Research on the Possible Detection of Gravitational Waves from the Merger of Two Neutron Stars

The Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), two months ago, notified Astronomers all over the world of the potential detection of gravitational waves from the merger of two neutron stars.

From that moment on August 17^th, the race commenced for detecting a visible counterpart, as unlike the colliding black holes responsible for LIGO's four earlier detections of gravitational waves, this event indeed was expected to generate an excellent explosion of visible light and various other types of radiation.

A small team headed by Ryan Foley, an Assistant Professor of Astronomy and Astrophysics at UC Santa Cruz, was the very first to identify the source of the gravitational waves, situated in a galaxy 130 million light-years away known as NGC 4993. Foley's team succeeded in capturing the first images of the event with the 1 meter Swope Telescope at the Carnegie Institution's Las Campanas Observatory in Chile.

This is a huge discovery. We're finally connecting these two different ways of looking at the universe, observing the same thing in light and gravitational waves, and for that alone this is a landmark event. It's like being able to see and hear something at the same time.

Ryan Foley, Assistant Professor of Astronomy and Astrophysics, UC Santa Cruz

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Enrico Ramirez-Ruiz, Theoretical Astrophysicist and Professor and Chair of Astronomy and Astrophysics at UC Santa Cruz and a member of Foley's team, stated that the observations have made room for a better understanding of the physics of neutron star mergers. Among other things, the results were also able to resolve a hotly debated question on the origins of gold and several other heavy elements in the universe, which Ramirez-Ruiz has been reviewing for years.

"I think this can prove our idea that most of these elements are made in neutron star mergers," he said. "We are seeing the heavy elements like gold and platinum being made in real time."

On October 16th, Foley's team published four papers in Science based on their observations and analysis, besides publishing three papers in Astrophysical Journal Letters, and they are Co-authors of several more papers in Nature and various other journals, including two key papers headed by the LIGO collaboration. One of the key Science papers presents the discovery of the first optical counterpart to a gravitational wave source, headed by UCSC Graduate Student David Coulter, and another, headed by Postdoctoral Fellow Charles Kilpatrick, presenting a modern comparison of the observations with theoretical models in order to confirm that it was a neutron-star merger. Foley's collaborators at the Carnegie Institution for Science headed two other Science papers.

By chance, the LIGO detection came on the closing day of a scientific workshop on, "Astrophysics with gravitational wave detections," which was organized by Ramirez-Ruiz at the Niels Bohr Institute in Copenhagen and where Foley had just presented a talk.

I wish we had filmed Ryan's talk, because he was so gloomy about our chances to observe a neutron star merger. But then he went on to outline his strategy, and it was that strategy that enabled his team to find it before anyone else.

Enrico Ramirez-Ruiz, Theoretical Astrophysicist and Professor and Chair of Astronomy and Astrophysics, UC Santa Cruz

Strategy

Foley's strategy deals with prioritizing the galaxies within the search field specified by the LIGO team, targeting those that could probably harbor binary pairs of neutron stars and getting as many of those galaxies as possible into every single field of view. The search field was covered more methodically by other teams, "like mowing the lawn," Foley said. His team discovered the source in the ninth field they observed, after waiting for 10 hours for the sun to set in Chile.

"As soon as the sun went down, we started looking," Foley said. "By finding it as quickly as we did, we were able to build up a really nice data set."

He pointed out that the source was sufficiently bright such that it could be seen by amateur Astronomers, and it possibly would have been visible from Africa hours before it was visible in Chile. The neutron star merger emitted gamma rays that were detected by the Fermi Gamma-ray Space Telescope at almost the same time as the gravitational waves, but when comparing LIGO with the Fermi data, no better information about the location of the source was provided the Fermi data than that provided by LIGO.

Foley's team took the first image of the optical source 11 hours following the LIGO detection and, after approving their discovery, the team announced it to the astronomy community an hour later. Dozens of other teams rapidly followed up with observations from other telescopes. Foley's team also succeeded in obtaining the first spectra of the source with the Magellan Telescopes at Carnegie's Las Campanas Observatory.

Rich data set

The gravitational wave source was called GW170817, and the optical source was called Swope Supernova Survey 2017a (SSS17a). Almost seven days later, the source had faded and was no longer found in visible light. However, while it was visible astronomers were able to collect a treasure trove of data on this amazing astrophysical phenomenon.

It's such a rich data set, the amount of science to come from this one thing is incredible.

Enrico Ramirez-Ruiz, Theoretical Astrophysicist and Professor and Chair of Astronomy and Astrophysics, UC Santa Cruz

Neutron stars are present among the most exotic forms of matter in the universe, almost entirely made up of neutrons and are so dense that a sugar cube of neutron star material would weigh almost a billion tons. The violent merger of two neutron stars ejects a large amount of this neutron-rich material, powering the synthesis of heavy elements in a process known as rapid neutron capture (r-process).

The radiation emitted by this looks nothing like an exploding star or an ordinary supernova. Astrophysicists like Ramirez-Ruiz have created numerical models in order to predict what such an event, known as a kilonova, would appear to be like, but this is indeed the very first time one has in fact been observed in such detail. Kilpatrick explained that the data fit remarkably well with the predictions of theoretical models.

"It doesn't look like anything we've ever seen before," he said. "It got very bright very quickly, then started fading rapidly, changing from blue to red as it cooled down. It's completely unprecedented."

A theoretical synthesis of data from across the spectrum, from radio waves to gamma rays, was headed by Ariadna Murguia-Berthier, a Graduate Student working with Ramirez-Ruiz, and published in Astrophysical Journal Letters, offering a coherent theoretical framework that will understand the complete range of observations. Their analysis indicates, for instance, that the merger activated a relativistic jet (material moving at near the speed of light) that produced the gamma-ray burst, while matter torn from the merger system and ejected at lower speeds drove the kilonova emissions and the r-process at optical, ultraviolet and infrared wavelengths.

Ramirez-Ruiz has calculated that a single neutron-star merger can produce an amount of gold equal to the mass of Jupiter. The calculations of heavy element production by SSS17a indicate that neutron star mergers can account for almost half of all the elements heavier than iron in the universe.

Not a joke

The detection came only one week before the end of LIGO's second observing run, which had started in November 2016. Foley was in Copenhagen, taking advantage of his one afternoon off to visit Tivoli Gardens along with his partner, when he received a text from Coulter notifying him about the LIGO detection. At first, he treated it to be a joke, but soon he was found pedaling his bicycle madly back to the University of Copenhagen in order to begin working with his team on an in depth search plan.

"It was crazy. We barely got it done, but our team was incredible and it all came together," Foley said. "We got lucky, but luck favors the prepared, and we were ready."

Foley's team at UC Santa Cruz is made up of Ramirez-Ruiz, Coulter, Kilpatrick, Murguia-Berthier, Professor of Astronomy and Astrophysics J. Xavier Prochaska, Postdoctoral Researcher Yen-Chen Pan and Graduate Students Matthew Siebert, Cesar Rojas-Bravo, and Enia Xhakaj. Other team members include Maria Drout, Ben Shappee and Tony Piro at the Observatories of the Carnegie Institution for Science; UC Berkeley astronomer Daniel Kasen; and Armin Rest at the Space Telescope Science Institute.

Their team is known as the One-Meter, Two-Hemisphere (1M2H) Collaboration as they use two one-meter telescopes, one in each hemisphere: the Nickel Telescope at UC's Lick Observatory and Carnegie's Swope Telescope in Chile. The UCSC group is partially supported by the National Science Foundation, Gordon and Betty Moore Foundation, Heising-Simons Foundation, and Kavli Foundation; fellowships for Foley and Ramirez-Ruiz from the David and Lucile Packard Foundation and for Foley from the Alfred P. Sloan Foundation; a Niels Bohr Professorship for Ramirez-Ruiz from the Danish National Research Foundation; and the UC Institute for Mexico and the United States (UC MEXUS).

A small team of UC Santa Cruz Astronomers was the first to observe the light from the violent merger of two neutron stars.