Breaking: the first light from two neutron stars merging

Previous detections of gravitational waves have all involved the merger of two black holes, a feat that won the 2017 Nobel Prize in Physics earlier this month. However, black hole mergers are not expected to produce any electromagnetic radiation (light), meaning they cannot be detected by conventional telescopes.

In contrast, binary neutron star (NS-NS) mergers have long been expected to produce an energetic explosion and a plume of radioactive material, generating light, but have never previously been detected. These mergers provide important clues as to how matter behaves under these extreme conditions.

The papers published online in Science on 16 October describe how light from the NS-NS merger was precisely located, subsequent observations at X-ray, ultraviolet (UV), optical, infrared (IR) and radio wavelengths, and theoretical analysis of the event.

First, a study by David Coulter et al. describes how a team of astronomers pinpointed the location of the merger during the critical few hours following detection of the gravitational waves. The team used the Swope 1-meter telescope in Chile to search for light emitted by the merger, searching the area of the sky that the gravitational waves could have come from.

Using a catalog of known nearby galaxies, they created a prioritized list of likely locations and began snapping images of them. In just their ninth image, they spotted a new source and identified the location of the event: within a galaxy called NGC 4993, about 130 million light years away. As the first team to locate the source, they dubbed it Swope Supernova Survey 2017a (SSS17a).

Immediately after SSS17a was identified, Maria Drout and colleagues began monitoring its brightness with the Swope and Magellan telescopes. Combining their observations with data from other facilities, they analyzed the UV, optical and IR brightness of the event from 11 hours to 18 days after the merger. The source quickly faded and changed from a blue to a red color – a sign that the material was expanding rapidly and cooling as it went.

They interpret the emission as a “kilonova,” an ejection of newly-produced heavy elements, whose radioactive decay powers the emission of UV, optical and infrared light. They estimate that the merger ejected material with a mass of 5% of our Sun, containing heavy chemical elements such as lanthanides.

A separate study by Phil Evans et al. describes how the Swift and NuSTAR space telescopes swung into action to observe the event at UV and X-ray wavelengths. The Swift satellite quickly detected a UV source at the location of the event, but it rapidly faded and had disappeared two days later.

The UV observations imply that material was ejected at a velocity that is a substantial fraction of the speed of light, and that it was hotter at early times than theoretical models of kilonovae had predicted. Neither Swift nor NuSTAR detected X-ray emission from the merger, but the authors were able to use its absence to determine the angle at which the binary system was tilted, estimating it to be 30 degrees from our line of sight.

Benjamin Shappee et al. studied the optical and near-infrared spectrum emitted by SSS17a between 12 hours and 18 days after the merger. They show that it behaves unlike any previously-known class of astronomical transient, such as supernovae or gamma-ray bursts. Their earliest observations show that the material ejected from the merging neutron stars was expanding at about 30% of the speed of light, much faster than a supernova.

As the material expands and cools, the spectrum becomes more complicated. They conclude that the ejecta contain two different components: one hot, blue and short-lived, whilst the other is cooler, redder and produces light for longer. There are no absorption lines due to the host galaxy in the spectrum. They note that no single previously-existing model can fully explain the spectroscopic changes they observed.

Gregg Hallinan, Alessandra Corsi, and colleagues monitored the event using numerous radio telescopes around the world. Sixteen days after the gravitational waves reached Earth, they detected the first radio waves from the event. They interpret the radio emission as produced when the expanding material slams into the surrounding gas within NGC 4993, which has a surprisingly low density.

The lack of radio emission at earlier times indicates that the relativistic jet, required to produce the observed gamma rays, cannot be aligned with the line of sight to Earth. They produce two different models that can explain the radio brightness whilst being consistent with observations at other wavelengths, and show how the two models make different predictions for how the radio emission will change over the next few months.

A study by Charles Kilpatrick et al. combines optical to near-infrared brightness and spectroscopy data with modeling, to determine the nature of the event independently of the gravitational wave signal. The observations of SSS17a match what scientists have previously predicted for kilonova events, but only if there are two distinct components with different masses, velocities and fractions of lanthanide elements.

These properties indicate that at least one of the merging objects must have been a neutron star, and the other one probably was too, thereby confirming the result found from the gravitational waves. They also calculate the amount of heavy elements produced in the SSS17a merger, estimate how frequently those events occur, and show that NS-NS mergers can be a major source of the lanthanide elements throughout the Universe, including those on Earth.

A paper by Mansi Kasliwal and colleagues brings together observations at X-ray, UV, optical, IR and radio wavelengths to produce a detailed theoretical model of the merger. Using data from 24 telescopes on seven continents, they reconstruct the total energy emitted by the event at each stage, then seek to simultaneously explain the observations at all wavelengths.

In their preferred model, a jet of material is produced that expands at close to the speed of light, but is directed away from our line of sight. Instead, we see emission from a “cocoon” of shocked material surrounding the jet, which expands over a wider angle.

They confirm this model with detailed simulations, and predict that about 30% of future NS-NS mergers will produce bright gamma rays that will reach Earth. Finally, they estimate the mass of heavy elements produced in the event and use that to calculate the rate of NS-NS mergers, showing both that they can be a major source of those elements and that many more detections can be expected.

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These ground-breaking results are put into a broader context in a related Perspective by Joshua Bloom and Steinn Sigurdsson.