GW170817, a titanic collision between two neutron stars detected in August 2017, ejected a structured relativistic jet with a speed greater than 99.97% the speed of light, according to a team of U.S. astronomers.
An artist’s impression of a jet emanating from NGC 4993. Image credit: J.A. Biretta et al / NASA / ESA / Hubble Heritage Team / STScI / AURA / Sci.News.
The binary neutron-star merger GW170817 occurred in NGC 4993, a lenticular galaxy approximately 130 million light-years from Earth.
It was the first event ever to be detected both by gravitational waves and electromagnetic waves, including gamma rays, X-rays, visible light, and radio waves.
The aftermath of the merger was observed by 70 orbiting and ground-based telescopes around the world.
Astronomers quickly aimed the NASA/ESA Hubble Space Telescope at the site of the explosion just two days later.
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 jets moving outward from its poles.
The roaring jet smashed into and swept up material in the expanding shell of explosion debris. This included a blob of material through which a jet emerged.
While the event took place in 2017, it has taken several years for scientists to come up with a way to analyze the Hubble data and data from other telescopes to paint this full picture.
The Hubble observation was combined with observations from multiple NSF’s radio telescopes working together for very long baseline interferometry (VLBI). The radio data were taken 75 days and 230 days after the explosion.
“I’m amazed that Hubble could give us such a precise measurement, which rivals the precision achieved by powerful radio VLBI telescopes spread across the globe,” said Dr. Kunal Mooley, an astronomer at Caltech and the National Radio Astronomy Observatory.
Dr. Mooley and colleagues used Hubble data together with data from ESA’s Gaia satellite, in addition to VLBI, to achieve extreme precision.
“It took months of careful analysis of the data to make this measurement,” said Dr. Jay Anderson, an astronomer at Space Telescope Science Institute.
By combining the different observations, they were able to pinpoint the explosion site.
The Hubble measurement showed the jet was moving at an apparent velocity of seven times the speed of light.
The radio observations show the jet later had decelerated to an apparent speed of four times faster than the speed of light.
In reality, nothing can exceed the speed of light, so this superluminal motion is an illusion.
Because the jet is approaching Earth at nearly the speed of light, the light it emits at a later time has a shorter distance to go. In essence the jet is chasing its own light.
In actuality more time has passed between the jet’s emission of the light than the observer thinks. This causes the object’s velocity to be overestimated — in this case seemingly exceeding the speed of light.
“Our result indicates that the jet was moving at least at 99.97% the speed of light when it was launched,” said Dr. Wenbin Lu, an astronomer at the University of California, Berkeley.
A paper on the findings was published in the journal Nature.
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K.P. Mooley et al. 2022. Optical superluminal motion measurement in the neutron-star merger GW170817. Nature 610, 273-276; doi: 10.1038/s41586-022-05145-7
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