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LIGO & Virgo Make 1st Detection of Gravitational Waves Produced by Colliding Neutron Stars

The Etelman Observatory of the University of the Virgin Islands

For the first time, scientists have directly detected gravitational waves — ripples in space and time — in addition to light from the spectacular collision of two neutron stars. This marks the first time in history that a cosmic event has been viewed in both gravitational waves and light.

The discovery was made using the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO), the Europe-based Virgo detector, and some 70 ground- and space-based observatories, including UVI's Etelman Observatory.

A neutron star is the corpse of an ancient giant star, and is about 20 kilometers, or 12 miles, in diameter (i.e. roughly the size of the island of St Thomas). Neutron stars are composed exclusively of very closely packed neutrons which makes their density almost unimaginably large. Comparing neutron stars to normal matter, a spoonful of neutron star “stuff” would weigh more than the largest cruise ships seen in the Virgin Islands.

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 collision, other forms of light (or electromagnetic radiation) including X-ray, ultraviolet, optical, infrared, and radio waves were detected.

The observations have given astronomers an unprecedented opportunity to probe a collision of two neutron stars. For example, observations made by the U.S. Gemini Observatory, the European Very Large Telescope, and NASA’s Hubble Space Telescope reveal signatures of recently synthesized material, including gold and platinum, solving a decades-long mystery of where in the Universe nearly half of all elements heavier than iron are produced.

The LIGO-Virgo results are published today in the journal Physical Review Letters. Additional papers from the LIGO and Virgo collaborations and the astronomical community have been either submitted or accepted for publication in various journals. Members of the Etelman Observatory participated in several of these papers, including one published by Nature today.

“It is tremendously exciting to experience a rare event that transforms our understanding of the workings of the universe,” says France A. Córdova, director of the National Science Foundation (NSF), which funds LIGO. “This discovery realizes a long-standing goal many of us have had, that is, to simultaneously observe rare cosmic events using both traditional as well as gravitational-wave observatories. Only through NSF’s four-decade investment in gravitational-wave observatories, coupled with telescopes that observe from radio to gamma-ray wavelengths, are we able to expand our opportunities to detect new cosmic phenomena and piece together a fresh narrative of the physics of stars in their death throes.”

A stellar sign

The gravitational signal, named GW170817, was first detected on Aug. 17 at 8:41 a.m. Eastern Daylight Time. The detection was made by the two identical 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 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.

The NSF-funded LIGO observatories were conceived, constructed, and operated by Caltech and MIT. Virgo is funded by the Istituto Nazionale di Fisica Nucleare (INFN) in Italy and the Centre National de la Recherche Scientifique (CNRS) in France, and operated by the European Gravitational Observatory. Some 1,500 scientists in the LIGO Scientific Collaboration and the Virgo Collaboration work together to operate the detectors and to process and understand the gravitational-wave data they capture.

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 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.

Theorists 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 — something that was only theorized before.

UVI and Etelman Observatory Participation

The Etelman Observatory, part of the University of the Virgin Islands, located in the hills of St Thomas, participated in this observing campaign, using its Virgin Islands Robotic Telescope (or VIRT). The observatory team, led by the Observatory Resident Astronomer Dr. Bruce Gendre, performed a set of observations and detected the counterpart of the gravitational wave emitter, together with numerous other teams. Making the observations was complicated by the rainy weather conditions typical of hurricane season in the USVI and were forced to cease due to the approach of hurricane Irma in early September.

"This is a truly landmark observation in the history of science and we at UVI and the Etelman Observatory are thrilled to have played a role in this work.", said Dr. David Morris, Assistant Professor of Physics at UVI and the Etelman Observatory Director. "Our entire research group, including scientists from the UVI physics faculty, OrangeWave Innovative Science, the US Air Force Academy, and the University of Toulouse aided in preparing Etelman Observatory to be a part of this discovery. In these clearly challenging times in the USVI, this is an achievement of which all Virgin Islanders can be proud. It reminds us of the exciting work being done by scientists, engineers, and students at UVI and in our new physics program. We are now eager to bring the Etelman Observatory back online this Fall and continue work in this exciting new field of gravitational wave astrophysics."

While trying to cope with two major hurricanes and their aftermaths, the absence of communications with all the other teams, damage to the observatory from the hurricanes, not to mention voracious post-hurricane mosquitoes, the VIRT team managed to send their results to Australian colleagues who combined the VIRT data with their own to study the evolution of the phenomenon. The observations taken at the Etelman Observatory filled gaps in the timeline of the optical evolution of the explosion and will help produce a more complete understanding of the merging process, and the formation of compact objects and heavy element production in the Universe.

“These observations are a milestone achievement in my eyes.", said Dr. Gendre, "I was working on the coordination of electromagnetic and gravitational waves observations since 2010. With this result, we now have in hands first quality data to understand how the matter reacts when located inside strong gravitational fields. It may even be possible to ultimately understand what is the internal structure of a neutron star, a mystery since the discovery of these events more than 50 years ago.”

The observations taken at the Etelman Observatory will be published in the Publications of the Astronomical Society of Australia, together with optical images taken in Australia and radio follow-up.

LIGO is funded by the NSF, and operated by Caltech and MIT, which conceived of LIGO and led the Initial and Advanced LIGO projects. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council) making significant commitments and contributions to the project. The Etelman Observatory and its VIRT are supported by grants from NASA-EPSCoR, NASA-MUREP, NSF, and the US Department of Defense.

Media Contacts

David C. Morris, UVI, Etelman Observatory Director

Bruce Gendre, UVI, Observatory Resident Astronomer