A neutron star is the remains of a once massive star that died in a supernova explosion. As a whole, neutron stars are considered some of the most extreme objects in the known universe – and this is especially true when these incredibly dense stellar remnants exist alongside companion stars (that are not yet “dead”) that are close enough to accommodate a neutron star’s enormous size. gravity to remove material from the second star. In other words, the companion star is like the stellar victim of the neutron star.
These “vampire neutron stars” are special because they come back to life like a cosmic Bela Lugosi. This is because falling material from a companion star triggers thermonuclear explosions on the surface of the neutron star. Some of this stolen matter is directed towards the poles of the neutron star, from where it explodes at near-light speeds in the form of powerful astrophysical jets. But what caused these jets to be ejected and how they were connected to these thermonuclear explosions remained a mystery.
But new research offers a clue to solving the puzzle.
Scientists have uncovered a way to measure the speeds of these jets and link the values to the characteristics of both a neutron star and the ill-fated binary star it feeds on. This could ultimately help resolve this dilemma regarding the jet, and perhaps provide information about other objects that also receive material from the companion star, such as supermassive black holes.
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“For the first time, we were able to measure the velocities of stationary jets ejected from a neutron star,” Thomas Russell, lead author of the study and a National Institute for Astrophysics (INAF) scientist, told Space.com. “These jets, like jets from black holes, are incredibly important to our universe because they transfer huge amounts of energy into their surroundings, affecting star formation, the growth of galaxies, and even how galaxies come together. Understand how these jets are ejected.”
Russell explained that scientists previously thought the jets might be ejected because the jets spin as material stripped from the victim star spirals inward. There was also the theory that the jets were due to the spin of the spinning object itself. .
This new research may help determine an answer as to which mechanism is predominantly responsible.
“Our discovery of the connection between thermonuclear explosions and jets now provides us with an easily accessible and reproducible probe to unravel the ejection mechanism of jets in neutron stars,” Russell continued. “Since we think jets are ejected in very similar ways for all types of objects, this will help us understand how jets are ejected from all objects, even supermassive black holes found at the centers of galaxies.”
How do neutron stars blow their tops?
To reach this conclusion, Russell and his colleagues examined two systems feeding neutron stars: X-ray binaries 4U 1728-34 and 4U 1636-536. Both systems are known to periodically explode with thermonuclear explosions.
Thermonuclear explosions on the surfaces of neutron stars are not a new phenomenon for scientists. These explosions have been analyzed for years, and Russell points out that astronomers have observed at least 125 “exploding” neutron stars in total.
“As the neutron star consumes material from a nearby star, the accreted material accumulates on the surface of the neutron star,” Russell said. “It will reach the entire surface of the neutron star in a few seconds.”
Outbursts associated with 4U 1728-34 and 4U 1636-536 are visible in the X-ray band; This means the team was able to use the European Space Agency’s International Gamma Ray Astrophysics Laboratory (INTEGRAL) space telescope to detect them.
“We found that these explosions caused an extra amount of material to be pumped into the jets for the tens of seconds that the explosions lasted,” Russell continued. “By using radio telescopes to watch the jets with the Australian Telescope Compact Array, we were able to track this extra material flowing down the jets, providing us with a cosmic speed camera to measure the jet speed.”
What they wanted to see were changes in radio emissions following X-ray bursts.
Indeed, the team detected an increase in radio brightness a few minutes after each thermonuclear explosion. This led the researchers to conclude that the evolution of the jets was closely related to thermonuclear explosions.
“We were surprised at how clear the response was in the jets. These were very bright and clear flares that flowed down the jet and were easily detected,” Russell said. “We expected some backlash, but we thought it would be much more subtle.”
Neutron star jets caught speeding up
The speed of these jets is the missing piece of the puzzle, leading to a connection between the jets’ violent ejections and explosive feeding events, the team says.
“Speed is incredibly important for understanding how jets are launched, and this new discovery opens a very accessible window into answering this question,” Russell said. “We can now apply this experiment to many other exploding neutron stars and then compare how the jet speed relates to the spin, mass, and possibly even the magnetic field of the neutron star, all of which are thought to be key components of the explosion jet being ejected.”
If the team sees a correlation between one of these features and the jet speed, it will reveal what the main ejection mechanism for these jets is; whether it is the rotation of the neutron star or the rotation of falling material.
This is the first time the speed of such a jet coming from a neutron star has been measured, but it is worth noting that it has also been measured for black holes before. However, Russell explained that neutron stars have a huge advantage over black holes for investigating jet ejection mechanisms.
“Neutron stars can have very precisely measured spins, well-defined masses, and possibly even known magnetic field strengths; all of which are much more difficult to measure in black holes,” he said. “So right now we can only begin to relate system properties to jets with neutron stars.”
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Ultimately, the team saw this result in two fed neutron star systems, but those are the only two they’ve looked at so far.
“We apply our new technique to as many exploding neutron stars as possible to reveal how jet velocities vary with different neutron star properties,” he concluded. “Once we create a sufficient sample, we will be able to unravel the fundamental aspects of jet production and reveal how jets are launched.”
The team’s research was published in the journal Nature on Wednesday, March 27.