LIVINGSTON, LA (WAFB) - LSU physicists at the Laser Interferometer Gravitational-Wave Observatory or LIGO in Livingston, LA were some of the first scientists to directly detect gravitational waves (ripples in space and time) and light from the collision of two neutron stars, according to the university. The discovery was made on August 17 using LIGO, the Europe-based Virgo detector, and 70 ground and space-based observatories.
"This first observation of gravitational waves caused by two neutron stars colliding is not only a breakthrough for the LIGO-Virgo Scientific Collaboration that detected this, but also for our colleagues who study neutron stars, gamma-ray flashes and other astronomical phenomena," said Gabriela González, LSU Department of Physics & Astronomy professor and former LIGO Scientific Collaboration spokesperson.
Neutron stars form when massive stars explode in supernovas, LSU explains. They are the smallest, densest stars known to exist. As these neutron stars spiraled together, they emitted gravitational waves that were detectable for about 100 seconds; when they collided, a flash of light in the form of gamma rays was emitted and seen on Earth about two seconds after the gravitational waves. In the days and weeks following the smashup, other forms of light, or electromagnetic radiation — including x-ray, ultraviolet, optical, infrared and radio waves — were detected.
Scientists say the two neutron stars were in their final moments of orbiting each other about 130 million years ago. At that time, the two stars were separated only by about 300 kilometers, or 200 miles, gathering speed while closing the distance between them. They spiraled faster and closer together before stretching and distorting the surrounding space-time, giving off energy in the form of powerful gravitational waves, before smashing into each other.
At the moment of the collision, the bulk of the two neutron stars merged into one ultradense object, emitting a "fireball" of gamma rays. The initial gamma-ray measurements, combined with the gravitational-wave detection, also provide confirmation for Einstein's general theory of relativity, which predicts that gravitational waves should travel at the speed of light.
"Gamma-rays are the high-energy electromagnetic counterparts to the LIGO discoveries. Collaborators around the world, including LSU researchers using a telescope on the International Space Station, have been eagerly looking for these events. Now, counterpart observations of this neutron star merger event with several telescopes from the radio to the gamma-ray regime are giving us valuable information about the nature of these exotic astrophysical sources," said Michael Cherry, LSU Department of Physics & Astronomy professor and collaborator on the CALET experiment on the International Space Station.
When the two stars collided which glows with light, is blown out of the immediate region and far out into space. The new light-based observations show that heavy elements, such as lead and gold, are created in these collisions and subsequently distributed throughout the universe, solving a decades-long mystery of where about half of all elements heavier than iron are produced. 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
The gravitational signal, named GW170817, was first detected on Aug. 17 at 7:47 a.m. Central Daylight Time; the detection was made by the two identical LIGO detectors, located in Hanford, Wash., and Livingston, La. The LIGO Livingston observatory is located on LSU property, and LSU faculty, students and research staff are major contributors to the international scientific collaboration.
The information provided by the third detector, Virgo, situated near Pisa, Italy, enabled an improvement in localizing the cosmic event. At the time, LIGO was nearing the end of its second observing run since being upgraded in a program called Advanced LIGO, while Virgo had begun its first run after recently completing an upgrade known as Advanced Virgo.
Each observatory consists of two long tunnels arranged in an "L" shape, at the joint of which a laser beam is split in two. Light is sent down the length of each tunnel, then reflected back in the direction it came from by a suspended mirror. In the absence of gravitational waves, the laser light in each tunnel should return to the location, where the beams were split at precisely the same time. If a gravitational wave passes through the observatory, it will alter each laser beam's arrival time, creating an almost imperceptible change in the observatory's output signal.
On Aug. 17, LIGO's real-time data analysis software caught a strong signal of gravitational waves from space in one of the two LIGO detectors. At nearly the same time, the Gamma-ray Burst Monitor on NASA's Fermi space telescope had detected a burst of gamma rays. LIGO-Virgo analysis software put the two signals together and saw it was highly unlikely to be a chance coincidence, and another automated LIGO analysis indicated that there was a coincident gravitational wave signal in the other LIGO detector. Rapid gravitational-wave detection by the LIGO-Virgo team, coupled with Fermi's gamma-ray detection, enabled the launch of follow-up by telescopes around the world.
The LIGO data indicated that two astrophysical objects located at the relatively close distance of 130 million light-years from Earth had been spiraling in toward each other. The two objects were estimated to be in a range from around 1.1 and 1.6 times the mass of the sun, in the mass range of neutron stars. A neutron star is about 20 kilometers, or 12 miles, in diameter and is so dense that a teaspoon of neutron star material has a mass of about a billion tons.
While binary black holes produce "chirps" lasting a fraction of a second in the LIGO detector's sensitive band, the Aug. 17 chirp lasted approximately 100 seconds and was seen through the entire frequency range of LIGO — about the same range as common musical instruments. Scientists could identify the chirp source as objects that were much less massive than the black holes seen to date.