(VIDEO) Radio observations confirmed the superfast jet of material from a neutron star merger.
It was the first event to be detected both by gravitational waves and electromagnetic waves – including gamma rays, X-rays, visible light and radio waves – and Texas Tech University faculty member Alessandra Corsi was instrumental in the detection.
“After our very first detection of a radio glow from the neutron star, our team continued to monitor this fantastic event for months,” said Corsi, an associate professor of physics in the Texas Tech Department of Physics & Astronomy. “We not only continued to track the evolution of the radio light curve, but also employed techniques such as radio polarimetry and Very Long Baseline Interferometry (VLBI) to probe the structure of the ejecta in detail. This VLBI result is particularly exciting as it reveals that jets formed in binary neutron star mergers can have a more complex structure than previously thought.”
The aftermath of the merger, called GW170817, was observed by orbiting and ground-based telescopes around the world. Scientists watched as the characteristics of the received waves changed with time, then used the changes as clues to reveal the nature of the phenomena that followed the merger.
One question that stood out, even months after the merger, was whether or not the event had produced a narrow, fast-moving jet of material that made its way into interstellar space. That was important because such jets are required to produce the type of gamma ray bursts that theorists had said should be caused by the merger of neutron-star pairs.
The answer came when astronomers used a combination of the Very Long Baseline Array (VLBA), the Karl G. Jansky Very Large Array (VLA) and the Robert C. Byrd Green Bank Telescope (GBT). They discovered that a region of radio emissions from the merger had moved, and the motion was so fast that only a jet could explain its speed.
“We measured an apparent motion that is four times faster than light,” said Kunal Mooley of the National Radio Astronomy Observatory (NRAO) and the California Institute of Technology (Caltech). “That illusion, called superluminal motion, results when the jet is pointed nearly toward Earth and the material in the jet is moving close to the speed of light.”
The astronomers observed the object 75 days after the merger, then again 230 days after.
“Based on our analysis, this jet most likely is very narrow, at most 5 degrees wide, and was pointed only 20 degrees away from the Earth's direction,” said Adam Deller of the Swinburne University of Technology and formerly of the NRAO. “But to match our observations, the material in the jet also has to be blasting outward at more than 97 percent of the speed of light.”
The scenario that emerged is that the initial merger of the two superdense neutron stars caused an explosion that propelled a spherical shell of debris outward. The neutron stars collapsed into a black hole whose powerful gravity began pulling material toward it. That material formed a rapidly-spinning disk that generated a pair of jets moving outward from its poles.
As the event unfolded, the question became whether the jets would break out of the shell of debris from the original explosion. Data from observations indicated that a jet had interacted with the debris, forming a broad “cocoon” of material expanding outward. Such a cocoon would expand more slowly than a jet.
“Our interpretation is that the cocoon dominated the radio emission until about 60 days after the merger, and at later times, the emission was jet dominated,” said Ore Gottlieb of the Tel Aviv University, a leading theorist on the study.
“We were lucky to be able to observe this event, because if the jet had been pointed much farther away from Earth, the radio emission would have been too faint for us to detect,” said Gregg Hallinan of Caltech.
The detection of a fast-moving jet in GW170817 greatly strengthens the connection between neutron star mergers and short-duration gamma-ray bursts, the scientists said. They added that the jets need to be pointed relatively closely toward the Earth for the gamma-ray burst to be detected.
“Our study demonstrates that combining observations from the VLBA, the VLA and the GBT is a powerful means of studying the jets and physics associated with gravitational wave events,” Mooley said.
Corsi and her colleagues reported their findings today (Sept. 5) in the online version of the journal Nature.
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