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Neutron star crash in a galaxy far, far... far away spews 'faster than light' radio signal jets at Earth

Open up your eyes and look around: It's just an illusion

A recently observed neutron star collision was so violent it sprayed jets of radio signals that appeared to travel faster than light, it has just emerged.

The cosmic prang – logged as GW170817 after the resulting gravitational wave detected in mid-2017 – involved two neutron stars running into one another in NGC 4993, a galaxy 130 million light years away. The crash blew out a stash of goodies for boffins on our home world to study.

Scientists said the gravitational wave signal lasted 100 seconds and was accompanied by powerful gamma ray bursts. However, they weren’t quite sure if this outpouring of energy also included super-fast jets of radio signals. Eggheads suggested these powerful emissions go hand in hand with gamma ray bursts typically seen from neutron star mergers.

Now, a group of researchers have confirmed in a paper published in Nature today that these incredible radio jets were, indeed, flung from the collision. In fact, they were so powerful, they appeared – keyword: appeared – to travel faster than the speed of light.

The spurt was most likely traveling at 97 per cent or more of the speed of light, but due to superluminal motion, appeared to break the universe's speed barrier.

"We measured an apparent motion that is four times faster than light," said Kunal Mooley, coauthor of the paper and a research fellow at the National Radio Astronomy Observatory (NRAO) and California Institute of Technology (Caltech) in the US. "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."


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The flares were observed 75 days after the merger was detected, and again 230 days later.

The team reckons a spherical cloud of debris ballooned after the two neutron stars crashed into each other. The stars then began collapsing into a black hole that started sucking up material. As the debris spiraled around the hole, an accretion disk was formed. The gravitational forces compressed the disk, the frictional forces heated it up, and a pair of powerful radio-signal jets shot out from its poles.

“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, a coauthor of the paper and a theoretical physicist at the Tel Aviv University in Israel.

Scientists got lucky with this particular merger. Since one of the electromagnetic jets was pointed towards Earth, they were able to detect it as well as measure its superluminal speed.

"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, another coauthor and a researcher at Caltech.

Radio signal bursts have been spotted before from certain giant galaxies, quasars, black holes, and pulsars. ®

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