Accurate measurements using the National Science Foundation's (NSF) collection of radio telescopes have shown that a narrow jet of particles traveling at near the speed of light ejected into interstellar space just after a pair of neutron stars merged in a galaxy 130 million light-years from Earth. The merger in August 2017 created gravitational waves, causing vibrations in space. This was the first event in which both gravitational waves and electromagnetic waves, including gamma rays, X-rays, visible light and radio waves, were detected immediately.
The effects of the merger, dubbed GW170817, could be observed through orbiting and ground-based telescopes around the world. The scientists noted that the characteristics of the resulting waves changed over time and used these changes to identify the nature of the phenomena that followed the merger.
One question that stood out, even months after the merger, was whether the event created a narrow, fast-moving stream of material that made its way into interstellar space. This was very important, because such jets are necessary to create the type of bursts of gamma radiation that theorists believed should have been caused by merging pairs of neutron stars.
The answer came when astronomers used a combination of NSF's very long baseline array (VLBA), Karl Jansky's large-scale array (VLA), and Robert S. Byrd's Green Bank Telescope (GBT). It was found that the location of the radio emission from the confluence moved in space, and the movement was so fast that only an airplane could explain its speed.
'We measured this movement, which turned out to be four times faster than light. This illusion, called superluminal motion, occurs when the jet is almost toward Earth and the material in the jet is approaching the speed of light, 'said Kunal Muli, National Radio Astronomy Observatory (NRAO) and Caltech.
Astronomers observed the object 75 days after the merger, then again 230 days later.
“Based on our analysis, this jet is likely very narrow, no more than 5 degrees wide, and only 20 degrees off Earth's direction,” said Adam Deller of Swinburne University of Technology. “But to match our observations, the material in the jet also had to explode outward at more than 97 percent faster than the speed of light.”
The current scenario of the event is that the initial merger of two superdense neutron stars caused an explosion that pushed a spherical shell of debris outward. Neutron stars collapsed into the black hole, whose powerful gravity began to pull material towards it. This material formed a rapidly rotating disk that generated a pair of jets moving outward from their poles.
As this event unfolded, the question arose as to whether the jets would emerge from the shell of the debris of the original explosion. Observational data showed that the jet interacted with space debris, forming a wide 'cocoon' of material, expanding outward. The cocoon expanded more slowly than the jets.
“Our interpretation is that the cocoon dominated the radio emission until about 60 days after the merger, and later the emissions were exposed to the jet,” said Ore Gottlieb of Tel Aviv University, the study's lead theorist.
“We were lucky that we were able to observe this event, because if the jet was far away from Earth, the radio emission would be too weak to detect it,” added Gregg Hollinan of Caltech.
At the moment, scientists are confident that the detection of a fast-moving jet in GW170817 significantly strengthens the connection between neutron star mergers and short-lived gamma-ray bursts. They now know that the jets must be relatively directed toward Earth to detect a gamma-ray burst.
