A jet of material released from the neutron star collision GW170817 travels at 99.97 percent of the speed of light and manages to appear to defy the laws of physics by traveling seven times faster. The jet was seen by the Hubble and Gaia space telescopes in the days shortly after the collision, but it took five years to process and analyze the images sufficiently to reveal what was going on.
The detection of GW170817, the first observed neutron star merger, transformed astronomy. It was the first time that gravitational wave detectors and telescopes operating in the electromagnetic spectrum worked together to study the same event, and 70 observatories participated. The cooperation was beyond the world—instruments in space were also distracted from what they were doing to capture as many photons as they could.
The combined masses of the two neutron stars created an object so dense that it collapsed into a black hole, which pulled material from the surrounding region into a rapidly spinning disk, whose poles released powerful jets. An analysis of these jets Nature reveals how close they came to the speed of light, but scientific research often works at a more impressive pace, and the work was only just published. Along the way, the authors support the hypothesis that gamma rays burst it is a consequence of neutron star mergers.
The paper’s authors combined data produced by Hubble and Gaia eight and 159 days after the collision, with radio studies made at 75 and 230 days. The radio wavelength observations were made with telescopes operating thousands of kilometers away using Very Long Baseline Interferometry (VLBI).
“I am amazed that Hubble could give us such a precise measurement, which rivals the precision achieved by the powerful VLBI radio telescopes scattered across the globe,” CalTech’s Dr. Kunal Mooley said in a statement.
The measurements from Hubble were accurate, but at first seem absurd, showing the jet moving at seven times the speed of light. As science fiction has accustomed us to concepts like “warp five,” the speed of light remains an insurmountable obstacle. Exceeding even 7 percent, let alone seven times, will shake physics to its core.
However, this is not the first time that astronomers have seen apparently “superluminous” motion and understood its cause. A jet is pointed almost Earthward, so the light emitted as it extends has a shorter distance to travel to reach us. The delay between the emission of light from the leading edge of the jet and its point of origin is masked to create the illusion of faster-than-light travel. By measuring the gap between the speed the jet appears to be traveling and its maximum possible speed, the authors were able to estimate the angle of the jet relative to Earth to be between 19 and 25 degrees. This in turn helped them estimate how close to the speed of light the jet really is.
“Our result shows that the jet was traveling at least 99.97% the speed of light when it was launched,” said Dr Wenbin Lu from the University of California, Berkeley.
Gamma-ray bursts are so powerful, astronomers were initially puzzled about what could create them. Jets produced by neutron star mergers became a favored explanation, but only now can we confirm that such jets exist and are fast enough to produce the explosions.
As the paper notes, this work is just the beginning, JWST will be able to see events like this with precision that far exceeds that of Hubble.
The study is published in Nature (open access).