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For the first time, scientists have directly detected gravitational waves—ripples in space and time—from the spectacular collision of two neutron stars. Light from the collision was also observed by telescopes, marking the first time that a cosmic event has been viewed in both gravitational waves and light.
This new field of gravitational-wave astronomy is providing opportunities to understand the universe in ways that cannot be achieved with traditional telescopes alone, and scientists believe this event will become one of the most studied astrophysical events in history.
The new discovery was made using the U.S.-based Laser Interferometer Gravitational-Wave Observatory, called LIGO; the Europe-based Virgo detector; and some 70 ground- and space-based observatories.
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, suggesting a resolution to a decades-long debate about how most of the 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 submitted or accepted for publication in various journals.
Discovery from collaboration
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.
Two West Virginia University researchers have been part of the international collaboration since before the first detection of gravitational waves, a discovery for which three of the original founders of LIGO were recently awarded the 2017 Nobel Prize for Physics.
Sean McWilliams, assistant professor of physics and astronomy, Zachariah Etienne, assistant professor of mathematics, and their team of students and postdocs in the Eberly College of Arts and Sciences focus on the analysis of gravitational-wave detections.
Their work focuses on modeling the sources of gravitational waves and using these models to measure characteristics of the sources based on the details contained in their gravitational-wave signals.
Etienne will deliver a talk on gravitational waves, the significance of the discovery and WVU’s work as part of the Department of Physics and Astronomy Colloquium Series on October 31 at 3:30 p.m. in room G09 of White Hall. The talk is open to the public and will be geared to both scientific and non-scientific audiences.
“These dense, massive neutron stars spiraled around each other with increasing speed, ripping each other apart in their collision,” said Etienne. “This incredible observation and our analysis provide a means to understand extreme matter inside neutron stars.”
With the first detection of gravitational waves, scientists had opened a new window to the universe, different from any existing way of gaining understanding.
“This new event bridges a gap,” said McWilliams. “It connects extreme gravity to extreme matter and electromagnetic energy so that events like this can act as a Rosetta stone for testing our understanding of how all three interact with one another.”
Etienne is principal investigator and McWilliams is co-principal investigator of an NSF-funded program to speed up data analysis, so that deeper insights into gravitational wave observations can be provided in weeks instead of months or years.
Together with Caleb Devine, a math alumnus; David Buch, an Honors College undergraduate from Beckley; Tyler Knowles, a math graduate student; and Serdar Bilgili, a physics and astronomy graduate student; Etienne and McWilliams sped up the codes that predict what the signal would look like coming from sources with different characteristics. McWilliams also contributed to the development of some of the models themselves.
“We were able to speed up parts of the data analysis by about a factor of 100,” said Etienne. “This helps to ensure timely dissemination of important results related to gravitational-wave discoveries to the scientific community.”
In addition, McWilliams, Paul Baker, physics and astronomy postdoctoral researcher; Belinda Cheeseboro, physics and astronomy graduate student; and Amber Lenon, physics and astronomy graduate student; worked to improve the tools used for “unmodeled searches,” in which scientists looked for anything unusual in the data.
“For this cosmic event, those types of tools are important for telling scientists how consistent the data is with our predictions, and therefore with general relativity itself,” said McWilliams.
For instance, scientists don’t know what happens at the end of these events, since the colliding neutron stars can immediately make a black hole, they can wait a while before collapsing into a black hole, or they can make a single stable gigantic neutron star.
“Unfortunately, we aren’t sensitive enough yet to detect any signal after the collision that would tell us exactly what happened, but the ‘unmodeled methods’ allow us to set limits on what could have happened,” said McWilliams.
A cosmic pairing
Neutron stars are the smallest, densest stars known to exist and are formed when massive stars explode in supernovae. A neutron star is less than 20 miles in diameter and is so dense that a teaspoon of neutron star material has a mass of about a billion tons.
As these neutron stars spiraled together, they emitted gravitational waves that were detectable for roughly 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.
The characteristic “chirps” of binary black holes discovered last year lasted a fraction of a second in the LIGO detector’s sensitive band, but the new detection’s chirp lasted much longer and was seen through the entire sensitive frequency range of LIGO — coincidentally about the same range of frequencies as the sound waves that fall within the human audible range.
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.
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.
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.
The LIGO data indicated that two astrophysical objects located at a relatively close distance of about 130 million light-years from Earth had been spiraling toward each other.
The data showed that the objects were not as massive as the binary black holes that LIGO and Virgo had detected in 2016. Instead, these 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 and too light to be black holes.
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.
But while one mystery appears to be solved, new mysteries have emerged that will yield new insights for years to come.
A fireball and an afterglow
Approximately 130 million years ago, two neutron stars were in their final moments of orbiting each other, separated only by about 200 miles and gathering speed while closing the distance between them.
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 ultra-dense 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.
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 show that heavy elements, such as lead and gold, are created in these collisions and subsequently distributed throughout the universe.
In the weeks and months ahead, telescopes around the world will continue to observe the afterglow of the neutron star merger and gather further evidence about various stages of the merger, its interaction with its surroundings, and the processes that produce the heaviest elements in the universe.
“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, 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.”
More about the LIGO-Virgo collaborations
LIGO is funded by the National Science Foundation, 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.
More than 1,200 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration and the Australian collaboration OzGrav. Additional partners are listed at http://ligo.org/partners.php.
The Virgo collaboration is funded by the Istituto Nazionale di Fisica Nucleare in Italy and the Centre National de la Recherche Scientifique in France. Virgo consists of more than 280 physicists and engineers belonging to 20 different European research groups: six from Centre National de la Recherche Scientifique in France; eight from the Istituto Nazionale di Fisica Nucleare in Italy; two in the Netherlands with Nikhef; the MTA Wigner RCP in Hungary; the POLGRAW group in Poland; Spain with the University of Valencia; and the European Gravitational Observatory, EGO, the laboratory hosting the Virgo detector near Pisa in Italy, funded by CNRS, INFN, and Nikhef.
Paper: “GW170817: Observation of gravitational waves from a binary neutron star merger.” (The paper will be available to read online at 10 a.m. EDT on Oct. 16, 2017.)
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