Merging neutron stars emit both gravitational waves and light
In a major milestone for astronomy, the merger of two neutron stars in the galaxy NGC 4993 produced both gravitational waves and light, enabling scientists to observe the event in various wavelengths and pinpoint its source.
About two seconds after 8:41 a.m. EDT (12:41 GMT) on August 17, 2017, scientists at NASA’s Fermi Gamma-ray Space Telescope detected a brief high-energy gamma-ray burst. At that exact time, however, their counterparts at the National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO) had already detected gravitational waves for the fifth time since the groundbreaking first detection in 2015.
That first detection earned the leading three LIGO scientists the 2017 Nobel Prize in physics.
As word of the gamma-ray burst spread worldwide, astronomers aimed both ground- and space-based telescopes at the source of the explosion, which was then observed by the Hubble, Spitzer, and Chandra space telescopes, the European Space Agency’s (ESA) INTEGRAL satellite, and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) in Hawaii.
The Virgo European Gravitational Observatory, through analysis of the gravitational wave data, subsequently identified the position from which they originated. This time, scientists took just 12 hours to pinpoint the source of both the gravitational waves and light as a kilonova in the constellation Hydra.
A kilonova is a supernova produced by the merger of either two neutron stars or a neutron star and a black hole in a binary system.
All four previous detections of gravitational waves came from merging black holes, from which light cannot escape, and therefore were not associated with any light emissions.
Neutron stars are the stellar remnants of stars with 10–60 solar masses that died in supernova explosions. Stars even more massive than this that undergo supernova explosions leave behind black holes.
The violent core collapse that occurs during a supernova explosion crushes the protons and electrons inside that core into neutrons, which are subatomic particles, leaving behind a small but extremely dense remnant.
Approximately 130 million years ago, the binary neutron stars that produced the gravitational waves dubbed GW170817 spiraled closer and closer to one another and eventually collided, in the process emitting gravitational waves and a gamma-ray burst.
The process released the neutrons from the extreme pressure they were under, causing them to not only turn back into protons and electrons but also form elements heavier than iron, including gold and platinum.
“Now, for the first time, we’ve seen light and gravitational waves produced by the same event. The detection of a gravitational-wave source’s light has revealed details of the event that cannot be determined from gravitational waves alone. The multiplier effect of study with many observatories is incredible,” stated Paul Hertz, director of NASA’s Astrophysics Division at its Washington, DC, headquarters.
LEFT & MIDDLE: On Aug. 17, 2017, the Laser Interferometer Gravitational-Wave Observatory detected gravitational waves from a neutron star collision. Within 12 hours, observatories had identified the source of the event within the galaxy NGC 4993, shown in this Hubble Space Telescope image, and located an associated stellar flare called a kilonova. Hubble observed that flare of light fade over the course of 6 days, as shown in these observations taken on August 22, 26, and 28 (insets). Image & Caption Credit: NASA / ESA; Acknowledgments: A. Levan (U. Warwick), N. Tanvir (U. Leicester), and A. Fruchter, and O. Fox (STScI). RIGHT: Swift’s Ultraviolet / Optical Telescope imaged the kilonova produced by merging neutron stars in the galaxy NGC 4993 (box) on Aug. 18, 2017, about 15 hours after gravitational waves and the gamma-ray burst were detected. The source was unexpectedly bright in ultraviolet light. It faded rapidly and was undetectable in UV when Swift looked again on Aug. 29. This false-color composite combines images taken through three ultraviolet filters. Inset: Magnified views of the galaxy. Image & Caption Credit: NASA/Swift
Andy Fruchter of the Space Telescope Science Institute reported that scientists suspect that the heaviest elements are produced in mergers of neutron stars. Jets that stream from the kilonova travel rapidly, heating up and gathering interstellar material and producing X-rays in the process.
Light emitted by the event was detected in various wavelengths. Using ground-based observatories and Hubble, scientists viewed the brightening in both visible and infrared light.
Over the next six days, the visible light faded.
Both the visible and infrared light are attributed to heating caused by the decay of radioactive elements in the explosion’s debris.
Surprisingly, the Swift satellite, which was launched to find the origin of gamma-ray bursts, also detected ultraviolet light (top-right image) when aimed at the source of the kilonova.
“We did not expect a kilonova to produce a bright UV emission. We think this was produced by the short-lived disk of debris that powered the gamma-ray burst,” said Swift principal investigator S. Bradley Cenko of NASA’s Goddard Space Flight Center in Maryland.
Swift also detected high-energy gamma rays coming from the site but did not detect X-rays. The latter was first observed (shown right) by the Chandra X-ray Observatory nine days after the kilonova. That delay might be the result of the jet hurled outward by the explosion not initially being angled toward Earth.
“The detection of X-rays demonstrates that neutron star mergers can form powerful jets streaming out at near light speed,” noted Eleonora Troja of Goddard, who found the X-rays with Chandra. “We had to wait for nine days to detect it because we viewed it from the side, unlike anything we had seen before.”
Hubble observations enabled scientists to capture the kilonova’s near-infrared spectrum, from which they could extrapolate its motion and composition.
The Spitzer Space Telescope observed the longest infrared wavelengths, allowing it to determine the amount of heavy elements produced.
While the spectrum resembles what scientists expected from the merger of two neutron stars, including signatures of heavy elements, it also shows uncommon isotopes, including those of elements that experience radioactive decay, and smeared spectral lines caused by the rapid movement of jets from the kilonova that scientists will spend many hours analyzing.
When it is launched in 2019, NASA’s James Webb Space Telescope will observe any remaining infrared afterglow.
This animation captures phenomena observed over the course of nine days following the neutron star merger known as GW170817. They include gravitational waves (pale arcs), a near-light-speed jet that produced gamma rays (magenta), expanding debris from a kilonova that produced ultraviolet (violet), optical and infrared (blue-white to red) emission, and, once the jet directed toward us expanded into our view from Earth, X-rays (blue).
Video & caption courtesy of NASA’s Goddard Space Flight Center / CI Lab
Video courtesy of NASA’s Marshall Center
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.