A jet of radiation from two colliding neutron stars appears to be travelling at seven times the speed of light, according to measurements from the Hubble Space Telescope. Although this is merely an optical illusion, as nothing can travel faster than the speed of light, it provides key insights into mysterious gamma ray bursts, which aren鈥檛 fully understood.
In 2017, astronomers at the Laser Interferometer Gravitational-Wave Observatory detected a wave, GW170817, produced from the collision of two neutron stars. This was followed up by dozens of Earth and space-based telescopes, which then captured a bright flash of light and powerful gamma rays 鈥� an incredibly fast jet called a gamma ray burst.
The initial measurements from this network of telescopes, which used a technique called Very Long Baseline Interferometry (VLBI), found that the jet was travelling at least 95 per cent the speed of light and traced its rough direction of travel. But given the 130 million light-year distance between Earth and the neutron stars, along with the relative dimness of the gamma ray burst, there were still many uncertainties about the jet.
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Now, at the California Institute of Technology and his colleagues have compared the VLBI data with measurements from the Hubble Space Telescope taken eight and then 159 days after the collision, to achieve a much higher level of precision than previous measurements.
The team found that the jet seems to be moving seven times faster than the speed of light, but this is an optical illusion caused by the fact that the light is pointing towards Earth in Earth鈥檚 direction of travel. A similar effect happens if you point a laser beam at the moon and sweep it across the surface 鈥� the dot of the laser appears to move faster than light, even though no individual photons do.
However, the highly precise measurement does tell us important things about the jet鈥檚 real speed, at 99.97 per cent the speed of light, and its direction of travel, as well as give a much more precise handle on its location, in a small region of the shell galaxy NGC 4993. 鈥淣ow, what we can do is really zoom in much further, 1000 or 10,000 times further, into this event and find out what the local properties of this event are,鈥� says Mooley.
鈥淚t鈥檚 really, really hard to actually see this without having super high-resolution data and very accurately measuring exactly where in the images your source is to detect this tiny effect,鈥� says at the University of Birmingham, UK. 鈥淪even times the speed of light sounds impressive, but it actually corresponds to movements of tiny fractions of a pixel over hundreds of days.鈥�
Nature
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