New source of gravitational waves is observed

Collaborations linked to 60 observatories involving scientists from different countries, including Brazil, observed the first electromagnetic counterpart of a binary neutron star merger that produced gravitational waves.

By Elton Alisson  |  Agência FAPESP – US physicists Rainer Weiss, Barry Barish and Kip S. Thorne were awarded the 2017 Nobel Prize in Physics for their contributions to the detection of gravitational waves. Now, a group of over 3,000 astronomers, including 60 from Brazil, have succeeded in observing – for the first time in visible light – a source of these space-time fluctuations, which Albert Einstein (1879-1955) predicted a century ago.

In an article published in October in Astrophysical Journal Letters, the group, which includes the three Nobel Laureates, announced the first-ever observations of several electromagnetic bands of a merger of two neutron stars, which are extremely dense celestial bodies created by the imploding cores of giant stars.

The event produced gravitational waves recorded in August 2017 by the US National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo near Pisa in Italy. This is the first time light associated with a gravitational-wave event has been detected. 

The discovery is also described in an another article in Astrophysical Journal Letters by a group of 55 astronomers, 17 of whom are from Brazil and are affiliated with the University of São Paulo’s Institute of Astronomy, Geophysics & Atmospheric Sciences (IAG-USP), the same university’s Physics Institute (IF-USP), the National Observatory (ON), and the Federal Universities of Sergipe (UFS), Santa Catarina (UFSC) and Rio de Janeiro (UFRJ).

The Brazilian researchers participated in the study in collaboration with colleagues from the US, Argentina, Chile, Spain and Germany through observations made using T80-South, a robotic telescope built with FAPESP’s support and installed at the Cerro Tololo Inter-American Observatory in Chile.

“This is the first time anyone has obtained the optical [visible] counterpart of a gravitational-wave event,” said Claudia Lucia Mendes de Oliveira, a professor at IAG-USP and one of the authors of the study. 

“On four previous occasions, gravitational waves were detected from collisions and mergers of black holes, which don’t emit electromagnetic radiation, so that there was no visible-light counterpart of the events that produced them,” Oliveira told Agência FAPESP.

Since the LIGO Collaboration’s February 2016 announcement of the first detection of gravitational waves created by a black hole merger, astronomers in various countries who specialize in observations in different bands of the electromagnetic spectrum (radio, visible, X-ray, gamma-ray) have endeavored to obtain the electromagnetic counterpart of a gravitational-wave event.

They redoubled their attention when LIGO and Virgo issued an alert on August 17 that a binary neutron star merger might have created gravitational waves detected by the observatories.

The event is believed to have occurred in NGC 4993, a galaxy located in the austral constellation of Hydra, some 130 m light-years from Earth. The emission of gravitational waves by the neutron star merger occurred approximately 2 seconds before a gamma-ray burst was detected by NASA’s Fermi space telescope.

Intrigued by these clues, astronomers who participate in collaborations linked to 60 observatories around the world began scanning a large section of the sky, equivalent to the angular size of 60 full moons (5 full moons fill approximately 1 square degree), using a large section of the electromagnetic spectrum to try to detect an object with decaying visible emissions or brightness. Called an optical transient, the object might have been produced by a merger of neutron stars.

A team of astronomers who use the Swope telescope at the Las Campanas Observatory in Chile were the first to report the detection of the optical transient, less than 11 hours after the neutron star merger.

A few minutes after the Chilean telescope detected the optical transient, five other teams working with other telescopes independently announced its detection.

“We received the information that an optical transient had been detected in the galaxy NGC 4993 on August 18,” Oliveira said. “T80-South is an autonomous telescope that can be pointed at a new source in a matter of seconds, so we were able to observe the object promptly through three color filters in the SDSS camera, known as the g, r and i filters.”

More than 20 researchers linked to the Pierre Auger Collaboration participated in the observations described in the first article, which has over 3,000 authors. They looked for neutrinos coming from the direction of NGC 4993, where the gravitational waves occurred, but did not find this type of particle.

“Our failure to observe neutrinos coming from the direction of NGC 4993 is also important to an understanding of the phenomenon observed,” said Carola Dobrigkeit, a professor at the University of Campinas’s Physics Institute (IFGW-UNICAMP) and one of the authors of the article. 

Cataclysmic events

According to the researchers, the observations of the optical transient in all wavelengths support the hypothesis that the object was produced by the merger of binary neutron stars. They were located in the region of the NGC 4993 galaxy, exactly where the source of the gravitational waves detected by LIGO and Virgo had been located.

The researchers considered the neutron star merger a “kilonova” because of the violence and brilliance of the gamma-ray explosion, however short-lived, and the emission of electromagnetic radiation due to the decomposition of heavy ions created by rapid neutron capture (known as the r-process) during the merger.

“We knew these objects existed, but we didn’t know much about them,” Oliveira said. “The new observations will enable us to measure the r-process and estimate the quantity of elements formed to compare them with the models.”

The observations will also help answer many questions about neutron stars, which are dead in the sense that they no longer produce internal energy by nuclear fusion. Thus, they correspond to one of the possible end-stages of the life of a high-mass star. A neutron star is created when a star with more than eight times the Sun’s mass dies in a supernova, a brilliant explosion during which the core collapses and the protons and electrons melt into each other to form neutrons. 

“Neutron stars have special physical properties, and poorly understood high-energy nuclear processes occur when they merge. These observations may provide more information to help us understand these processes better,” said Alberto Molino Benito, another author of the study who is doing postdoctoral research at IAG-USP with a scholarship from FAPESP.

The article “Multi-messenger observations of a binary neutron star merger” by B. P. Abbott et al. can be read in Astrophysical Journal Letters at iopscience.iop.org/article/10.3847/2041-8213/aa91c9. 

The article “Observations of the first electromagnetic counterpart to a gravitational wave source by the TOROS Collaboration” can be read in Astrophysical Journal Letters at iopscience.iop.org/article/10.3847/2041-8213/aa9060.