Scientists worldwide reported in October the detection of both gravitational waves—ripples in space and time—and light originating from the spectacular collision of two neutron stars, marking the first time that a cosmic event has been viewed in both gravitational waves and light. The discovery was made beginning on Aug. 17 using the US-based Laser Interferometer Gravitational-Wave Observatory (LIGO), the European-based Virgo detector and some 70 ground- and space-based observatories.
Neutron stars are the smallest, densest stars known to exist, 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 the neutron stars in question spiraled together, they emitted gravitational waves that were detectable for roughly 100 seconds. And 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.
The observations have given astronomers an unprecedented opportunity to probe a collision of two neutron stars. For example, observations made by the US Gemini Observatory, the European Very Large Telescope and NASA’s Hubble Space Telescope revealed signatures of recently synthesized material, including gold and platinum, solving a decades-long mystery of where about half of all elements heavier than iron are produced.
“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.”
The gravitational signal, named GW170817, was first detected on Aug. 17 at 8:41 a.m. Eastern Daylight Time by the two identical LIGO detectors, located in Hanford WA and Livingston LA. Information provided by Virgo, a detector 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 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.
“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,” says Laura Cadonati, professor of physics at Georgia Tech and 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.”
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.
“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,” says David Shoemaker, spokesperson for the LIGO Scientific Collaboration and senior research scientist in MIT’s Kavli Institute for Astrophysics and Space Research. “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.”
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 the NASA Fermi telescope 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.
“For decades we’ve suspected short gamma-ray bursts were powered by neutron star mergers,” says Fermi project scientist Julie McEnery of NASA’s Goddard Space Flight Center. “Now, with the incredible data from LIGO and Virgo for this event, we have the answer. The gravitational waves tell us that the merging objects had masses consistent with neutron stars, and the flash of gamma rays tells us that the objects are unlikely to be black holes, since a collision of black holes is not expected to give of light.”
But while one mystery appears to be solved, new mysteries have emerged. The observed short gamma-ray burst was one of the closest to Earth seen so far, yet it was surprisingly weak for its distance. Scientists are beginning to propose models for why this might be, McEnery says, adding that new insights are likely to arise for years to come.
Each electromagnetic observatory will be releasing its own detailed observations of the astrophysical event. In the meantime, 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, 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 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.
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.
The kilonova was first identified in Dark Energy Camera (DECam) images by Ohio University astronomer Ryan Chornock, who instantly alerted his colleagues by email. “I was flipping through the raw data, and I came across this bright galaxy and saw a new source that was not in the reference image [taken previously],” he says. “It was very exciting.”
DECam is the primary observing tool of the Dark Energy Survey (DES). Once the crystal clear images from DECam were taken, a team led by professor Edo Berger, from the Harvard-Smithsonian Center for Astrophysics (CfA), went to work analyzing the phenomenon using several different resources. Within hours of receiving the location information, the team had booked time with several observatories, including NASA’s Hubble Space Telescope and the Chandra X-ray Observatory.
LIGO/Virgo works with dozens of astronomy collaborations around the world, providing sky maps of the area where any detected gravitational waves originated. The team from DES and CfA had been preparing for an event like this for more than two years, forging connections with other astronomy collaborations and putting procedures in place to mobilize as soon as word came down that a new source had been detected. The result is a rich data set that covers “radio waves to X-rays to everything in between,” Berger says. “This is the first event, the one everyone will remember. I’m extremely proud of our entire group, who responded in an amazing way. I kept telling them to savor the moment. How many people can say they were there at the birth of a whole new field of astronomy?”
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 event, its interaction with its surroundings, and the processes that produce the heaviest elements in the universe.
“When we were first planning LIGO back in the late 1980s, we knew that we would ultimately need an international network of gravitational-wave observatories, including Europe, to help localize the gravitational-wave sources so that light-based telescopes can follow up and study the glow of events like this neutron star merger,” says Caltech’s Fred Raab, LIGO associate director for observatory operations. “Today we can say that our gravitational-wave network is working together brilliantly with the lightbased observatories to usher in a new era in astronomy, and will improve with the planned addition of observatories in Japan and India.”