Radio ‘eyes’ unlocking secrets of neutron-star collision
When a pair of superdense neutron stars collided and potentially formed a black hole in a galaxy 130 million light-years from Earth, they unleashed not only a train of gravitational waves but also an ongoing torrent of radio waves that are answering some of the biggest questions about the nature of such a cataclysmic event.
On August 17, the LIGO and VIRGO gravitational-wave observatories combined to locate the source of faint ripples in spacetime caused by the merger of the two neutron stars. This was only the fourth direct detection of gravitational waves, which were predicted more than a century ago by Albert Einstein's General Theory of Relativity. In this case, for the first time, electromagnetic radiation — light, gamma rays, and radio waves — were detected coming from the same object that emitted the gravitational waves.
"The story that now is unfolding is more complete than for any previous event in astronomical history. With information provided by both gravitational waves and electromagnetic waves, which are completely different phenomena, it's like being able to both see and hear the same event for the first time," said Gregg Hallinan, of Caltech.
"The gravitational waves confirmed that we're seeing the merger of two neutron stars, and the radio waves we're observing are going to answer important questions about how much energy was in the explosion, how much mass was ejected, and what kind of environment this explosion occurred in," said Alessandra Corsi, of Texas Tech University.
Hallinan, Corsi and collaborators coordinated various radio telescopes across the globe to observe the galaxy in which the explosion, called GW170817, occurred just a day after the gravitational waves were detected. The first detection of radio waves came from the National Science Foundation's Karl G. Jansky Very Large Array (VLA) on September 2. This radio discovery was made by the Jansky VLA mapping of Gravitational Wave bursts as Afterglows in Radio (JAGWAR) team, where Hallinan played a key role. Independent confirmation came subsequently from a smaller team led by Corsi, also observing with the VLA. The Australia Telescope Compact Array (ATCA) also detected radio emission from the object on September 5.
"This event is the first unambiguous detection of a merger of two neutron stars," said Dale Frail of the National Radio Astronomy Observatory (NRAO). "Such mergers are thought to be the cause of one type of Gamma Ray Burst, and we now have the opportunity to test the theories that have been proposed about how those bursts and their afterglows work," he added.
So far, the scientists said, the evidence provided by the radio observations indicates that the explosion either produced a jet of particles moving at nearly the speed of light that we are seeing at an angle widely separated from the jet's axis, or that there is a "cocoon" of material expanding more slowly from the explosion.
"The radio waves are still coming in, and will continue to do so for months or even years," Corsi said. "Their behavior over that time will tell us which story is correct."
In addition, the radio observations will tell the scientists how dense the environment around the explosion is, and into which the debris is expanding.
"Another exciting possibility is that if the radio emission is coming from a cocoon, we may be able to directly image it with high-resolution radio telescope systems using antennas separated by thousands of miles," said Kunal Mooley, of the University of Oxford in the UK, who is the principal investigator of the JAGWAR program. "We should know about this in just a few months."
"Radio telescopes now are our key to learning the physics of this explosion. The radio emission came late to the party, but it's the last to leave! We will continue to learn important facts about this event in the coming months," Hallinan said.
The object already has faded below the sensitivity limits of most ground-based telescopes except at radio wavelengths, where the VLA will continue to play a very important role.
"This event is our first look at the neutron-star collision process that creates heavy elements such as gold, which are included in the material moving outward from the explosion and producing the radio waves. That means that there is a little bit of neutron-star merger material in all of us," Mooley said.
Hallinan and Corsi's team reported their results in the journal Science.
Another team of VLA observers also detected radio emission from the event.
The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.