Gravitational Wave Source in NGC 4993
Acknowledgment: A. Levan (U. Warwick), N. Tanvir (U. Leicester), and A. Fruchter and O. Fox (STScI)
Neutron Star Collision Creates Kilonova
Release images
For the first time, NASA scientists have detected light tied to a
gravitational-wave event, thanks to two merging neutron stars in the
galaxy NGC 4993, located about 130 million light-years from Earth in the
constellation Hydra.
Shortly after 8:41 a.m. EDT on Aug. 17, NASA's Fermi Gamma-ray Space
Telescope picked up a pulse of high-energy light from a powerful
explosion, which was immediately reported to astronomers around the
globe as a short gamma-ray burst. The scientists at the National Science
Foundation’s Laser Interferometer Gravitational-Wave Observatory (LIGO)
detected gravitational waves dubbed GW170817 from a pair of smashing
stars tied to the gamma-ray burst, encouraging astronomers to look for
the aftermath of the explosion. Shortly thereafter, the burst was
detected as part of a follow-up analysis by ESA’s (European Space
Agency’s) INTEGRAL satellite.
NASA's Swift, Hubble, Chandra, and Spitzer missions, along with dozens of ground-based observatories, including the
NASA-funded PanSTARRS survey, later captured the fading glow of the blast's expanding debris.
"This is extremely exciting science," said 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."
Neutron stars are the crushed, leftover cores of massive stars that
previously exploded as supernovas long ago. The merging stars likely had
masses between 10 and 60 percent greater than that of our Sun, but they
were no wider than Washington, D.C. The pair whirled around each other
hundreds of times a second, producing gravitational waves at the same
frequency. As they drew closer and orbited faster, the stars eventually
broke apart and merged, producing both a gamma-ray burst and a rarely
seen flare-up called a "kilonova."
"This is the one we've all been waiting for," said David Reitze,
executive director of the LIGO Laboratory at Caltech in Pasadena,
California. "Neutron star mergers produce a wide variety of light
because the objects form a maelstrom of hot debris when they collide.
Merging black holes — the types of events LIGO and its European
counterpart, Virgo, have previously seen — very likely consume any
matter around them long before they crash, so we don't expect the same
kind of light show."
"The favored explanation for short gamma-ray bursts is that they're
caused by a jet of debris moving near the speed of light produced in the
merger of neutron stars or a neutron star and a black hole," said Eric
Burns, a member of Fermi's Gamma-ray Burst Monitor team at NASA's
Goddard Space Flight Center in Greenbelt, Maryland. "LIGO tells us there
was a merger of compact objects, and Fermi tells us there was a short
gamma-ray burst. Together, we know that what we observed was the merging
of two neutron stars, dramatically confirming the relationship."
Within hours of the initial Fermi detection, LIGO and the Virgo
detector at the European Gravitational Observatory near Pisa, Italy,
greatly refined the event's position in the sky with additional analysis
of gravitational wave data. Ground-based observatories then quickly
located a new optical and infrared source — the kilonova — in NGC 4993.
To Fermi, this appeared to be a typical short gamma-ray burst, but it
occurred less than one-tenth as far away as any other short burst with a
known distance, making it among the faintest known. Astronomers are
still trying to figure out why this burst is so odd, and how this event
relates to the more luminous gamma-ray bursts seen at much greater
distances.
NASA’s Swift, Hubble and Spitzer missions followed the evolution of
the kilonova 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.
When Swift turned to the galaxy 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," said
Goddard’s S. Bradley Cenko, principal investigator for Swift. "We think
this was produced by the short-lived disk of debris that powered the
gamma-ray burst."
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.
NASA’s Chandra X-ray Observatory clearly detected X-rays nine days
after the source was discovered. Scientists 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."
On Aug. 22, NASA’s Hubble Space Telescope began imaging the kilonova
and capturing its near-infrared spectrum, which revealed the motion and
chemical composition of the expanding debris.
"The spectrum looked exactly like how theoretical physicists had
predicted the outcome of the merger of two neutron stars would appear,"
said Andrew Levan at the University of Warwick in Coventry, England, who
led one of the proposals for Hubble spectral observations. "It tied
this object to the gravitational wave source beyond all reasonable
doubt."
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, Spitzer 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 Sept. 30
observation 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," says Mansi Kasliwal, Caltech
assistant professor and principal investigator of the Spitzer observing
program.
Numerous scientific papers describing and interpreting these
observations have been published in Science, Nature, Physical Review
Letters and The Astrophysical Journal.
NASA's Hubble Studies Source of Gravitational Waves
On August 17, 2017, weak ripples in the fabric of space-time known as
gravitational waves washed over Earth. Unlike previously detected
gravitational waves, these were accompanied by light, allowing
astronomers to pinpoint the source. NASA’s Hubble Space Telescope turned
its powerful gaze onto the new beacon, obtaining both images and
spectra. The resulting data will help reveal details of the titanic
collision that created the gravitational waves, and its aftermath.
The Laser Interferometer Gravitational-Wave Observatory (LIGO)
detected gravitational waves at 8:41 a.m. EDT on August 17. Two seconds
later, NASA’s Fermi Gamma-ray Space Telescope measured a short pulse of
gamma rays known as a gamma-ray burst. Many observatories, including
space telescopes, probed the suspected location of the source, and
within about 12 hours several spotted their quarry.
In a distant galaxy called NGC 4993, about 130 million light-years
from Earth, a point of light shone where nothing had been before. It was
about a thousand times brighter than a variety of stellar flare called a
nova, putting it in a class of objects astronomers call “kilonovae.” It
also faded noticeably over 6 days of Hubble observations.
“This appears to be the trifecta for which the astronomical community
has been waiting: Gravitational waves, a gamma-ray burst, and a
kilonova all happening together,” said Ori Fox of the Space Telescope
Science Institute, Baltimore, Maryland.
The source of all three was the collision of two neutron stars, the
aged remains of a binary star system. A neutron star forms when the core
of a dying massive star collapses, a process so violent that it crushes
protons and electrons together to form subatomic particles called
neutrons. The result is like a giant atomic nucleus, cramming several
Suns’ worth of material into a ball just a few miles across.
In NGC 4993, two neutron stars once spiraled around each other at
blinding speed. As they drew closer together, they whirled even faster,
spinning as fast as a blender near the end. Powerful tidal forces ripped
off huge chunks while the remainder collided and merged, forming a
larger neutron star or perhaps a black hole. Leftovers spewed out into
space. Freed from the crushing pressure, neutrons turned back into
protons and electrons, forming a variety of chemical elements heavier
than iron.
“We think neutron star collisions are a source of all kinds of heavy
elements, from the gold in our jewelry to the plutonium that powers
spacecraft, power plants, and bombs,” said Andy Fruchter of the Space
Telescope Science Institute.
Several teams of scientists are using Hubble’s suite of cameras and
spectrographs to study the gravitational wave source. Fruchter, Fox, and
their colleagues used Hubble to obtain a spectrum of the object in
infrared light. By splitting the light of the source into a rainbow
spectrum, astronomers can probe the chemical elements that are present.
The spectrum showed several broad bumps and wiggles that signal the
formation of some of the heaviest elements in nature.
“The spectrum looked exactly like how theoretical physicists had
predicted the outcome of the merger of two neutron stars would appear.
It tied this object to the gravitational wave source beyond all
reasonable doubt,” said Andrew Levan at the University of Warwick in
Coventry, England, who led one of the proposals for Hubble spectral
observations. Additional spectral observations were led by Nial Tanvir
of the University of Leicester, England.
Spectral lines can be used as fingerprints to identify individual
elements. However, this spectrum is proving a challenge to interpret.
“Beyond the fact that two neutron stars flung a lot of matter out
into space, we’re not yet sure what else the spectrum is telling us,”
explained Fruchter. “Because the material is moving so fast, the
spectral lines are smeared out. Also, there are all kinds of unusual
isotopes, many of which are short-lived and undergo radioactive decay.
The good news is that it’s an exquisite spectrum, so we have a lot of
data to work with and analyze.”
Hubble also picked up visible light from the event that gradually
faded over the course of several days. Astronomers believe that this
light came from a powerful “wind” of material speeding outward. These
observations hint that astronomers viewed the collision from above the
orbital plane of the neutron stars. If seen from the side (along the
orbital plane), matter ejected during the merger would have obscured the
visible light and only infrared light would be visible.
“What we see from a kilonova might depend on our viewing angle. The
same type of event would appear different depending on whether we’re
looking at it face-on or edge-on, which came as a total surprise to us,”
said Eleonora Troja of the University of Maryland, College Park,
Maryland, and NASA’s Goddard Space Flight Center, Greenbelt, Maryland.
Troja is also a principal investigator of a team using Hubble
observations to study the object.
The gravitational wave source now is too close to the Sun on the sky
for Hubble and other observatories to study. It will come back into view
in November. Until then, astronomers will be working diligently to
learn all they can about this unique event.
The launch of NASA’s James Webb Space Telescope also will offer an
opportunity to examine the infrared light from the source, should that
glow remain detectable in the months and years to come.
Contact
Christine Pulliam / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4366 / 410-338-4514
cpulliam@stsci.edu / villard@stsci.edu
Felicia Chou
NASA Headquarters, Washington, D.C.
202-358-0257
felicia.chou@nasa.gov
Dewayne Washington
Goddard Space Flight Center, Greenbelt, Maryland
301-286-0040
dewayne.a.washington@nasa.gov
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