. Córdova, director of the National Science Foundation. The organization she now runs took a multimillion dollar risk that the LIGO detector would bring us to this point. If the recent Nobel Prizes to the LIGO team didn't already validate that risk, then this set of discoveries surely does.

There are two press conferences about to happen, and several papers will be released shortly. If any contain important information, we'll have further coverage.

UPDATES:

Researchers are saying that the resulting object is either one of the heaviest neutron stars yet observed, or the lightest black hole. Data is ambiguous at this point, but they're hoping further observations will sort this out.

The neutron stars have been orbiting each other for 11 billion years, as their stars went supernova early in the Universe's history. The second explosion sent the neutron stars on an erratic orbit through their galaxy through this entire time.

A 16-inch telescope could have picked up the source, meaning that these events will be in the range of amateur astronomers. Plans are to open the LIGO trigger system notifications to the public, letting the amateurs in on the earliest observations.