Four Columbian College astrophysicists are part of a global group of scientists who collaborated to identify and study the first confirmed observation of two merging neutron stars, a so-called kilonova. The existence of a kilonova—an explosive event roughly 1,000 times brighter than a nova—had long been suggested but was never definitively witnessed until now.
“For the first time ever, we have detected both gravitational waves and electromagnetic radiation at the same time from a single source,” said Professor of Physics Chryssa Kouveliotou, head of the GW Astrophysics group. “With these observations, we put the astrophysical context to the gravitational waves. This is a monumental result—the result of the decade.”
The four CCAS astrophysicists involved in the finding are Kouveliotou, Assistant Professor of Physics Alexander van der Horst, Associate Professor of Honors and Physics Bethany Cobb Kung and Assistant Professor of Physics Sylvain Guiriec.
The merger of two neutron stars was predicted to generate both gravitational waves and emit electromagnetic radiation at various wavelengths. Researchers also suspected that these mergers are the source of mysterious short gamma-ray bursts. The recent observations confirmed those predictions.
In August, scientists were alerted to a gravitational wave event by both the Advanced Laser Interferometer Gravitational-Wave Observatory in the United States and the Advanced Virgo Interferometer in Italy. Two orbiting space observatories—the European Space Agency’s INTErnational Gamma Ray Astrophysics Laboratory and NASA’s Fermi Gamma-ray Space Telescope—detected a short gamma-ray burst coming from the same area of the sky as the gravitational wave event.
In the days following the initial detection, scientists aimed their instruments at the object, located some 130 million light-years away in the constellation Hydra. The effort included, among others, the European Southern Observatory’s telescopes in Chile, Gemini South and the Cerro Tololo Inter-American Observatory (CTIO), which detected the light from this gravitational wave event. Together these observations confirmed that a kilonova had been detected.
Gamma-ray bursts are the most powerful, brightest explosions in the universe. These short, transient events can last a few seconds or minutes, providing information about the formation and expansion of our universe. Kouveliotou, a leading expert on gamma ray bursts, worked closely with the research group led by Elena Pian of Istituto Nazionale di Astrofisica in Italy to study the optical and infrared electromagnetic radiation emitted from the merger. Since each element in the universe emits a unique spectrum, this data can help researchers determine how and where rare elements on Earth originate. The work published in Nature by Kouveliotou, Pian and other scientists confirms that neutron-star mergers are major cosmic production sites of heavy elements such as gold, platinum and lanthanides.
Other members of the GW Astrophysics Group studied different aspects of the kilonova. Van der Horst and Kouveliotou co-authored a paper that examined the polarimetry—the measurement of the polarization of transverse waves—of the light emitted by the neutron-star merger. By studying the light emitted by the event in this way, scientists are learning more about how the light is produced.
“Polarimetry is a very powerful tool to determine properties of the source producing the light that we observe,” said van der Horst. “While this particular event did not have a strong polarimetry signal, future observations will be able to pin down the structure of the source with such observations.” Van der Horst is also leading a science team working on OCTOCAM, an instrument being built at the Gemini South Observatory that will allow researchers to do in-depth studies on objects such as kilonovae once they are detected. OCTOCAM is scheduled for completion in 2021.
Cobb Kung used a telescope at CTIO to observe the infrared glow from the kilonova over a course of 10 nights. She was a part of a team of astronomers that studied the composition of the material released by the merging of the two stars, further confirming that a kilonova had been witnessed.
“Near infrared observations of the event were absolutely critical,” Cobb Kung said, “because the optical light from the source faded away very quickly, and one of the key signatures of a kilonova is its behavior at infrared wavelengths.”
Guiriec, who is also an assistant researcher at NASA Goddard Space Flight Center, analyzed the data from the gamma ray burst discovered by NASA’s Fermi telescope in coincidence with the gravitational wave signal.
"This is a new era for astrophysics. We can now observe objects that we cannot see. It is like waking up one morning with a new sense to perceive the universe with brand new perspectives" Guiriec said.