Faster than the speed of light? Neutron-star merger shot from a jet with seemingly impossible speed

The simulated radio images in this not-to-scale artist’s illustration show fast jets blasting from the black hole created by the merger of two neutron stars, a dramatic event observed in August 2017. In the 155 days between the two observations, the jet seemed to move 2 light years is a distance that would require to travel four times faster than the light. This “superluminal motion” is an illusion created when the beam is pointed almost in the direction of the Earth; it’s actually the move of approximately 97 percent of the speed of light.

(D. Berry, D. Gottlieb, K. Mooley, G. Hallinan, NRAO/AUI/NSF)

The dramatic neutron-star merger that astronomers spotted last year generated a beam of material that seemed to move at four times the speed of light, a new study reports.

“Seemed” is the operative word here, of course; the laws of physics tell us that nothing can travel faster through space than light. Therefore, the superluminal motion was an illusion, which was caused by the jet’s (still very fast) speed, and the fact that it destroyed almost directly at us, researchers said.

“Based on our analysis, this jet most likely is very narrow, at most 5 degrees wide, and it was emphasised that only 20 degrees away from the Earth direction,” study co-author Adam assembled at cambridge, of the Swinburne University of Technology in Australia, said in a statement from the National Radio Astronomy Observatory (NRAO), a facility of the US National Science Foundation (NSF). [Gravitational waves from neutron stars: The Discovery Explained]

“But to our observations, the material in the jet also has to emit to the outside on more than 97 percent of the speed of light,” he added.

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Assembled at cambridge, and his colleagues, led by Kunal Mooley, of the NRAO and the California Institute of Technology in Pasadena used a variety of telescopes to the study of the aftermath of the neutron-star collision, a historical event known as GW170817.

GW170817 was the first documented collision of two neutron stars, the superdense remains of massive stars that died in supernova explosions. GW170817, which are located about 130 million light-years from Earth, also opened the era of the “multimessenger astronomy”: it was the first event that has ever been discovered via gravitational waves (the ripples in space-time first predicted by Albert Einstein a century ago) and electromagnetic radiation.

The name GW170817, by the way, with a nod to these gravitational waves, as well as the date on which the astronomers observed the event — Aug. 17, 2017.

Scientists believe that the merger generated a powerful explosion that ejected a shell of material far out into space. Within this shell, the merged neutron stars made of a single black hole, which started to suck a lot of gas and dust. This material formed a rapidly spinning disk around the black hole; it didn’t take long, two-jets began to radiate from this disc poland, research team members said.

It was unclear whether these jets broke through the remains of the shell is created by the initial explosion. But the observations of the Mooley and his team — made of 75 days and 230 days after the first detection of GW170817 — indicate that this indeed happened.

First, the jets interaction with the displaced dirt to a kind of cocoon, which moved much slower than the jets themselves. But the jets finally broke free in interstellar space.

“Our interpretation is that the cocoon dominated the radio emission up to about 60 days after the merger, and in more recent times the issue was jet-dominated,” study co-author Ore Gottlieb, a theorist at the University of Tel Aviv in Israel, said in the same statement.

Now for the faster-than-light part: In the 155 days between the two observations, the jet in the direction of the Earth seemed like a jump forward in time by 2 light-years — a distance that suggests that it was traveling at four times the speed of light. But again, this was only an illusion.

The new results suggest that neutron-star mergers are important sources of short-duration gamma-ray bursts, study team members said. Merger-generated jets must be nearly aligned with Earth for these bursts of high-energy light to be detected, they will be added.

“The merger case, it was important for a number of reasons, and it remains a surprise for the astronomers for more information” Joe Pesce, NSF program director for NRAO, said in the same statement. “Jets are enigmatic phenomena in a number of environments, and now these beautiful observations in the radio part of the electromagnetic spectrum provide a fascinating insight into them, to help us understand how they work.”

The new study today published online (Sept. 5) in the journal Nature.

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