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A jet of charged particles moving at almost the speed of light broke the debris left by the melting of the neutron stars that produced the gravitational waves detected by the LIGO – Virgo collaboration on August 17, 2017.
The event, cataloged GW170817, was a Rosetta stone for astronomers as it allowed them to observe the same event using gravitational waves and electromagnetic radiation ranging from a gamma ray burst (GRB ) radio remanence. This was a first for the new and exciting field of multimodal astronomy.
The collision of two neutron stars is called a kilonova and is thought to produce a black hole or a "hypermassive" neutron star. Part of the neutron-rich debris forms a shell or cocoon around the black hole or neutron star. The remainder spirals on the remaining object, producing a jet of charged particles that burst into space almost at the speed of light.
Incandescent cocoon
As the jet falls into the cocoon, it starts to shine, a phenomenon that astronomers call the remanence. However, it was unclear whether the jet could penetrate through the cocoon and emerge from the other side.
Astronomers, led by Kunal Mooley of Caltech, attempted to answer this question by observing the persistence using the high-sensitivity network of ten radio telescopes in the United States, which is the longest-running picture. New Mexico and Green Bank Observatory in West Virginia. They found that afterglow had an onset time, taking 150 days to reach maximum intensity, while other observed remnants became visible after a week or two.
The more the Earth is off-axis with respect to the direction of the jet, the lower the remanence. The length of the delay implies that we see the remanence and the jet from an angle of view of 20 ° and that the jet and the remanence had to be intrinsically very luminous so that we can detect them at this angle.
Faster than light
Mooley's team also found that the radio remanence associated with GW170817 exhibited a superluminal movement – that is, apparently faster than light – between 75 and 230 days after the kilonova. The superluminal movement is an illusion caused by a very narrow jet – in this case calculated as being just 5 ° wide – moving at a rate slightly less than the speed of light towards us.
These measures allowed the Mooley team to better understand GW170817. The detection of gravitational waves indicated that it was a fusion of binary neutron stars, and the superluminal motion showed that the jet could cross the cocoon.
"By gathering this information, we now have strong evidence that binary neutron star fusions produce efficient streams," says Mooley. World of Physics.
"It's probably the best estimate of a GRB jet we've ever had," says Ore Gottlieb of Tel Aviv University, co-author with Mooley of an article describing the results of Nature.
Find
The surprise was to find how narrow the angle of opening of the jet and the angle of vision of the jet and remanence were. If the angle of view was 30 degrees instead of 20 degrees, says Gottlieb, "we would have missed it."
They estimate that only a GRB runs on 1000 with these narrow and very bright jets is directed towards the Earth. "It's plausible that they're much more common in the universe," says Gottlieb.
Nial Tanvir, who is an astronomer at the University of Leicester and did not participate in this research, describes the results as a "good result" to answer the question of whether the relativistic jets produced by star fusions neutrons can cocoons.
"This seems consistent with the long-held idea that many, if not all, neutron-star fusions produce these very fast jets, which, if viewed from a near-axis point of view will look like gamma rays, "he says.
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