John Simonetti part of worldwide effort in detection of two merging neutron stars and resulting explosion
An international group of scientists, including Virginia Tech’s John Simonetti, recently announced they had detected a kilonova — a collision of two neutron stars that unleashed a set of gravitational waves into outer space.
The discovery — made in light waves and gravitational waves — set the science world abuzz with headlines ranging from scientific journals to the New York Times. Simonetti, a professor and associate chair in the College of Science’s Department of Physics, said, “No other worldwide astronomical effort has ever come close to this coordinated effort.”
The unprecedented detection happened on Aug. 17. Scientists, using the Laser Interferometer Gravitational-wave Observatory (LIGO for short), detected gravitational waves, aka ripples in space-time, from the collision of two neutron stars 130 million years ago. Additional astronomers, representing some 70 observatories, announced they detected the resulting explosion. This marks the first time a cosmic event has been viewed in both gravitational waves and light.
This news comes on the heels of a spectacular month for LIGO researchers, who recently won the 2017 Nobel Prize in Physics for their fall 2015 discovery of a gravitational wave event caused by the spiraling merger of two black holes that took place 1.3 billion years ago. Since 2015, LIGO scientists have detected other black hole collisions from the gravitational waves emitted during those mergers.
The 2015 discovery by LIGO confirmed predictions of Albert Einstein’s century-old theory of general relativity, which included the existence of gravitational waves — ripples in the space-time fabric.
LIGO scientists said the latest effort was made using its U.S.-based gravitational-wave observatory, the Europe-based Virgo gravitational-wave detector, and some 70 ground- and space-based astronomical observatories, including the Long Wavelength Array (LWA) in New Mexico, the radio telescope used by Simonetti and his team.
“These world-wide, coordinated, multi-messenger observations, with so many independent science teams, including Virginia Tech, are unprecedented in astronomical history,” Simonetti said.
In a news release, LIGO stated that neutron stars are the smallest, densest stars known to exist and are formed when massive stars explode in supernovas. As these neutron stars spiraled together, they emitted gravitational waves that were detectable for about 100 seconds on Aug. 17. When they collided, a flash of light in the form of gamma rays was emitted and seen on Earth for about two seconds following the gravitational waves. In the following days and weeks, other forms of light, or electromagnetic radiation — including X-ray, ultraviolet, optical, infrared, and radio waves — were detected, according to LIGO.
Using the LWA, Simonetti and his team looked for the resulting fireworks caused by the material and energy explosively ejected during the collision.
“Our observations were among the first follow-up observations after the communication of the gravitational wave event by the LIGO/Virgo Collaboration to the collaborating astronomers, certainly the first in the radio spectrum,” Simonetti said. “Our observations were at the lowest radio frequencies. Both the quickest response and the lowest frequencies may provide vital information to help understand what occurred when these two neutron stars merged.”
In the latter stages of such a dramatic ejection of energy and material, some of the light, particularly at radio wavelengths, is probably caused by the collision of the ejected high energy particles with the surrounding, ambient interstellar gas, said Simonetti. Think of it in an analogy as seeing not the blast of a bomb’s explosion, but instead the thudding effects of the shrapnel as it hits nearby buildings.
“This merger of two neutron stars is the sort of event LIGO and Virgo were designed to detect,” Simonetti said. “This is the type of merger event that should be more common, compared to black hole mergers.”
The results of the complete “multi-messenger” campaign were published in The Astrophysical Journal Letters. Joining Simonetti as co-authors are physics undergraduate researcher Kaila Nathaniel, a junior from Springfield, Virginia; doctoral alumnus Michael Kavic, now an associate professor at Long Island Univerity in Brooklyn, New York; and physics doctoral student Jr-Wei Tsai.
“Much more work will need to be done,” Simonetti said. “We will be further analyzing our data and will produce a separate paper. Additionally, we will continue to observe to see if brighter radio emission is released that we might detect at low frequencies. There is some indication that the radio emission is growing in strength.”