Edo Berger from the Harvard-Smithsonian Center for Astrophysics explains this historic event and what we have already learned from it.
By Shari Lifson, Peter Michaud, and Joan Najita
The end of August was a very exciting time for several of AURA’s observatories. While many people were relaxing at the beach and enjoying the last days of summer, a historic scientific story was unfolding. On the night of August 17th, the National Science Foundation (NSF) supported Laser Interferometer Gravitational-Wave Observatory (LIGO) detected a new gravitational wave, but this one was different. Researchers realized this gravitational wave resulted from a collision of two neutron stars, and observing the aftermath was a real possibility. Teams of astronomers around the globe leapt into action and contacted observatories where they had rapid “Target of Opportunity” plans already in place. With help from the VIRGO Interferometer in Italy, the observatories responded quickly and the hunt was on.
Before VIRGO came on line and joined LIGO in detecting gravitational waves, it was very difficult to find just where in the sky those waves were coming from. Now, with the addition of information from VIRGO, the portion of the sky that needed to be searched was still large, but not impossible.
Edo Berger from the Harvard-Smithsonian Center for Astrophysics, and his team chose the Dark Energy Camera (DECam) on the Blanco telescope at NSF’s Cerro Tololo Inter-American Observatory (CITO) to begin the search. The wide-field Dark Energy Camera is capable of scanning large portions of the sky very quickly, and allowed the team to find a new bright source they believed, caused the gravitational wave.
Continued imaging with DECam and spectroscopy with the SOAR telescope, also located at CTIO, over the following ten nights revealed that the detected optical emission matched the expected brightness and color evolution of a merging neutron star binary, a “kilonova.”
SOAR Observatory Director Jay Elias, who coordinated the SOAR follow up effort, commented that “When we started the follow up, we were concerned that the light we were seeing might be from an unrelated supernova from the same region of the sky rather than the optical counterpart. That explanation became more unlikely as the observations progressed.”
Mansi Kasliwal from Caltech led another team in the race to find and analyze the light from the neutron star collision. Both Kasliwal’s team and Berger’s team also used the 8-meter NSF funded Gemini South telescope to follow the evolution of the infrared spectrum of the source. Using the spectra from SOAR and Gemini, the teams studied in detail the composition of the material ejected in the merger.
Gemini Observatory director Laura Ferrarese recounted the challenges faced by the flood of requests for observations once the source was pinpointed. “We had four teams involved with the follow-ups and we were able to coordinate the observations, which was challenging because the target could only be observed for two hours in the beginning of the night. We didn’t have a lot of time, but luckily Gemini is very flexible, we can change instrument set ups very quickly so we are very well positioned for these types of events.” Ferrarese added that the greatest challenge involved scheduling the observations so that all of the teams would receive the data they needed – a task that, in her words, “…required lots of coordination, and a good dose of diplomacy!”
The challenges extended to the observations themselves, according to Gemini astronomer Hwihyun Kim, who was instrumental in obtaining the Gemini data which primarily used the FLAMINGOS-2 infrared imager and spectrograph. “We were very lucky with observing this target,” said Kim. “It was not always easy to see the source but the field had a very bright star that helped our pointing even when the object was getting lost in the glow of twilight.” Kim adds that everyone in the control room was nervous as the observation window got shorter and shorter each night. “Each night we pointed the telescope until we hit the absolute lowest limit that the telescope could reach.” See interview of Kim and Lopez.
On August 22nd NASA’s Hubble Space Telescope, science operations conducted by Space Telescope Science Institute, also trained its eye on the kilonova and gathered data on the spectrum, movement, and chemical composition of the ejected material from the collision. The spectrum from Hubble and Gemini confirm for the first time that heavy elements, including gold and platinum, are produced by these intense neutron star collisions.
AURA Board member and LIGO Executive Director David Reitze commented, “The LIGO-Virgo gravitational-wave detectors, in conjunction with one of the largest coordinated multi-wavelength astronomical observing campaigns ever carried out, have for the very first time seen the cataclysmic collision of two neutron stars. Through the astronomical observations, we’ve learned that gold and other heavy elements are produced in the resulting maelstrom, so this discovery is a scientific goldmine – literally! This is the first time that an astronomical event has ever been seen simultaneously in gravitational waves and light, paving the way for a new and richer way to study the universe.”
The collaboration foretells a bright future for wide-field imagers and facilities optimized for the study of time variable events. According to Berger, this discovery “demonstrates the power and importance of DECam for optical follow-up of gravitational wave sources.” Looking to the future, toward future LIGO and Virgo observing runs, Berger anticipates that “with its high sensitivity and ability to survey large areas of sky, DECam will play an almost unique role in the identification of future gravitational wave events.”
The National Science Foundation supports LIGO as well as facilities involved in pinpointing and studying the optical counterpart to GW170817, including CTIO and Gemini Observatory.
The Cerro Tololo Inter-American Observatory (CTIO) is part of the National Optical Astronomy Observatory (NOAO), which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation.
The Southern Astrophysical Research (SOAR) telescope, is a joint project of the Ministério da Ciência, Tecnologia, Inovaçãos e Comunicações do Brasil (MCTIC/LNA), the University of North Carolina at Chapel Hill (UNC), the Michigan State University (MSU), and NOAO.
Gemini South is part of the Gemini Observatory, which is operated by AURA, under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia e Inovação (Brazil).
The Hubble Space Telescope (HST) is a cooperative program of the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA) to operate a space-based observatory for the benefit of the international astronomical community. Responsibility for conducting and coordinating the science operations of the Hubble Space Telescope rests with the Space Telescope Science Institute (STScI) on the Johns Hopkins University Homewood Campus in Baltimore, Maryland. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc. (AURA).
LIGO is funded by the NSF, and operated by Caltech and MIT, which conceived of LIGO and led the Initial and Advanced LIGO projects. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council) making significant commitments and contributions to the project. More than 1,200 scientists and some 100 institutions from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. Additional partners are listed at http://ligo.org/partners.php.
The Virgo collaboration consists of more than 280 physicists and engineers belonging to 20 different European research groups: six from Centre National de la Recherche Scientifique (CNRS) in France; eight from the Istituto Nazionale di Fisica Nucleare (INFN) in Italy; two in the Netherlands with Nikhef; the MTA Wigner RCP in Hungary; the POLGRAW group in Poland; Spain with the University of Valencia; and the European Gravitational Observatory, EGO, the laboratory hosting the Virgo detector near Pisa in Italy, funded by CNRS, INFN, and Nikhef.