Matter ejected from collapsing neutron stars appeared to break the speed of light : ScienceAlert

When astronomers around the world watched the epic collision of two neutron stars in 2017, the main event was just the beginning. The aftermath, both immediate and long-term, of such a massive, never-before-seen merger was inevitably fascinating, interesting, and deeply informative.

And now scientists have revealed a boring one. As the two neutron stars collided, they ejected a jet of material that, to our eyes, appeared to be hurtling into space at seven times the speed of light.

This, of course, is impossible according to our current understanding of physics. It’s a phenomenon known as superluminal speed, which despite its name is actually an illusion based on our viewing angle.

However, even when its speed was corrected, the jet was found to be insanely fast.

“Our result shows that the jet was traveling at least 99.97 percent of the speed of light when it was launched,” says astronomer Wenbin Lu of the University of California, Berkeley.

Data on the jet was obtained by the Hubble Space Telescope, which took a series of observations over about 8 days and then again about 159 days after the merger, seen here on Earth in August 2017.

Other telescopes were watching, including the European Space Agency’s Gaia satellite and some radio telescopes from the National Science Foundation. By gathering their data, the researchers could construct a kind of measurement called very long baseline interferometry (VLBI).

Based on these observations and months of analysis, a team led by Caltech astronomer Kunal Mooley was able to first detect and then track the motion of a jet that erupted when the two superdense stellar cores merged.

Superluminal motion occurs when something comes towards us at a high enough speed, very close to our field of vision. As the object gets closer, the distance required for its light to travel to us decreases – something we usually don’t have to consider in our everyday lives, where light appears to move instantaneously (compared to our slow motions).

In this case the jet moves almost as fast as the light it emits, creating the illusion that its own light appears to cover greater distances than it does (and therefore moves at an impossible speed).

Therefore, revealing the true speed of the jet requires accurate data and a lot of difficulty.

The Hubble data showed a superluminous speed seven times that of light. The VLBI data, taken between 75 and 230 days after the merger, and covered in a previous publication, showed that the jet later slowed to a superluminal speed four times the speed of light.

“I am amazed that Hubble could give us such a precise measurement, which rivals the precision achieved by the powerful VLBI radio telescopes scattered around the globe,” says Mooley.

The result further constrains the angle at which we see the jet and strengthens the link between neutron star mergers and short-duration gamma-ray bursts. This connection requires a relativistic jet, which is exactly what Mooley and his colleagues have measured.

“We have demonstrated in this work that precision astrometry with space-based optical and infrared telescopes is an excellent means of measuring the proper motions of jets in neutron star mergers,” they write in their paper.

“The James Webb Space Telescope (JWST) should be able to perform astrometry much better than that with the Hubble Space Telescope, due to its larger collection area and smaller pixel size… The combination of optical astrometry and VLBI radio measurements (with current facilities observational ) may be even stronger and could provide strong constraints on the viewing angles of neutron star mergers as far away as 150 Mpc [roughly 500 million light-years].”

Now, we just have to wait for another neutron star collision…

The team’s research was published in Nature.

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