A jet of “superluminal” matter from a collision of neutron stars

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In August 2017, astronomers around the world watched closely as a binary neutron star system merged. This rare and impressive event, called GW1708171, released an incredible amount of energy comparable to a supernova explosion. The persistence of the phenomenon revealed a beam of matter which acted much faster than light itself!

Neutron stars are extremely dense objects that result from the gravitational collapse of massive stars – which are both too small to form a black hole and too large to become a white dwarf. When two of these ultracompact objects collide, they greatly perturb the spacetime around them: event GW1708171 was the first combined detection of gravitational waves and gamma-ray radiation originating from a neutron star merger.

The consequences of this merger were observed collectively by 70 observatories around the world, on Earth and in space, across a broad band of the electromagnetic spectrum – improving our understanding of these spectacular collisions. In particular, the event generated a jet of matter moving at a speed close to that of light. Thanks to the Hubble telescope, which was able to point to this area of ​​space only a few days after the collision, astronomers were able to accurately measure the speed of this jet.

An illusion of superluminal speed

While the event took place in 2017, it took several years for researchers to find a way to analyze the data from Hubble and other telescopes. The results of these studies have just been published in Nature. The authors used data from Hubble, as well as data from the European Space Agency’s Gaia satellite and Very Long Baseline Interferometry (VLBI) radio telescopes to achieve extreme precision. ” It took months of careful data analysis to make this measurement said Jay Anderson of the Space Telescope Science Institute in Baltimore.

By combining the various observations, they were able to precisely locate the site of the explosion and managed to reconstruct the scenario. The host galaxy (NGC 4993) is 41 megaparsecs from Earth. The two neutron stars collapsed into a black hole, which irreversibly attracted matter to itself. The accretion disk thus formed, in rapid rotation, generated jets moving outward from the poles.

The superfast jet, tracked by astronomers, slammed into, then swept away material in its path, breaking through the expanding shell made of debris from the explosion. Data collected by Hubble (8 and 159 days after the merger) showed that this jet was moving at an apparent speed of seven times the speed of light! Radio observations, made 75 days and 230 days after the merger, show that it then slowed to an apparent speed four times the speed of light.

Of course, according to the laws of physics, this is impossible because nothing can exceed the speed of light. This superluminal speed is actually an illusion, tied to our viewing angle.

A way to determine the expansion rate of the universe

The jet is heading towards Earth at nearly the speed of light, and the light it emits has less and less distance to travel as it gets closer to Earth. ” It’s like the jet is chasing its own light “, the researchers explain. In reality, more time has passed since the emission of light from the jet than the observer thinks. ” This results in an overestimation of the object’s speed, which in this case appears to exceed the speed of light », concludes the team.

No superluminal jet therefore, but an impressive speed nonetheless: the measurements reveal that the jet was traveling at least 99.97% of the speed of light when it was emitted!

These results reinforce the previously assumed connection between neutron star mergers and short-lived gamma-ray bursts (GRBs): the researchers proved here that this cosmic collision was indeed the cause of a relativistic jet, typical of GRBs. ” We demonstrated in this work that precision astrometry with optical and infrared space telescopes is an excellent way to measure proper jet motions in neutron star mergers. “, they write in their article.

This work thus opens the way for more precise studies of neutron star mergers, which are detected on Earth by gravitational wave observatories. Scientists hope to collect enough data in the future on relativistic jets to more precisely determine the expansion rate of the universe – because there is currently an inexplicable difference between the values ​​of the Hubble constant measured via two separate methods.

The James Webb Space Telescope should be able to perform much better astrometry than that achieved by the Hubble Space Telescope due to the larger collection area and smaller pixel size “, the researchers note. The only thing left is to wait for the instrument to catch another neutron star collision to try to solve the mystery.

Source: K. Mooley et al., Nature

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