On the morning of August 17, gravitational waves arrived at Earth, tripping ultrasensitive detectors waiting for exactly this type of event.
First, the US-based Laser Interferometer Gravitational-Wave Observatory, or LIGO, caught a strong signal of gravitational waves from space. At nearly the same time, the Gamma-ray Burst Monitor on NASA’s Fermi space telescope had registered a burst of gamma rays. A third detector, Virgo, situated near Pisa, Italy, provided clues to the location of the cosmic event.
An alert went out to collaborators worldwide and within hours some 70 instruments turned their sights on the location a mere 130 million light-years away.
“The evidence that these new gravitational waves are from merging neutron stars has been captured, for the first time, by observatories on Earth and in orbit that detect electromagnetic radiation, including visible light and other wavelengths,” says Chad Hanna, assistant professor of physics and of astronomy & astrophysics at Penn State and a primary data analyst with LIGO.
The historic blip
“Near the very end, the two neutron stars were orbiting each other almost at the speed of light,” explains David Sand, assistant professor at the University of Arizona.
This animation captures phenomena observed over the course of nine days following the neutron star merger known as GW170817. They include gravitational waves (pale arcs), a near-light-speed jet that produced gamma rays (magenta), expanding debris from a kilonova that produced ultraviolet (violet), optical and infrared (blue-white to red) emission, and, once the jet directed toward us expanded into our view from Earth, X-rays (blue). (Credit: NASA’s Goddard Space Flight Center/CI Lab)
Sand is principal investigator of a supernova survey instrument that he describes as a “souped-up backyard telescope.” Sporting a puny, 16-inch primary mirror and tucked away in a dome atop a mountain in the Chilean Atacama Desert, the instrument is rigged with custom-made electronics enabling it to scan the night sky on its own, without human supervision.
About 11 hours after the gravitational waves came in, Sand’s automated telescope detected a blip—like a star that wasn’t there before—in the location that the LIGO and Virgo teams relayed.
That blip, according to Laura Cadonati, a professor of physics at Georgia Tech and deputy spokesperson for the LIGO Scientific Collaboration, “will be remembered as one of the most studied astrophysical events in history.”
“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,” says Vicky Kalogera of Northwestern University, the leading astrophysicist in the LIGO Scientific Collaboration.
Unexpected UV emission
About 16 minutes after notification from LIGO/Virgo, the Swift Gamma Ray Burst Explorer, a NASA satellite in low Earth orbit, moved to the new target. Swift, which orbits the Earth every 96 minutes and can maneuver to observe a target in as little as 90 seconds, began its search for an electromagnetic counterpart.
Initially, because of the predictions of theoretical models, the researchers thought that the electromagnetic radiation they would see would be X-rays. This is why NASA’s NuSTAR, (Nuclear Spectroscopic Telescope Array), which looks at X-rays, also searched the sky for electromagnetic signals. Neither Swift nor NuSTAR detected any X-rays.
“For gamma ray bursts, models predict that an early X-ray emission would be seen,” says Aaron Tohuvavohu of Swift science operations and Penn State. “But there were none detectable from this event until 9 days post-merger.”
Instead, Swift identified a rapidly fading ultraviolet afterglow. “The early UV emission was unexpected and very exciting,” Tohuvavohu adds.
Gamma ray bursts appear as a directional burst of energy from collapsed massive stars. Any type of detector must be within a certain arc of the burst to see it. The afterglow of the explosion, is however, more omnidirectional.
“Whatever we thought was going to happen, wasn’t what actually happened,” says Jamie A. Kennea, head of the Swift Science Operations team and associate research professor of astronomy and astrophysics at Penn State. “The next neutron star-neutron star merger event could look very different.”
Gold and platinum from a kilonova
The neutron stars’ spiral death dance ended with an extremely violent and bright collision—powerful enough to forge gold, platinum, lead, and other heavy elements.
Astronomers at Las Cumbres Observatory in Santa Barbara, California activated their robotic network of 20 telescopes around the world and were one of six teams to co-discover a new source of light in that region and localize it to the galaxy NGC 4993.
“Such a gravitational wave signal had never been seen before but was unmistakably generated by two neutron stars spiraling together,” explains Iair Arcavi, a postdoctoral fellow in the University of California, Santa Barbara’s physics department and leader of the LCO follow-up effort that appears in the journal Nature.
A tablespoon full of neutron stars weighs as much as Mt. Everest.
The outburst that occurs right after two neutron stars merge is called a kilonova, a phenomenon that had long been theorized though never conclusively observed—until now. Unlike traditional ground-based facilities with single telescopes, the LCO network could observe the phenomenon every few hours for five consecutive days. During that time, the light from the explosion dimmed by a factor of 20, fading at an unprecedented rate for something so luminous.
“For years, we’ve heard theorists predict how a kilonova should look. I couldn’t believe we were finally seeing one for the first time,” Arcavi says.
The name originates from the prediction that a kilonova would be a thousand times brighter than a nova, though dimmer than a supernova. Kilonovae are thought to be the primary source of all the elements heavier than iron in the universe. For example, most of the gold on Earth may have been created in a kilonova.
“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,” says Raffaella Margutti, an assistant professor of physics and astronomy at Northwestern and co-leader of two observational efforts covering the electromagnetic spectrum that follow up on the August 17 gravitational-wave signal.
What’s so special about neutron stars?
A neutron star results when the core of a large star collapses and the atoms get crushed. The protons and electrons squeeze together and the remaining star is about 95 percent neutrons. A tablespoon full of neutron stars weighs as much as Mt. Everest.
“The window is open and there are going to be mind-blowing surprises.”
“Neutron stars have some of the strongest gravity you’ll find—black holes have the strongest—and thus they give us handles on studying strong-field gravity around them to see if it deviates at all from General Relativity,” says Mandeep Gill, the outreach coordinator at KIPAC at SLAC and Stanford University, and a member of the Dark Energy Survey collaboration.
Astronomers proposed the existence of neutron stars in 1934. They were first found in 1967, and then in 1975 a radio telescope observed the first instance of a binary neutron star system. From that discovery, Roger Blandford, professor of physics at Stanford, and colleagues confirmed predictions of the General Theory of Relativity.
Blandford says the calculations related to the system Advanced LIGO saw are even more complicated because the stars are much closer together and could only be completed by a computer. This observation continues to support the General Theory of Relativity but Gill is hopeful that additional binary neutron star systems may begin to inform extension to the theory that could reveal how it fits with quantum theory, dark energy, and dark matter.
“One of the things I find terribly exciting about these observations is that not only do they confirm aspects of astronomical and relativistic precepts but they actually teach us things about nuclear physics that we don’t properly understand,” says Blandford.
As we observe more of these systems, which scientists anticipate, we may finally understand long-standing mysteries of neutron stars, like whether they have earthquakes on their crust or if, as suspected, they have small mountains that send out their own gravitational wave signal.
“Even though we’ve been doing astronomy since the dawn of civilization, every time we turn on new instruments, we learn new things about what’s going on in the universe,”says Brian Lantz, a senior research scientist who leads the Engineering Test Facility for LIGO at Stanford.
“If the elements heavier than iron are actually made in events like this, that stuff is here on Earth and it’s likely that was generated by events like this. It gives you sort of a way to reach out and touch the stars.”
“This is only a beginning. There are many innovations to come and I don’t know where we’re going to be in 10 years, 20 years, 30 years,” says Peter Michelson, a professor of physics at Stanford University who helms the Fermi Large Area Telescope. “The window is open and there are going to be mind-blowing surprises. That, to me, is the most exciting.”