A neutron star is the remains of a massive star that once died during a supernova explosion. Overall, neutron stars are considered some of the most extreme objects in the known universe – and that’s especially true when these incredibly dense stellar remnants sit next to companion stars (that haven’t yet ‘died’) close enough to reveal the immense size of to observe a neutron star. gravity to remove material from that second star. In other words, the companion star resembles the neutron star’s stellar victim.
These ‘vampire neutron stars’ are special because they come back to life like a cosmic Bela Lugosi. This is because the infalling material from a companion star causes thermonuclear explosions on the surface of the neutron star. Some of this stolen matter is directed to the neutron star’s poles, where it erupts at near-light speeds in the form of powerful astrophysical jets. But what causes these jets to launch – and how they connect to these thermonuclear bursts – is still a mystery.
However, new research has provided a clue to the puzzle.
Scientists have unveiled a way to measure the speeds of those jets and tie the values to the qualities of both a neutron star and the unfortunate binary star it enjoys. This could ultimately help solve this jet-related dilemma, and perhaps also provide information about other objects that remove matter from a companion star, such as supermassive black holes.
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“For the first time, we have been able to measure the speeds of the steady jets launched from a neutron star,” lead author and National Institute for Astrophysics (INAF) scientist Thomas Russell told Space.com. ‘These jets, like those of black holes, are incredibly important in our universe because they transfer enormous amounts of energy to their surroundings, influencing star formation, galaxy growth and even the way galaxies cluster together. But we don’t really do that. understanding how these jets are launched.”
Russell explained that scientists previously thought the jets could be launched by the rotation of material stripped from a victim star as that material spirals inward. There was also the theory that the jets are connected to the spin of the rotating object itself. .
This new research could help answer the question of which mechanism is mainly responsible.
“Our discovery of a link between the thermonuclear explosions and the jets now provides us with an easily accessible and repeatable probe to disentangle the launch mechanism of the jets in neutron stars,” Russell continued. ‘Since we think that jets are launched in very similar ways for all types of objects, this will help us understand how the jets are launched from all objects, even the supermassive black holes that reside at the centers of galaxies.’
How do neutron stars blow their tops?
To reach their conclusion, Russell and colleagues examined two systems with feeding neutron stars: the X-ray binaries 4U 1728-34 and 4U 1636-536. Both systems are known to erupt periodically with thermonuclear eruptions.
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 ‘bursting’ neutron stars in total.
“As the neutron star eats matter from a nearby star, the accumulated material accumulates on the surface of the neutron star,” Russell said. the entire surface of the neutron star in a few seconds.”
Bursts associated with 4U 1728-34 and 4U 1636-536 are visible in the X-ray band, meaning 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 cause some extra material to be pumped into the jets during the tens of seconds that the eruptions last,” Russell continued. “By using radio telescopes to monitor the jets with the Australia Telescope Compact Array, we were able to track this extra material as it flowed through the jets, essentially giving us a cosmic flash camera to measure the jet speed.”
What they wanted to see were changes in the radio emissions after the X-ray flashes.
The team found an increase in radio brightness within minutes of each thermonuclear explosion. This led the researchers to conclude that the evolution of jets is closely linked to thermonuclear explosions.
“We were surprised at how clear the response was in the jets. These were very bright and clear flares that flowed through the jet and were easily visible,” Russell said. “We expected a response, but thought it would be much more subtle.”
Neutron star jets caught speeding
The speeds of these jets were the missing piece of the puzzle, according to the team, leading to a link between the jets’ violent ejections and explosive feeding events.
“Velocity is incredibly important for understanding how the jets are launched, and this new discovery opens a very accessible window to answer that question,” Russell said. ‘We can now apply this experiment to many other bursting neutron stars, and we can then compare how the jet speed correlates with the neutron star’s spin, mass and possibly even magnetic field, all of which are thought to be key ingredients in its formation. of the neutron star. jet launch.”
Should the team see a connection between any of these properties and the speed of the jets, it will reveal what the main launch mechanism for these jets is – whether it is the rotation of the neutron star or the rotation of the infalling material.
This is the first time the speed of such a jet from a neutron star has been measured, but it is worth noting that it has been measured before for black holes. However, Russell explained that neutron stars have a huge advantage over black holes when it comes to using them as probes to investigate jet launching mechanisms.
“Neutron stars can have very precisely measured spins, well-determined masses and possibly even known magnetic field strengths, all of which are much more difficult to measure in black holes,” he said. “So it is currently only with neutron stars that we can connect the system properties to the jets.”
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Overall, the team has now seen this result in two feeding neutron star systems, but these are the only two they have looked at so far.
“We apply our new technique to as many other bursting neutron stars as possible to reveal how the jet velocities vary with the different properties of neutron stars,” he concluded. “Once we build enough samples, we can unravel the key properties for jet production and reveal how the jets are launched.”
The team’s research was published Wednesday (March 27) in the journal Nature.