A long time ago in a galaxy 130 million light years away …
… two neutron stars locked in a gravitational embrace whizzed around each other at blinding speed, sending gravitational waves – ripples in the “fabric” of space and time – billowing through the cosmos. The pair collided in a smash-up finale that scattered gold, platinum, uranium, and other heavy metals into space, and sparked a dazzling cosmic light show that radiated across the electromagnetic spectrum.
Astronomers learned about this neutron star collision, called a “kilonova,” when gravitational waves arrived at Earth on Aug. 17 – 130 million years after the event. The waves buzzed through the Laser Interferometer Gravitational-Wave Observatory (LIGO), with detectors in Louisiana and Washington, for 100 seconds. The Virgo interferometer in Italy received a much shorter, weaker signal in its blind spot. Together, the LIGO-Virgo network localized the event on a banana-sized section of sky in the southern hemisphere, so astronomers could steer telescopes in the right direction to pinpoint its location.
Less than two seconds after the gravitational wave signal stopped, a short gamma ray burst reached NASA’s Fermi Gamma-ray Space Telescope, which orbits 300 miles above Earth. The gravitational waves meant something big happened, like the merger of two black holes. But the companion light waves meant the source was something else; in this case, a neutron star merger. Those multiple signals from one stellar event opened the door to a new field of cosmic study: multi-messenger astronomy.
A New View of the Universe.
“It’s basically a completely new way of studying astronomical objects,” explains Vicky Kalogera, Ph.D., Director of Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), and the lead Northwestern scientist on the International LIGO Scientific Collaboration (LSC). “It’s like the change we experienced when we transitioned from having silent movies to also having sound. In fact, gravitational wave detectors are more like our ears, so it’s truly this transition from having pure light and images to having light, images, and sound together.”
The discovery brings a number of firsts, including detecting a neutron star collision, receiving a notably long gravitational wave signal, confirming that short gamma ray bursts originate from neutron star mergers, and observing every type of light wave emanating from a single event.
“It’s the first time we are seeing the death spiral of two neutron stars, spinning around each other at about the speed of light and colliding,” Dr. Kalogera says. “But the story does not end there because for the first time we also have this cascade of light emission that starts two seconds after the gravitational wave ends. We get gamma rays and other light all the way through the electromagnetic spectrum. And the signal is much longer so we can study it in much more detail.”
Within 16 days of detecting gravitational waves and gamma rays from the collision, astronomers had seen the kilonova’s aftermath in visible light, X-rays, ultraviolet waves, infrared light, and radio waves.
The kilonova itself would have been a cause for celebration, says Shane Larson, Ph.D., a gravitational wave astrophysicist at Northwestern and a member of the LIGO Scientific Collaboration. But the multi-messenger detection also confirmed one of Albert Einstein’s predictions a century ago: that gravitational waves would travel at the speed of light.
“Because light waves, which travel at the speed of light, arrived within two seconds
of the gravitational waves, we know gravitational waves also travel at the speed of light,” Dr. Larson explains. “It’s the best experiment so far that has demonstrated this.”
Supernova Remnants That Pack a Punch.
When a star 10 to 20 times the mass of our sun prepares to die, intense heat in its core combines electrons and photons into neutrons. After the star’s exterior explodes away (a “supernova”), the neutron-rich core remains.
This much smaller neutron star – a “stellar skeleton,” says Dr. Larson – packs 1.5 times the mass of our sun into a ball only 15 miles wide, with a fearsome gravity pull. A spoonful of neutron star is so dense it would weigh 30 times the total weight of every person on Earth. “If you could walk along the surface of a neutron star, and you had the misfortune of falling off a one millimeter cliff, by the time you reach the bottom you’d be traveling 130,000 miles per hour,” Dr. Larson says.
Heavy Metal Stars
The kilonova detection solved a mystery regarding where the precious metals for our jewelry come from. Stars 10 or more times the mass of our sun are known to synthesize lighter elements such as nitrogen, oxygen, aluminum and sodium within their cores, through a process called nuclear fusion. Scientists long theorized that these stars’ supernova explosions also produced heavier elements – gold, silver, platinum, uranium, titanium, and dozens more elements that appear after iron on the Periodic Table. But the kilonova changed that.
“In recent years, it’s been theorized that these neutron star collisions have the right chaotic conditions – high pressure and temperature – and the right material to create heavy elements,” explains Wen-fai Fong, Ph.D., a Hubble Postdoctoral Fellow at Northwestern and an electromagnetic astronomer who studies cosmic explosions. “With this event we were able to take a spectrum at optical and infrared wavelengths and actually see signatures of heavy elements being created in this merger.” This was the first evidence that neutron star mergers, not supernovas, created heavy elements, Dr. Fong says.
Kilonovas fling clouds of heavy metal particles into space – particles much too small to make into a watch or wedding ring. Eventually some of those particles will be picked up by a star and integrated into new planets.
“During formation, planets take up whatever material is around them, like hydrogen, helium, and trace amounts of other elements,” Dr. Fong explains, which coalesce into metals and elements the way they did on our own planet billions of years ago. “That’s why we say all the platinum and gold on Earth came from a neutron star merger.”
CIERA Scientists First on the Cosmic Scene.
Dr. Fong and her Northwestern colleague, Rafaella Margutti, Ph.D., an Assistant Professor in Physics and Astronomy, were on one of the first three scientific teams in the world to see the kilonova through a telescope. They used the Dark Energy Camera in Chile to locate the gravitational wave’s “optical counterpart” – the kilonova’s visible light – in Galaxy NGC 4993, in the constellation Hydra.
The event was right above Earth’s horizon and could be seen for only about an hour each night after sunset. By now it has disappeared behind the sun and will reappear in early December, after which the team can start assessing exactly what the merger produced: a larger neutron star or a black hole. Current observations favor the latter.
“There’s still tons of material that was ejected from this neutron star merger, and that’s what’s causing all this electromagnetic light,” Dr. Fong explains. “We need to wait for that cloud of material to disperse to see what’s inside.” There is no extra energy source to indicate a large neutron star, she says, although one may have formed and then quickly collapsed into a black hole.
Radio waves will help solve the mystery. Dr. Fong and Dr. Margutti have remote access to radio telescopes at the Very Large Array in New Mexico – a group of 27 radio antennas that work together as one large telescope – to observe the event around the clock. By next summer, they may be closer to an answer.
“From the electromagnetic side, we have every reason to believe a black hole was created” Dr. Fong says. “We’re very excited for it to come out from behind the sun. It could yield unexpected surprises.”
Scientists On Both Sides of the Kilonova
Northwestern has multiple specialists involved with the kilonova detection, plus several students and postdoctoral researchers. Along with the astronomers, NU’s LIGO team includes Selim Shahriar, an instrumentation expert in the Department of Electrical Engineering in the McCormick School of Engineering.
“This recent discovery was especially important at Northwestern because we have faculty on both the LIGO gravitational-wave and electromagnetic discovery teams,” says Jay Walsh, NU’s Vice President for Research. “Our LIGO group, led by Vicky Kalogera, brings together world-class gravitational-wave and electromagnetic astronomers, as well as experts in engineering and computer science, positioning us well to make an ongoing impact as this new age of astronomy gets underway.”
Multi-Messenger Astronomy: The Universe Speaking Many Languages.
Because it generated gravitational and electromagnetic waves, Dr. Fong likes to think of the neutron star merger as speaking multiple languages. This new multi-messenger astronomy brings together physicists, engineers, computer programmers, observational astronomers, and more.
“It is incredibly exciting to have opened up a new window of discovery, to be able to interpret the multiple languages that the Universe is speaking — gravitational waves and electromagnetic radiation — and have the tools to answer these questions within our lifetimes,” enthuses Dr. Fong, who views it as a rare opportunity to work with many different disciplines. “This event is just the beginning. We have so much more to learn.”