EVANSTON - An international research collaboration, including four Northwestern University astronomers, is the first to detect the spectacular collision of two neutron stars using both gravitational waves and light.
The discovery ushers in an exciting new era in astronomy — multi-messenger astronomy with gravitational waves — less than two years after the first detection of gravitational waves opened a new window onto the universe.
Northwestern’s Vicky Kalogera, the leading astrophysicist in the LIGO Scientific Collaboration (LSC), was one of six experts on a special panel at the National Science Foundation press conference announcing the news at the National Press Club in Washington, D.C.
Gravitational waves were directly detected for the first time Sept. 14, 2015, by the Laser Interferometer Gravitational-Wave Observatory (LIGO), confirming Einstein’s theory of general relativity. Following the discovery, the observatory’s architects were awarded the 2017 Nobel Prize in Physics.
The historic discovery of colliding neutron stars — which happened the morning of Aug. 17 with the longest gravitational-wave signal detected to date and a short gamma ray burst signal — was made by thousands of scientists and engineers using the U.S.-based LIGO; the Europe-based Virgo gravitational wave detector; and some 70 ground- and space-based observatories, including NASA’s Hubble Space Telescope.
The neutron stars’ spiral death dance in a nearby galaxy, 130 million light-years from Earth, ended with an extremely violent and bright collision — powerful enough to forge gold, platinum, lead and other heavy elements.
“Mergers of double neutron stars were predicted over many decades to drive such powerful explosions, but this multi-messenger discovery brings two key pieces of the puzzle together for the first time,” Kalogera said. “Our discovery confirms a lot of our theoretical predictions, including that double neutron stars give rise to gamma rays, optical, infrared, X-rays and radio waves. At the same time, there are hints in these observations that are providing new mysteries we still need to understand.”
Northwestern's leadership position
Northwestern holds a unique leadership position in the discovery of the rare neutron star merger with four astronomers in the worldwide collaborations that made the discovery — two on the gravitational-wave side, including Kalogera, and two on the electromagnetic radiation side. They and their research groups worked within their collaborations in leadership roles, producing several of the key science publications.
“Northwestern has played a singular role in this discovery, and that really is not an exaggeration,” Kalogera said. She is the Daniel I. Linzer Distinguished University Professor of Physics and Astronomy and director of Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) in Northwestern’s Weinberg College of Arts and Sciences.
Neutron stars are the smallest, densest stars known to exist and are formed when massive stars explode in supernovae. A neutron star is about 12 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 detected on Aug. 17 spiraled toward each other, they emitted gravitational waves — ripples in space and time — that were detectable on Earth for about 100 seconds; when the stars collided, a flash of light in the form of gamma rays was emitted and seen on Earth about two seconds after the gravitational waves were observed. 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.
“This combination of light and gravitational waves is brand new and very exciting — we’ve never had this kind of observation before,” Kalogera said. “With the gravitational-wave signals from three detectors, two in the U.S. and one in Italy, we were able to tell our electromagnetic colleagues, working across the electromagnetic spectrum, where in the sky to point their telescopes to find the pair of neutron stars.”
At Northwestern, Kalogera has built a strong and unique research team that covers both areas of astronomy critical to this discovery — and with many more discoveries to come. Kalogera leads Northwestern’s LIGO Scientific Collaboration (LSC) team, which includes fellow theoretical astrophysicist Shane L. Larson. On the electromagnetic side are Raffaella Margutti and Wen-fai Fong, two new faculty hires in astronomy at the University.
Selim Shahriar, professor of electrical engineering and computer science at Northwestern’s McCormick School of Engineering, also is a member of Northwestern’s LSC team and part of the Aug. 17 discovery. More than a dozen Northwestern postdoctoral fellows and graduate and undergraduate students also are involved.
Margutti and Fong are leading two observational efforts covering the electromagnetic spectrum that follow up on the Aug. 17 gravitational-wave signal. Each of them is leading one of the eight publications coming out of the electromagnetic collaboration.
“The universe produces very different radiations — it is trying to communicate with us in multiple ways,” said Margutti, an expert in stellar explosions. “Before LIGO, it was the equivalent of astronomers using one eye to look at our universe. Now, with gravitational wave detectors, we have a new independent channel of information, providing us with a second eye for investigating the mysteries of the universe.
“Thanks to this multi-messenger event, we know for a fact that neutron star mergers can produce heavy elements such as gold, silver and iron, which are so important to us on this planet,” she said.
Margutti, an assistant professor of physics and astronomy and a CIERA member, said once word of the gravitational-wave signal spread to observational astronomers on Aug. 17, they had to rush to point their telescopes at the right area of the sky before sunset so they would be ready. For Margutti and Fong, that telescope was the 4-meter Victor Blanco telescope in Chile. The visible light signal came shortly after sunset, loud and clear, only 12 hours after the gravitational-wave signal reached Earth.
Several days after the neutron star merger, Margutti and Fong also saw electromagnetic waves with the Chandra X-ray Observatory in space (X-rays) and with the Very Large Array in New Mexico (radio waves).
The gravitational-wave signals and electromagnetic radiation signals each contribute different critical pieces of information about the same cosmic event. Gravitational waves tell the scientists what the objects are, the masses of the objects and roughly where in the sky the event happened. Electromagnetic waves are used to pinpoint the location in the sky exactly — what galaxy produced the neutron star merger and where in the galaxy it happened.
“We’ve always predicted that if you had light and gravity together, the two pieces of information could tell you something more,“ said Larson, a research associate professor of physics and astronomy, associate director of CIERA and a LIGO Scientific Collaboration (LSC) member.
“This is the event we’ve all been waiting for — binary neutron stars — because all four previous gravitational wave detections have been binary black holes,” Larson said. “Black holes don’t emit light, but neutron stars are made of matter that does emit light. These skeletons of dead stars are telling us something about how stars live and evolve in the universe and how they fill the stellar graveyard with whatever it is that they’ve done during their lives.”
The gravitational-wave signal, named GW170817, originated in galaxy NGC 4993 in the constellation Hydra. It was first detected Aug. 17 at 8:41 a.m. Eastern Daylight Time 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 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. Rapid gravitational wave detection by the LIGO-Virgo team, coupled with Fermi’s gamma-ray detection, enabled the launch of follow-up observations by telescopes around the world, including in Chile and New Mexico, the location of two of the telescopes with which the Northwestern astronomers work.
“I remember waking up that day to a flurry of emails saying there is a gamma-ray burst in conjunction with a gravitational wave event,“ said Fong, a Hubble Postdoctoral Fellow at CIERA whose expertise is in gamma-ray bursts. “I thought, ‘This is what we’ve been waiting for our entire careers.’
“To be involved in electromagnetic follow up of gravitational wave sources — that’s essentially what I was hired to do here at Northwestern,“ said Fong, who will become an assistant professor of physics and astronomy at Northwestern in fall 2018. “Never did I realize that 17 days after I started, the event of the century would happen. Up until now, there have been decades of theories predicting what these sources should look like, and it’s very exciting because this event remarkably matches these theories.”
Only the beginning
And this multi-messenger discovery of colliding neutron stars is only the beginning for all the astronomers, scientists and engineers around the globe involved in studying the cosmos. Astronomers know that two neutron stars merged into one compact object, but what exactly is it? And why was the gamma ray burst so surprisingly faint? These are only a few of the questions raised. Plus, there are many more extreme cosmic events to discover.
“One might say, ‘Okay, you’ve done your job, now let’s go home,’” Kalogera said. “But in reality, this is just the beginning for us. The more sources like this we detect, the more we can learn. The universe doesn’t stop with one such collision, and not all collisions will be the same. We know that from the binary black holes. We’re bound to find new mysteries."