LIGO Detects Collision of Neutron Stars

An illustration of a gamma-ray burst shows a blue cloud of bright light against a dark background.
Image: LIGO
The neutron-star merger detected by LIGO and other observatories on 17 August 2017 was followed 2 seconds later by a gamma-ray burst.

Do you have a piece of gold or platinum jewelry? Every single one of the gold or platinum atoms in it were probably formed by the violent collision of two neutron stars in the distant past of our galaxy. Such collisions have long been predicted and modeled by astrophysicists, but scientists have finally detected one as it happened, in a galaxy 130 million light-years away.

As announced this morning at a press conference hosted by the U.S. National Science Foundation, detectors in Europe and the United States picked up minute vibrations in the fabric of space at 8:41 a.m. eastern time on 17 August. These vibrations were gravity waves, and they are produced when astronomically massive objects experience significant accelerations, which means an awful lot of energy is involved.

Software constantly scans the signals emerging from the detectors. That morning it matched those vibrations with the predicted signature of a neutron star collision, and kicked off an unprecedented flurry of activity at telescopes around the world and in space.

Previous observations of gravity waves using the twin LIGO detectors in Livingston, La., and Hanford, Wash., couldn’t resolve where in the sky the waves were coming from, but just a few weeks ago LIGO was joined by the Virgo gravity-wave detector near Pisa in Italy. The gravity waves from the neutron star arrived first at the LIGO Livingston detector, then 8 milliseconds (ms) later were picked up by the Hanford detector, followed 6 ms later by the Virgo detector.

The gravity waves were observed for only about 100 seconds, but these seconds mark the end of a death spiral that was some 11 billion years in the making. For nearly all of that time since the stars formed in supernovas, the stars would have quietly orbited each other, but their mutual orbit was ultimately unstable. As they moved closer together, their orbital speed inexorably increased: During their final 100 seconds, researchers estimated the stars would have orbited each other about 1,500 times, moving at close to the speed of light. Once the neutron stars collided in what has been dubbed a “kilonova,” the gravity wave signal abruptly cut off.

However, this was only the start of the international campaign to glimpse the aftermath. A huge wave of radioactive material is ejected during a neutron star collision, forming elements heavier than iron, including the aforementioned gold and platinum but also elements like radon and uranium. The amount of material produced by the 17 August kilonova was later estimated to be about 16,000 times the mass of Earth, with the gold and platinum portion of that adding up to around 10 times the mass of Earth.

By combining the timing information from the gravity-wave detectors, the signal was localized to an area of the sky covering about 28 square degrees in the Hydra constellation, and astronomers around the world were alerted. For an astronomer, this is still a huge area—for comparison, the moon is only about half a degree in diameter as observed from Earth—but it was small enough that they had a real chance of spotting electromagnetic (EM) emissions, ranging from gamma rays through the visible light spectrum and into the near infrared.

This began with gamma rays. Gamma-ray bursts are often mysterious explosions that so far have only been observed in distant galaxies. Several orbiting telescopes are designed to detect these bursts, and after the gravity-wave alert was sent out by LIGO and Virgo, the operators of the Fermi Gamma-ray Space Telescope noticed that they had spotted a relatively faint burst that had occurred 1.7 seconds after the collision. Hundreds of bursts at this level of brightness are detected every year, so this burst would likely have gone unnoticed if not for the timing of the gravity-wave event. Fermi was able to determine that the burst came from the same patch of sky as the gravity waves, marking the first time that a gravity-wave event had been associated with EM emissions of any sort.

To really pin down the location of the kilonova required rapid and intensive optical and near-optical observations. There was no shortage of participants: Speaking at the press conference, Andy Howell of the Las Cumbres Observatory Global Telescope Network said that the LIGO alert was essentially saying that a “giant explosive train wreck that makes gold” had occurred in the Hydra constellation, which was much better than previous gravity-wave events, where location estimates had boiled down to “kind of hard to say.” Dozens of remotely operated and highly automated telescopes around the world and especially in Chile (the first major observatory site that could see the Hydra rise in the sky) swung into action.

These telescopes are designed to detect transient astronomical events by repeatedly imaging swaths of the sky. Comparing images taken at different times allows events like distant supernovae to be spotted. Within a few hours, the blue glow of the kilonova had been detected in the elliptical galaxy NGC4993.

The kilonova rapidly faded in intensity over the next few days, turning redder in the process. Later on, both X-rays and radio waves were also detected. Because astronomers have been able to obtain so much scientific data from so many different sources—which they are calling “multi-messenger” observations—the “combined information is bigger than the sum of its parts,” said Laura Cadonati, deputy spokesperson for LIGO.

Even with robotic telescopes on the ground and in space, monitoring the August kilonova aftermath required considerable human activity and coordination. This meant that several hours elapsed before optical telescopes could be brought to bear. With such a rapidly evolving event, it is almost certain that interesting data was lost. In the future astronomers would like to create even more automated systems to capture such events.

Amatuer astronomers may also play a role in these campaigns. Several of the participants at the conference noted the while the kilonova was too dim to be seen with the naked eye, it could be seen using a 16-inch telescope of the sort owned by many amatuers. LIGO gravity-wave alerts are not currently public, but if that changes, then it’s entirely likely that the first person to see the light from future neutron-star collisions will be a backyard stargazer.

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