UH astronomers and their international collaborators announced the discovery and study of the first binary neutron star merger detected in gravitational waves in articles published today in Science, Nature, and the Astrophysical Journal. Two neutron stars — the leftover remnants of massive stars that used up all of their fuel — were orbiting each other in a death spiral, emitting gravitational waves until they finally collided and merged. The cataclysmic coalescence ejected a few percent of the neutron stars’ material into space at about one-quarter the speed of light.
This rare neutron-rich material produced new and highly radioactive atomic nuclei, which rapidly decayed in an eerie glow called a kilonova. The study of this event shows that at least some of the elements heavier than iron were originally created in binary neutron star mergers like this one.
“We are made of star stuff.” Carl Sagan famously said. This reflected astronomers’ understanding that much of the material in our bodies and in the Earth, originated in stars. For decades, it was thought that some of the elements heavier than iron — such as silver — came from the dying explosions of massive stars. Now, UH astronomers together with their international collaborators have seen for the first time a different way that such elements are created and dispersed in the universe.
The amazing sequence of events began August 17 at 02:41 HST with alerts from the Nobel Prize-winning Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Gamma-ray Burst Monitor aboard NASA’s Fermi Observatory. The two LIGO detectors in Hanford, Washington, and Livingston, Louisiana, together with the Virgo detector in Pisa, Italy, spotted a burst of gravitational waves they named GW170817. This burst came from the last few minutes of the two neutron stars’ lives, as they circled each other, getting ever closer until they finally collided. Gravitational waves are ripples in the fabric of space-time, where space itself expands and contracts in a strange undulation; in a merger like this they increase in frequency like running your fingers up the keyboard of a piano.
Next, Fermi witnessed a burst of high-energy gamma rays from the same direction as GW170817, a mere 1.7 seconds after the crescendo of gravity waves. However the LIGO/Virgo detector could only provide a rough estimate of the position of the source in the sky. Actually finding the precise location of GW170817 and studying this extraordinary event would require an armada of telescopes and astronomers from around the world.
Dr. Benjamin Shappee, who just joined the faculty of the Institute for Astronomy, is a member of the One-Meter-Two-Hemisphere (1M2H) discovery team that leapt into action. This small team, a collaboration between Carnegie Observatories and UC Santa Cruz, swiftly used the Carnegie Institute’s Swope and Magellan telescopes to image galaxies that might have been the host of the neutron star merger. Less than 11 hours after the LIGO/Virgo alert, the 1M2H team found a new bright object, Swope Supernova Survey 17a (SSS17a) in the galaxy NGC 4993.
“It is amazing that with the multitude of teams searching for this source, our small team of mostly young astronomers was the first to image, discover and report it,” said Dr. Shappee. Based on the coordinates provided by Ben and his team, astronomers and observatories worldwide quickly followed up to confirm and study SSS17a.
Pan-STARRS, the University of Hawaii’s sky survey facility, being further west than Chile, had to wait for nightfall in Hawaii to scan the region. But this delay turned out to be an important advantage. When first observed, it was unclear if SSS17a was the true visible counterpart to the gravity wave source — it could have been an unrelated supernova. Pan-STARRS observed SSS17a six hours after the 1M2H discovery, when it was no longer visible from Chile. Pan-STARRS, which has previously surveyed most of the sky and is renowned for its precise calibration, was able to use its “before” images along with the new data to quickly and precisely measure that SSS17a had significantly faded in the few hours since its discovery.
“A new astronomical object fading this fast is unheard of, and the Pan-STARRS Team alerted the worldwide community to the unique nature of SSS17a,” said Ken Chambers, Director of the Pan-STARRS Observatory, “This was the signature of a kilonova.” These observations were critical for both studying the physics of a kilonova and allowing astronomers around the world to be confident that SSS17a was the true counterpart to GW170817. Now the teams knew this was the right object to focus their precious resources upon as it faded from view.
Every kind of telescope in the world and in orbit changed course to observe SSS17a (later given the IAU designation of AT2017gfo), including the NASA Fermi (gamma-rays), Chandra (x-rays), and Swift (Ultraviolet and X-ray) Observatories; the Hubble Space Telescope; and radio telescopes such as the NRAO Jansky Very Large Array, along with many others in a massive international effort unprecedented in the history of astronomy.
The picture that emerged over the following two weeks was that the initially bluish object changed into a redder and strangely colorful object the likes of which has never been seen before. The final piece of the puzzle came from spectra taken by the UH astronomers and their international collaborators on a variety of telescopes. The neutron-rich material ejected from the merger is a fertile environment for the kind of nuclear reactions that build larger and larger nuclei from smaller ones — and some of these were seen. “We see fingerprints of key elements that are heavier than iron,” said Chambers. The result is a fundamental change in our understanding of the origin of at least some of the heavier elements, many of which are common on Earth and are even in our bodies. “It is exciting to play a part in a story that will make it into the textbooks” said Shappee.
Founded in 1967, the Institute for Astronomy at the University of Hawaii at Manoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Maunakea. The Institute operates facilities on the islands of Oahu, Maui, and Hawaii.
Benjamin Shappee was partially supported by NASA during this work through Hubble Fellowships awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. The 1M2H is a collaboration between Carnegie Observatories and the University of California Santa Cruz. The Pan-STARRS1 observations were supported in part by the NASA Grant No. NNX14AM74G issued through the SSO Near Earth Object Observations Program, and the Queen’s University Belfast. The Pan-STARRS1 Surveys were made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, the Queen’s University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, and the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation Grant No. AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), and the Los Alamos National Laboratory. The Pan-STARRS1 Surveys are archived at the Space Telescope Science Institute (STScI) and can be accessed through MAST, the Mik
ulski Archive for Space Telescopes. Additional support for the Pan-STARRS1 public science archive is provided by the Gordon and Betty Moore Foundation.
This work was also supported in part by the European Southern Observatory and we acknowledge ESO program 199.D-0143 and 099.D-0376. We acknowledge the Leibniz-Prize to Prof. G. Hasinger (DFG grant HA 1850/28-1) for support of GROND observations.
