Here’s how that’s possible…

When astronomers around the world watched the epic collision between two neutron stars in 2017, the main event was just the beginning. The after-effects, both immediate and longer-term, of such a massive, never-before-seen merger were bound to be exciting, interesting, and deeply informative.

And now scientists have revealed a doozy. As the two neutron stars slammed together, they ejected a jet of material that, to our eyes, appeared to blast 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 in spite of its name is actually an illusion based on our viewing angle.

However, even once its velocity was corrected, the jet was found to be insanely fast.

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

The data on the jet was obtained by the Hubble Space Telescope, which took a set of observations at around 8 days and then again at around 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 a number of radio telescopes from the National Science Foundation. Pooling their data, 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 astronomer Kunal Mooley of Caltech was able to first identify and then track the movement of a jet that erupted when the two ultradense stellar cores came together.

Superluminal motion occurs when something is coming towards us at a sufficiently high speed, very close to our line of sight. As the object nears, the distance required for its light to travel to us shortens – something we don’t usually need to take into account in our day-to-day lives, where light seems to move instantly (compared with our slow movements).

In this case the jet is moving nearly as fast as the light it emits, creating the illusion of its own light appearing to cover longer distances than it does (and therefore move at an impossible speed).

Unveiling the true speed of the jet therefore requires precise data, and a lot of number crunching.

The Hubble data showed a superluminal speed of seven times faster than light. The VLBI data, obtained between 75 and 230 days post-merger, and covered in a previous paper, showed that the jet later slowed down to a superluminal speed four times faster than light.

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

The result further constrains the angle at which we are viewing the jet, and strengthens the link between neutron star mergers and short-duration gamma-ray bursts. This connection requires a relativistic jet, and that 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, owing to the larger collecting area and smaller pixel size… The combination of optical astrometry and radio VLBI measurements (with current observing facilities) may be even more powerful, and could deliver strong constraints on the viewing angles of neutron-star mergers located 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 has been published in Nature.