For the first time, astronomers have observed a celestial event through both conventional telescopes and gravitational waves. The collision of two super-dense neutron stars just 120 million light-years from Earth was captured by both gravity wave observatories (Laser Interferometer Gravitational Observatory, LIGO in the U.S., and Virgo in Italy) and telescopes including the DLT40 survey based in Chile. The results are published Oct. 16 in a collection of papers in The Astrophysical Journal, Physical Review Letters, Nature and other journals.

“This opens a new field,” said Stefano Valenti, assistant professor of physics at UC Davis and a co-leader of the DLT40 observing team. “Now we can use optical data to understand observations from gravitational waves, and vice versa.”

LIGO and Virgo are based on Einstein’s General Theory of Relativity, which holds that gravity can be understood as a wave passing through space-time like a ripple across a pond. The instruments look for gravitational waves by bouncing laser beams between mirrors miles apart: A tiny change in the distance between the mirrors could signal a passing gravitational wave. LIGO includes two instruments, in Livingston, Louisiana, and in Hanford, Washington.

The DLT40 survey uses the PROMPT telescope in Chile to survey galaxies within 40 megaparsecs (about 130 million light-years) of Earth.  

LIGO caught the first observed gravity wave in September 2014, when the ripple from the collision of two massive black holes over a billion light-years away passed through the Earth. The new event is much smaller but much closer, just within the range of DLT40.

Valenti’s team was making a nightly survey for supernovae in nearby galaxies when they were notified that the gravitational observatories had detected an event. They directed the telescope to the area of sky LIGO and Virgo had identified, and found a flash of light that could be the signature of colliding neutron stars.

Valenti, who was on shift at the time, took a second image and notified his colleagues.

“It was so exciting, the most exciting week of my career,” Valenti said.

“We thought it was too good to be true, but then we saw it evolving into something we had never seen before and it was clear this was something new,” said Professor David Sand, University of Arizona and principal investigator for the DLT40 project.

Neutron stars densest objects known

A neutron star is the densest known object in the universe, short of a black hole. They are thought to form when a star explodes as a supernova, and some of the remains collapse inward. They are enormously dense: Theoretically, a spoonful of neutron star-stuff would weigh billions of tons.

Isolated neutron stars do not give off visible light, but they can emit pulses of radio waves and X-rays.

However, theory predicts that colliding neutron stars should throw off gamma rays, X-rays and visible light as well as a powerful gravitational wave. The new event is very similar to the predictions, Valenti said.

“Gamma ray bursts have been seen before, but this is the first proof from two different perspectives of a merger of two neutron stars,” he said. The optical data helps astronomers to interpret the LIGO data and vice versa, Valenti said.

The researchers hope to use the combination of gravitational and optical astronomy to find more such objects and learn more about them and their place in the universe.

The DLT40 collaboration includes UC Davis, the University of Arizona and the University of North Carolina. DLT40 team shares credit for the optical observation with other groups involved in the LIGO Scientific Collaboration.

LIGO is funded by the NSF, and operated by Caltech and MIT. 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 online. 

The Virgo collaboration consists of more than 280 physicists and engineers belonging to 20 different European institutions including: the Centre National de la Recherche Scientifique (CNRS) in France; Istituto Nazionale di Fisica Nucleare (INFN), Italy; Nikhef, The Netherlands; the MTA Wigner RCP in Hungary; the POLGRAW group in Poland; University of Valencia, Spain; and the European Gravitational Observatory, EGO, the laboratory hosting the Virgo detector near Pisa in Italy, funded by CNRS, INFN and Nikhef.

Audio: Stefano Valenti talks about the discovery of colliding neutron stars