In August of 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected waves that were believed to be caused by a neutron star merger. This “kilonova” event, known as GW170817, was the first astronomical event to be detected in both gravitational and electromagnetic waves – including visible light, gamma rays, X-rays, and radio waves.
In the months that followed the merger, orbiting and ground-based telescopes around the world have observed GW170817 to see what has resulted from it. According to a new study by an international team of astronomers, the merger produced a narrow jet of material that made its way into interstellar space at velocities approaching the speed of light.
The study which describes their findings, titled “Superluminal motion of a relativistic jet in the neutron-star merger GW170817“, recently appeared in the journal Nature. The study was led by Kunal Mooley, a Jansky Research Fellow at Caltech and the National Radio Astronomy Observatory (NRAO); Adam Deller, from OzGrav and the Swinburne Univeristy’s Center for Astrophysics and Supercomputing; and Ore Gottlieb, a PhD student from Tel Aviv University.
They were joined by members from the NRAO, the California Institute of Technology (Caltech), the Onsala Space Observatory, The Hebrew University of Jerusalem, Texas Tech University, and Princeton University. For the sake of their study, the team combined data from the NSF’s Very Long Baseline Array (VLBA), the Karl G. Jansky Very Large Array (VLA), and the Robert C. Byrd Green Bank Telescope (GBT).
Using this data, they were able to resolve a long-standing mystery about the merger, which was whether or not it had produced a jet of material streaming from its poles. Scientists suspected that this was the case because such jets are required to produce the gamma ray bursts that are thought to be caused by the merger of neutron-star pairs.
After observing the object 75 days after the merger, and then again after 230 days, the team discovered that a region of radio emission from the merger had moved at incredible speeds. These observations could only be explained by the presence of a powerful jet. As Dr. Mooley explained in an NRAO press release:
“We measured an apparent motion that is four times faster than light. 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.”
“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,” added Adam Deller. “But to match our observations, the material in the jet also has to be blasting outwards at over 97 percent of the speed of light.”
From this new data, a new scenario emerged that explains what happened after the kilonova event. Essentially, the merger caused an explosion that propelled a spherical shell of debris outward. Meanwhile, the merged neutron stars collapsed to form a black hole that began pulling material towards it. This resulted in material falling into a rapidly-spinning disk around the black hole that generated a pair of jets shooting outwards from its poles.
As Gregg Hallinan of Caltech pointed out, the positioning of the jets was very fortunate. “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,” he said.
Data from these latest observations also showed that the jet was interacting with the debris shell, which formed a “cocoon” of material that is expanding outwards more slowly than the jets. This helped to resolve another mystery, which was whether or not the radio sources being detected were the result of interaction with the cocoon or coming from the jet of material. As Ore Gottlieb explained:
“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.”
According to the research team, this study bolsters the theory that there is a connection between neutron star mergers and short-duration gamma-ray bursts. It also demonstrated that jets need to be pointed relatively close towards Earth in order for these bursts to be detectable by our observatories. As Mooley explained:
“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.”
In addition, the observations of these jets – which were conducted in the radio part of the spectrum – are providing new and fascinating insights into this astronomical phenomenon. In the end, this is just the latest surprise that GW170817 has provided astronomers with since it was first detected.
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