The LIGO Scientific Collaboration, Virgo Collaboration, and their partners have directly detected gravitational waves — ripples in space and time — from a pair of inspiraling neutron stars. Named GW170817, the event was not only ‘heard’ in gravitational waves but also seen in light by dozens of space- and ground-based telescopes.
The GW170817 gravitational signal was first detected on August 17, 2017, at 8:41 a.m. EDT (5:41 a.m. PDT, 12:41 p.m. GMT, 2:41 p.m. CEST).
The detection was made by the two Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors, located in Hanford, Washington, and Livingston, Louisiana. The information provided by the third detector, Virgo, situated near Pisa, Italy, enabled an improvement in localizing the event.
At nearly the same time, NASA’s Fermi Space Telescope detected a burst of gamma rays. LIGO-Virgo analysis software put the two signals together and determined their appearance was highly unlikely to be chance.
The LIGO data indicated that two astrophysical objects located at the relatively close distance of about 130 million light-years from Earth had been spiraling in toward each other.
It appeared that the objects were not as massive as binary black holes — objects that LIGO and Virgo have previously detected.
Instead, the inspiraling objects were estimated to be in a range from around 1.1 to 1.6 times the mass of the Sun, in the mass range of neutron stars.
While binary black holes produce ‘chirps’ lasting a fraction of a second in the LIGO detector’s sensitive band, the GW170817 signal lasted approximately 100 seconds and was seen through the entire frequency range of LIGO.
“It immediately appeared to us the source was likely to be neutron stars, the other coveted source we were hoping to see — and promising the world we would see,” said Dr. David Shoemaker, spokesperson for the LIGO Scientific Collaboration.
“From informing detailed models of the inner workings of neutron stars and the emissions they produce, to more fundamental physics such as general relativity, this event is just so rich. It is a gift that will keep on giving.”
“Our background analysis showed an event of this strength happens less than once in 80,000 years by random coincidence, so we recognized this right away as a very confident detection and a remarkably nearby source,” said Professor Laura Cadonati, deputy spokesperson for the LIGO Scientific Collaboration.
“This detection has genuinely opened the doors to a new way of doing astrophysics. I expect it will be remembered as one of the most studied astrophysical events in history.”
Astrophysicists have predicted that when neutron stars collide, they should give off gravitational waves and gamma rays, along with powerful jets that emit light across the electromagnetic spectrum.
The gamma-ray burst detected by Fermi is what’s called a short gamma-ray burst; the new observations confirm that at least some short gamma-ray bursts are generated by the merging of neutron stars.
“For decades, we’ve suspected short gamma-ray bursts were powered by neutron star mergers. Now, with the incredible data from LIGO and Virgo for this event, we have the answer,” said Fermi project scientist Dr. Julie McEnery, of NASA’s Goddard Space Flight Center.
Fermi was able to provide a localization that was later confirmed and greatly refined with the coordinates provided by the combined LIGO-Virgo detection.
With these coordinates, observatories around the world were able, hours later, to start searching the region of the sky where the signal was thought to originate.
A new point of light, resembling a new star, was first found by optical telescopes. Ultimately, about 70 observatories on the ground and in space observed the event at their representative wavelengths.
“This detection opens the window of a long-awaited ‘multi-messenger’ astronomy,” said Dr. David H. Reitze, executive director of the LIGO Laboratory.
“It’s the first time that we’ve observed a cataclysmic astrophysical event in both gravitational waves and electromagnetic waves — our cosmic messengers. Gravitational-wave astronomy offers new opportunities to understand the properties of neutron stars in ways that just can’t be achieved with electromagnetic astronomy alone.”
A general picture is emerging among all observatories involved that further confirms that the initial gravitational-wave signal indeed came from a pair of inspiraling neutron stars.
“Approximately 130 million years ago, the two neutron stars were in their final moments of orbiting each other, separated only by about 200 miles (300 km) and gathering speed while closing the distance between them,” the researchers explained.
“As the stars spiraled faster and closer together, they stretched and distorted the surrounding space-time, giving off energy in the form of powerful gravitational waves, before smashing into each other. At the moment of collision, the bulk of the two neutron stars merged into one ultradense object, emitting a ‘fireball’ of gamma rays.”
“Theorists have predicted that what follows the initial fireball is a ‘kilonova’ — a phenomenon by which the material that is left over from the neutron star collision, which glows with light, is blown out of the immediate region and far out into space.”
The new light-based observations also show that heavy elements, such as lead and gold, are created in these collisions and subsequently distributed throughout the Universe.
The LIGO-Virgo results are published in the Physical Review Letters.
B.P. Abbott et al (LIGO Scientific Collaboration and Virgo Collaboration). 2017. GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral. Phys. Rev. Lett 119 (16); doi: 10.1103/PhysRevLett.119.161101