Astronomers Feast on First Light From Gravitational Wave Event
16 October 2017
Gemini Observatory "pulled all of the stops" to bring a gravitational wave source into focus and capture early optical and infrared light from the merger of two neutron stars. The critical ground-based observations spanned almost a month during the summer of 2017 and allowed astronomers to dissect the first electromagnetic light emissions ever associated with a gravitational wave event.
The first-ever detection of optical and infrared light linked to a gravitational wave event initiated a time-critical sequence of observations at the Gemini South telescope in Chile. "Gemini pulled all the stops to get these data," said Ryan Chornock of Ohio University who analyzed the resulting flood of data in his team's study of the event. The Gemini data allowed multiple research teams to form a complete picture of the aftermath from the gravitational wave event (GW170817) localized by LIGO (Laser Interferometer Gravitational-Wave Observatory), Virgo, and the Fermi Gamma-ray Space Telescope on August 17, 2017. The Gemini imaging and spectroscopy spanned a period of 25 nights – while the object's light gradually faded from view.
Researchers from around the world announced their results today at press conferences in Washington D.C., Caltech, and one hosted by the European Southern Observatory in Europe. Well over a dozen papers are also accepted for publication in the journals Nature, Science, and The Astrophysical Journal Letters.
Mansi Kasliwal, Assistant Professor of Astronomy at Caltech presented her team's findings at the Caltech press conference in Pasadena and recalls the excitement of the discovery. "Within 23 minutes of submitting our observing proposal to hunt for infrared photons it was approved by the Gemini Director!" says Kasliwal Principal Investigator of a the worldwide GROWTH (Global Relay of Observatories Watching Transients Happen) team studying the event. "On that first night the 8-meter Gemini South telescope successfully captured some of the first infrared photons ever seen from a neutron-neutron star merger - it was thrilling!"
Harvard astronomer Edo Berger, who presented at the D.C. press conference, describes the Gemini observations as, "... collectively the longest-running, and finest, infrared imaging and spectroscopy of this object that we have available." Berger adds that the data directly demonstrate that the much-speculated mechanism of a neutron star binary merger caused this ripple in space and time. In the process the event formed and dispersed heavy elements, like gold, into space. "Here, for the first time, using Gemini we showed the direct signature of the formation of heavy elements," said Berger, who continues, "[this] solves the decades-long mystery of the origin of the heaviest elements in the periodic table." (see the interview of Prof. Edo Berger on the right)
Leo Singer, of NASA's Goddard Space Flight Center, and a collaborator with Kasliwal in the GROWTH group adds, "Continued monitoring over many subsequent nights at Gemini allowed us to paint a stunning infrared portrait of neutron star mergers." In agreement with other researchers, the GROWTH team concluded that these neutron-neutron star mergers are primary sites for the production of elements heavier than iron. According to Kasliwal, "Each of these events is capable of forging over ten thousand times the Earth's mass in heavy elements such as gold and platinum - cosmic bling!"
Folding the Gemini data into observations from radio to X-rays, Eleonora Troja, of the University of Maryland, joined Berger in presenting her findings at the D.C. press conference. Troja's team focused on the time evolution of the event starting with the very early Gemini observations in the optical (visible) part of the spectrum. "It surprised me very much when I saw how bright this was in the optical," says Troja. "The question we asked is if this really was a so-called kilonova when a neutron pair merge, or some kind of exotic transient or supernova making fun of us!"
Troja and her team concluded from the optical spectra that this was not like anything they had seen before. "We are just beginning our effort to model and understand these explosions and the physics behind them," said team member Brad Cenko form NASA's Goddard Space Flight Center, and he continues, "...we need to add to our models an outflow of slower and more transparent material to account for the bright optical light component." Troja concludes, "This outflow is likely responsible for the production of less precious metals, such as silver and tin."
"The joint detection of light and gravitational waves from cosmic sources is one of the holy grails of present-day astronomy," exclaimed Marcelle Soares-Santos (Fermi National Accelerator Laboratory), the first author of the paper from Berger's team that reports their discovery of the optical counterpart. Both signals, light and gravitational waves, contribute unique information about extreme astrophysical events. As Soares-Santos explained, "Gravitational waves tell us about the motions and masses of the neutron stars, and light reveals the astrophysics of the event – what happened exactly as the stars merged, the mass of heavy elements produced."
"This is a game-changer for astrophysics," said Andy Howell who leads the supernova group at the Las Cumbres Observatory and is a coauthor on a paper in The Astrophysical Journal Letters based on the Gemini data. He adds, "One hundred years after Einstein theorized gravitational waves we've seen them and traced them back to their source to find an explosion with new physics of the kind we only dreamed about before."
"It is tremendously exciting to experience a rare event that transforms our understanding of the workings of the universe," says France A. Córdova, director of the National Science Foundation (NSF), which funds LIGO and a majority of the international Gemini Observatory. "This discovery realizes a long-standing goal many of us have had, that is, to simultaneously observe rare cosmic events using both traditional as well as gravitational-wave observatories. Only through NSF's four-decade investment in gravitational-wave observatories, coupled with telescopes that observe from radio to gamma-ray wavelengths, are we able to expand our opportunities to detect new cosmic phenomena and piece together a fresh narrative of the physics of stars in their death throes."
Gemini Observatory director Laura Ferrarese recounts the challenges faced by the flood of requests for observations once the source was pinpointed. "Several teams contacted us with requests to observe the source," according to Ferrarese. "Everybody at Gemini was terribly excited: we all knew that we were witnessing a historical event!" Ferrarese adds 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 above.
"Gemini's unique combination of depth and high-cadence through the hard work of staff like Kim have generated a totally unique data set for this fascinating event," said Nathaniel Butler from Arizona State University, and also part of the team with Troja and Cenko. Butler concludes, "The Gemini observations will provide a critical perspective on gravitational waves for years to come."
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