For the first time, astronomers have observed the source of gravitational waves, created by the merger of two neutron stars. The cataclysmic aftermaths of this kind of merger — long-predicted events called kilonovae — eject heavy elements such as gold and platinum into space. This discovery also provides the strongest evidence yet that short-duration gamma-ray bursts are caused by mergers of neutron stars.
This artist’s impression shows two neutron stars at the point at which they merge and explode as a kilonova. Such a very rare event is expected to produce both gravitational waves and a short gamma-ray burst, both of which were observed on August 17, 2017, by LIGO and Virgo collaborations. Image credit: ESO / L. Calcada / M. Kornmesser.
On August 17, 2017, LIGO and Virgo collaborations detected gravitational waves passing the Earth. This event, the fifth ever detected, was named GW170817.
About two seconds after the detection, ESA’s INTEGRAL telescope and NASA’s Fermi Gamma-ray Space Telescope observed a short gamma-ray burst.
In the night following the initial discovery, astronomers started their hunt to locate the source of the event.
They found it in the lenticular galaxy NGC 4993, located about 130 million light-years from Earth in the constellation Hydra.
NASA’s and ESA’s space missions, along with dozens of ground-based observatories, later captured the fading glow of the blast’s expanding debris.
“We managed to get the first observational proof for a kilonova, the visible counterpart of the merging of two extremely dense objects — most likely two neutron stars,” the astronomers said.
“Such mergers were first suggested more than 30 years ago but this marks the first firm observation of such an event.”
“This is extremely exciting science,” said Dr. Paul Hertz, director of NASA’s Astrophysics Division at the agency’s headquarters in Washington.
“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.”
On August 17, scientists detected gravitational waves from the collision between two neutron stars. Within 12 hours observatories had identified the source of the event within the lenticular galaxy NGC 4993, shown in this image gathered with Hubble. The associated stellar flare, a kilonova, is clearly visible in the Hubble observations. This is the first time the optical counterpart of a gravitational wave event was observed. Hubble observed the kilonova gradually fading over the course of six days, as shown in these observations taken in between 22 and 28 August (insets). Image credit: NASA / ESA / A.J. Levan, University of Warwick / N.R. Tanvir, University of Leicester / A. Fruchter & O. Fox, STScI.
The NASA/ESA Hubble Space Telescope, NASA’s Swift and Spitzer missions and followed the evolution of the kilonova (known as DLT17ck, SSS17a, or AT 2017gfo) to better understand the composition of this slower-moving material, while Chandra searched for X-rays associated with the remains of the ultra-fast jet.
Hubble captured images of NGC 4993 in visible and infrared light, witnessing a new bright object within the galaxy. The images showed that the object faded noticeably over the six days of the Hubble observations.
Using Hubble’s spectroscopic capabilities, the astronomers also found indications of material being ejected by the kilonova as fast as one-fifth of the speed of light.
“It was surprising just how closely the behavior of the kilonova matched the predictions,” said University of Leicester Professor Nial Tanvir, member of the Hubble observing team.
“It looked nothing like known supernovae, which this object could have been, and so confidence was soon very high that this was the real deal.”
Astronomers think a kilonova’s visible and infrared light primarily arises through heating from the decay of radioactive elements formed in the neutron-rich debris. Crashing neutron stars may be the Universe’s dominant source for many of the heaviest elements, including platinum and gold.
Because of its Earth-trailing orbit, NASA’s Spitzer Space Telescope was uniquely situated to observe the kilonova long after the Sun moved too close to the galaxy on the sky for other telescopes to see it.
Spitzer’s observation on September 30 captured the longest-wavelength infrared light from the kilonova, which unveils the quantity of heavy elements forged.
“Spitzer was the last to join the party, but it will have the final word on how much gold was forged,” said Dr. Mansi Kasliwal, Caltech assistant professor and principal investigator of the Spitzer observing program.
When NASA’s Swift Gamma-Ray Burst Mission turned to NGC 4993 shortly after Fermi’s gamma-ray burst detection, it found a bright and quickly fading ultraviolet (UV) source.
“We did not expect a kilonova to produce bright UV emission. We think this was produced by the short-lived disk of debris that powered the gamma-ray burst,” said Goddard’s S. Bradley Cenko, principal investigator for Swift.
“Over time, material hurled out by the jet slows and widens as it sweeps up and heats interstellar material, producing so-called afterglow emission that includes X-rays. But the spacecraft saw no X-rays — a surprise for an event that produced higher-energy gamma rays.”
Chandra clearly detected X-rays nine days after the source was discovered. Astronomers think the delay was a result of our viewing angle, and it took time for the jet directed toward Earth to expand into our line of sight.
“The detection of X-rays demonstrates that neutron star mergers can form powerful jets streaming out at near light speed,” said Goddard’s Eleonora Troja, who led one of the Chandra teams and found the X-ray emission.
“We had to wait for nine days to detect it because we viewed it from the side, unlike anything we had seen before.”
This composite shows images of NGC 4993 from several different ESO telescopes and instruments. They all reveal a faint source of light close to the center. This is a kilonova, the explosion resulting from the merger of two neutron stars. Image credit: VLT / VIMOS / MUSE / MPG / ESO / GROND / VISTA / VIRCAM / VST / OmegaCAM.
Next to NASA’s and ESA’s observatories, ESO’s Very Large Telescope, ESO’s New Technology Telescope, ESO’s VLT Survey Telescope, the MPG/ESO 2.2-m telescope, the Atacama Large Millimeter/submillimeter Array, the Visible and Infrared Survey Telescope for Astronomy, the Rapid Eye Mount (REM) telescope, the Swope Telescope, the LCO 0.4-m telescope, the American DECcam, and the Pan-STAARS survey all helped to identify and observe the event and its after-effects over a wide range of wavelengths.
Scientific papers describing and interpreting these observations are published in Science, Nature, Nature Astronomy, and the Astrophysical Journal Letters.
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Stefano Valenti et al. 2017. The Discovery of the Electromagnetic Counterpart of GW170817: Kilonova AT 2017gfo/DLT17ck. ApJL 848, L24; doi: 10.3847/2041-8213/aa8edf
A.J. Levan et al. 2017. The Environment of the Binary Neutron Star Merger GW170817. ApJL 848, L28; doi: 10.3847/2041-8213/aa905f
Jens Hjorth et al. 2017. The Distance to NGC 4993: The Host Galaxy of the Gravitational-wave Event GW170817. ApJL 848, L31; doi: 10.3847/2041-8213/aa9110
S. Covino et al. The unpolarized macronova associated with the gravitational wave event GW 170817. Nature Astronomy, published online October 16, 2017; doi: 10.1038/s41550-017-0285-z
E. Pian et al. Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger. Nature, published online October 16, 2017; doi: 10.1038/nature24298
N.R. Tanvir et al. 2017. The Emergence of a Lanthanide-rich Kilonova Following the Merger of Two Neutron Stars. ApJL 848, L27; doi: 10.3847/2041-8213/aa90b6
E. Troja et al. The X-ray counterpart to the gravitational-wave event GW170817. Nature, published online October 16, 2017; doi: 10.1038/nature24290
B.P. Abbott et al. A gravitational-wave standard siren measurement of the Hubble constant. Nature, published online October 16, 2017; doi: 10.1038/nature24471
B.P. Abbott et al. 2017. Multi-messenger Observations of a Binary Neutron Star Merger. ApJL 848, L12; doi: 10.3847/2041-8213/aa91c9
B.P. Abbott et al. 2017. Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A. ApJL 848, L13; doi: 10.3847/2041-8213/aa920c
C.D. Kilpatrick et al. Electromagnetic evidence that SSS17a is the result of a binary neutron star merger. Science, published online October 16, 2017; doi: 10.1126/science.aaq0073
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