spaceSpace and Physics

A Near-Light-Speed Jet Was Released By Last Year's Historic Neutron Star Merger


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockSep 6 2018, 16:09 UTC

Artist's impression of jets of material from first confirmed neutron star merger. Mark Garlick/University of Warwick

We now know what happened right after the neutron star collision that was discovered last year. The event, GW170817, was the fifth ever gravitational wave detection and the first one that was also studied with regular observatories. Astronomers were able to track the light emission in several different wavelengths and study the aftermath of such a momentous explosion.

The collision produced a short gamma-ray burst (GBR), the first evidence that GRBs are produced by neutron star collisions. These GRBs are some of the most energetic events in the universe and astronomers jumped at the chance to observe one. The analysis, published in Nature, showed that the stellar merger created a cocoon of material from which a high-speed jet emerged.


“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,” lead author Kunal Mooley, of the National Radio Astronomy Observatory (NRAO) and Caltech, said in a statement.

The first analysis of the events couldn’t confirm the existence of a jet, so two models emerged, one where the jet pierced the cocoon and one where the cocoon was too dense for it to break through. The astronomers observed the source 75 and 230 days after the collision and found evidence of the superluminal motion.

“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, 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 outwards at over 97 percent of the speed of light.”

The jet is moving really fast at a small angle with respect to us, but due to projection effects, it looks like it moves faster than the speed of light. D. Berry, O. Gottlieb, K. Mooley, G. Hallinan, NRAO/AUI/NSFF

The incredible speed is a testament to how energetic these events are. GW170817 was produced by two neutron stars with almost three times the mass of the Sun combined. The collision ended in a black hole, but not before throwing a lot of material into space, including approximately 10 Earth masses of gold and platinum.


“The merger event was important for a number of reasons, and it continues to surprise astronomers with more information,” said Joe Pesce, NSF Program Director for NRAO. “Jets are enigmatic phenomena seen in a number of environments, and now these exquisite observations in the radio part of the electromagnetic spectrum are providing a fascinating insight into them, helping us understand how they work.”

The two LIGO and the Virgo detectors that were responsible for the observation of the gravitational waves are currently in a technical upgrade rest. They will restart in a few months and researchers hope to soon spot many more of these neutron star mergers.

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