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The top spectrogram shows the characteristic “chirp” of a gravitational wave signal seen by LIGO as both the signal’s frequency and intensity rise sharply in the final moments of the death spiral of two colliding neutrons stars. The bottom two optical images from the Dark Energy Camera show the resulting transient kilonova source near galaxy NGC 4993 at first detection and its absence after rapidly fading (kilonova location marked by the reticle).
For the first time, scientists have directly detected gravitational waves — ripples in space and time — in addition to light from the spectacular collision of two neutron stars.
This marks the first time that a cosmic event has been viewed in both gravitational waves and light. The discovery was made using the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO); the Europe-based Virgo detector; and some 60 ground- and space-based telescopes.
Neutron stars are the smallest, densest stars known to exist and are formed when massive stars explode in supernovae. As these neutron stars spiraled together, they emitted gravitational waves that were detectable for about 100 seconds. When they collided, a flash of light in the form of gamma rays was emitted and seen on Earth about two seconds after the gravitational waves. In the hours, days and weeks following the smashup, other forms of light, or electromagnetic radiation — including ultraviolet, optical, infrared, and radio waves — were detected as part of the afterglow.
"We are remarkably lucky that LIGO's first detected neutron star merger brought with it an immediately detected gamma ray burst, followed by an afterglow seen in radio waves, in infrared, visible and ultraviolet light, and in X-rays. Nature has smiled on us once again," said University of Michigan physics professor Keith Riles, leader of the Michigan Gravitational Wave Group and a member of the LIGO Scientific Collaboration's Detection Committee, which validated the discovery.
"The chirp we saw in our data was simply beautiful—nearly too good to be true," said Richard Gustafson, a senior research scientist in the Michigan Gravitational Wave Group, who is stationed full-time at the LIGO Hanford Observatory.
A team of scientists, including other U-M physicists and astronomers, using the Dark Energy Camera, or DECam (the primary observing tool of the Dark Energy Survey), was among the first to observe the fiery aftermath of this burst of gravitational waves, capturing images of the optical counterpart within 12 hours of the initial collision.
U-M physics and astronomy professor David Gerdes, a member of the DECam team, called this one of the most important astronomical discoveries of his career.
"Together with the LIGO team, we've helped give birth to an entirely new field: the study of the most extreme astrophysical environments using both gravitational and electromagnetic waves," he said. "We've dreamed of this for decades."
A worldwide consortium of scientists, including members from the University of Michigan, built the Dark Energy Camera, one of the most powerful digital imaging devices in existence. It was integrated and tested at Fermilab, the lead laboratory on the Dark Energy Survey, and is mounted on the National Science Foundation’s 4-meter Blanco telescope, part of the Cerro Tololo Inter-American Observatory in Chile, a division of the National Optical Astronomy Observatory. The DES images are processed at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign.
Professor Riles noted, "This event will prove to be a figurative gold mine, in more than one sense, " alluding to the evidence from the light-based observations of the post-merger "kilonova", indicating signatures of recently synthesized material, including gold and platinum. Theorists have suggested that the merger of neutron stars leads to kilonovae that produce a substantial fraction of the Universe’s elements heavier than iron. A single neutron star merger might produce an amount of gold comparable to the total mass of the Earth. "Good luck collecting it, though!" Riles added.
The kilonova was clearly evident from the DECam images taken in multiple wavelengths as soon as the sky became dark enough for the target region to be observed. The significance of the discovery was immediately apparent, and the team exchanged excited messages throughout the night. They continued to observe the fading afterglow over the ensuing weeks until the object disappeared from sight.
The results of the new discovery have been published in a series of articles in journals that include Physical Review Letters, Astrophysical Journal Letters and Nature. One paper focuses on “multi-messenger astronomy” (MMA), combining results from gravitational waves and from follow-up by electromagnetic and neutrino telescopes, now published in a landmark Astrophysical Journal Letters article co-authored by more than 3,000 scientists worldwide.
Professor Riles, a member of the LSC Executive Committee, led the LIGO/ Virgo internal review of the MMA paper, which was written by a 12-person team co-chaired by the heads of the LIGO Hanford and LIGO Livingston Observatories, Michael Landry and Joseph Giaime. Riles remarked, "Producing a paper with such diverse content, authorship and astronomical perspective required a remarkable effort, one marked by many late nights and intense discussions among the paper-writing team." He added, "It was a genuine pleasure to work with such talented and hard-working colleagues on this special paper, one which will attract enormous interest from astronomers in the years to come."
U-M physics professor Gregory Tarlé, another member of the DECam team, remarked, "It was immensely exciting to see the world’s observatories mobilized by the LIGO trigger to study this first binary neutron star merger and the subsequent kilonova. These combined observations and more like them to come may expose subtle changes in the properties of the mysterious dark energy that will ultimately lead to an understanding of the very fabric of spacetime."
"The era of multi-messenger astronomy is now well under way, and it’s gratifying that U-M scientists are participating in it on both sides," noted Riles who recalled earlier groundbreaking multi-messenger or multi-wavelength observations in which U-M physicists played key roles. One example included the detection of neutrinos from Supernova 1987A with the IMB (Irvine-Michigan-Brookhaven) underground water tank experiment in 1987 (led by physics professor emeritus John Van der Velde), an event which opened a new field of neutrino astronomy. Another example was the first rapid optical detection of a gamma ray burst aftermath in 1999 by physics professor Carl Akerlof’s Robotic Optical Transient Search Experiment (ROTSE) team, pioneering work that prepared the ground for August’s rapid follow-up of gravitational wave detections by teams all over the globe.
The gravitational wave signal, named GW170817, was first detected on Aug. 17 at 8:41 a.m. Eastern Daylight Time; the detection was made by the two identical LIGO detectors, located in Hanford, Washington, and Livingston, Louisiana. A third detector, Virgo, situated near Pisa, Italy, recovered only a small signal but provided crucial information for localizing the cosmic event.
On Aug. 17, one of LIGO’s observatories caught a strong signal of gravitational waves from space. Around the same time, the Gamma-ray Burst Monitor on NASA’s Fermi space telescope had detected a burst of gamma rays. LIGO’s real-time data analysis software noted the gamma-ray burst event and subsequently found a signal in LIGO’s second interferometer, causing the team to immediately notify the astronomical community.
The LIGO data indicated that two astrophysical objects located at the relatively close distance of 130 million light-years from Earth had been spiraling in toward each other. It appeared that the objects were not as massive as binary black holes — objects that LIGO and Virgo had previously detected. Instead, the inspiraling objects were estimated to be around 1.1 and 1.6 times the mass of the sun, in the mass range of neutron stars. A neutron star is about 20 kilometers, or 12 miles, in diameter and is so dense that a teaspoon of neutron star material has a mass of about a billion tons.
Though the LIGO detectors first picked up the gravitational wave in the United States, Virgo, in Italy, played a key role in the story. Because of its orientation with respect to the source at the time of detection, Virgo recovered a small signal; combined with the signal sizes and timing in the LIGO detectors, this allowed scientists to triangulate the position in the sky. After performing a thorough vetting to make sure the signals were not an artifact of instrumentation, scientists concluded that a gravitational wave came from a relatively small patch in the southern sky.
With these coordinates, a handful of observatories around the world were able, hours later, to start searching the region of the sky where the signal was thought to originate. A new point of light, resembling a new star, was first found by optical telescopes, including by the DECam team. Ultimately, about 60 observatories on the ground and in space observed the event at their representative wavelengths.
In the weeks and months ahead, telescopes around the world will continue to observe the afterglow of the neutron star merger and gather further evidence about various stages of the merger, its interaction with its surroundings, and the processes that produce the heaviest elements in the Universe.
Other members of the Michigan Gravitational Wave Group include graduate students Ansel Neunzert, Orion Sauter and Jonathan Wang, along with undergraduates Sophie Hourihane, Paul Huang, Humza Khan, Jessica Leviton, Eilam Morag and Kaushik Rao.
Other members of the Michigan DECam team include faculty members Gus Evrard, Dragan Huterer, and Chris Miller, research scientist Michael Schubnell, postdoctoral fellow Ed Lin, graduate students Rutuparna Das, Anthony Kremin, and Stephanie Hamilton, and undergraduates Waleed Al-Rawi, Kyle Franson, Tali Khain, and Lynus Zullo.
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 and the Australian collaboration OzGrav. 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.
The Dark Energy Survey is a collaboration of more than 400 scientists from 26 institutions in seven countries. Its primary instrument, the 570-megapixel Dark Energy Camera, is mounted on the 4-meter Blanco telescope at the National Optical Astronomy Observatory’s Cerro Tololo Inter-American Observatory in Chile, and its data are processed at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign.
Funding for the DES Projects has been provided by the U.S. Department of Energy Office of Science, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, ETH Zurich for Switzerland, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, the Center for Cosmology and Astro-Particle Physics at the Ohio State University, the Mitchell Institude for Fundamental Physics and Astronomy at Texas A&M University,Financiadora de Estudos e Projetos, Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro, Conselho Nacional de Desenvolvimento Científico e Tecnológico and the Ministério da Ciência e Tecnologia, the Deutsche Forschungsgemeinschaft and the collaborating institutions in the Dark Energy Survey, the list of which can be found at www.darkenergysurvey.org/collaboration.