The jet velocities of a neutron star have been measured for the first time

Astronomers have found a way to measure the jets produced by an incoming neutron star. It is hoped that once a large sample of such measurements have been made, it will be possible to answer a question that has plagued astronomers since the jets were discovered: what accelerates these jets so spectacularly? According to one theory, they are magnetic fields around the star; another suggests it is the star itself.

Many black holes, especially the supermassive black holes (SMBH) at the heart of galaxies, accelerate jets of material to astonishing speeds. It is less known that some neutron stars do the same. Dr. Tom Russell of the Istituto di Astrofisica Spaziale e Fisica Cosmica told IFLScience that even white dwarfs produce jets occasionally.

The jets are the product of accreting neutron stars, stars that gradually attract more material, for example from a companion star that is torn apart by their more powerful gravity. Only a small minority of neutron stars do this, but that still means tens of thousands in the Milky Way alone.

The material in the accretion disk slowly spirals inwards until it falls on the neutron star. “That’s a very stable and steady process,” Russell told IFLScience. However, once it hits the star, it builds up until it reaches a critical density and undergoes a thermonuclear burst, accompanied by gamma and X-rays. How often this happens depends on the accretion rate and possibly other factors around the star, but in the case of 4U 1728-34 these outbursts occur every few hours.

Russell is part of a team that realized that, using a combination of telescopes operating at different wavelengths, they could use these bursts to measure the speeds of the jets. “The explosion tells us when the enhanced jets were launched, and we simply time them as they move downstream – just as we would time a 100-meter sprinter as they move between the starting blocks and the finish line,” says Professor James Miller-Jones from Curtin University, hub of the International Center for Radio Astronomy Research, said in a statement.

Calculating speed requires knowledge of both distance and time. Russell explained to IFLScience that the frequency at which the jet radiates changes with distance from the star. “Thanks to previous studies on black holes and neutron stars and theory, we can calculate the distance associated with a certain frequency,” he said.

Putting all this together, the team arrived at a speed of 38 percent of the speed of light (114,000 kilometers or 70,836 miles per second) for 4H 1728-34. That’s paltry compared to black holes, whose jets are believed to travel at more than 99 percent of the speed of light. Given the much lower escape velocity of a neutron star, the difference is not surprising.

The most important finding should come once the work is extended to more stars. “If the star itself is responsible, we should see a direct relationship between the speed of a jet aircraft and the rotation of a neutron star,” Russell said. The spins of neutron stars are much easier to measure than those of black holes, making the comparison simple. If the relationship is not found, magnetic fields are likely responsible.

“The beauty of this work is that it is highly reproducible,” Russell said. “We need two telescopes looking at a neutron star at the same time, but don’t rely on a lot of theory to get the result.”

In this case, those two telescopes were the gamma-ray space telescope Integral and the Australian Telescope Compact Array, a set of six dishes that can work together. Many hours of time were required to obtain the results, but this may decrease with increasing experience.

This type of work requires combining telescopes at opposite ends of the electromagnetic spectrum with one of the telescopes represented by the Australian Telescope Compact Array, located in the Gomeroi country pictured here.

This type of work requires combining telescopes at opposite ends of the electromagnetic spectrum with one of the telescopes represented by the Australian Telescope Compact Array, as shown here.

Image credit: Alex Cherney

The jets produced by SMBHs can determine the evolution of a galaxy, so understanding them is even more important.

“Despite [black holes and neutron stars] with completely different physical characteristics, i.e. an event horizon versus a stellar surface, very few differences between the emitted jets have been identified, apart from the jets that appear to be generally brighter in black hole systems at similar X-ray luminosities,” said the authors. Note: This suggests that the lessons learned here may have broader applications.

The research has been published in Nature.

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