How exactly is the inside of a neutron star?
A neutron star is what remains after a massive star goes supernova. It's a tightly packed, ultra-tight body made of – you guessed it – neutrons. That's actually not entirely true.
Mathematical models show that neutron stars consist of layers, and in these layers there are other things than just neutrons. But when you look deeper into a neutron star, you see more and more densely packed neutrons and less of everything else. Once you get to the core, it's mostly neutrons.
<img src = "https://www.universetoday.com/wp-content/uploads/2019/08/Neutron_star_cross_section.png" alt = "We are not sure exactly how inside a neutron star looks, but mathematical Models indicate that they are like this. Image Credit: By Robert Schulze ̵
1; Own Work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11363893ebrit19659005 ?? We are not sure, exactly what the interior of a neutron star looks like, but mathematical models suggest they are like this. Image Credit: By Robert Schulze – Own Work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid = 11363893
But it is what looks deeper into a neutron star is the problematic part. Nobody has ever seen the inside of one.
Astronomers are stuck observing the exterior of neutron stars to try to understand them. and mathematical models help, but there is no substitute for actual observation. Fortunately, sometimes neutron star rnor of "glitches", and these glitches are an opportunity to learn something about these ultra-dense bodies.
Neutron stars rotate. They can also emit electromagnetic radiation from their poles, and when that radiation is pointed at the earth intermittently during the rotation of the star, we can see the rays. These neutron stars are called pulsars.
In this animation of a neutron pulse is pink gamma radiation. The green ones are the narrow rays of radio waves that can only be detected when pointed to the earth. Video: NASA
For the most part, the rotation is very regular and very fast. But sometimes they rotate faster, and it happens when parts of the star's interior move toward the outside. For a brief astronomical moment, this glitch can allow astronomers to gain some insight into these confusing objects.
2016, astronomers using Mt. Nice telescope observed Vela Pulsar glitching. Vela Pulsar is about 1000 light years away, in the constellation Vela. It is the brightest pulsar in the sky in radio frequencies, and it is also the most famous of all glittering pulsars. Only about 5% of pulse glitch, and Vela glitches every three years.
<img src = "https://www.universetoday.com/wp-content/uploads/2011/11/Vela_Pulsar_jet.jpg" alt = "This Chandra image shows Vela Pulsar as a bright white spot in the middle of the image, surrounded by hot gas that appears in yellow and orange. A jet of material pours from the hot gas up to the right. Image Credit: By NASA / CXC / PSU / G.Pavlov et al. – http: //heasarc.gsfc. nasa.gov/docs/objects/heapow/archive/compact_objects/vela_pulsar_jet.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=135898  This Chandra image shows Vela Pulsar as a bright white spot in the middle of the image, surrounded by warm gas that appears in yellow and orange. A jet of material pours from the hot gas up to the right. Image Credit: By NASA / CXC / PSU / G.Pavlov et al. – http://heasarc.gsfc.nasa.gov/docs/objects/heapow/archive/compact_objects/vela_pulsar_jet.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=135898  This neutron star, which just like all n eutron stars are only several kilometers in diameter, normally rotating about 11 times per second. But during the glitch 2016, the star's rotation accelerated. It was the first time glittering live was observed.
In a paper published in the journal Nature Astronomy, a team of researchers analyzed the data from the glitch 2016. The paper is called "Rotational evolution of the Vela pulse during the 2016 glitch." The first author is Dr. Greg Ashton from Monash School of Physics and Astronomy.
The main finding of their reanalysis is that glitch is more than just a simple increase in rotational speed. The star quickly spun up before relaxing for fast speeds. According to the authors, Vela's behavior during the glitch gave them a glimpse into the makeup of the interior of the neutron star.
They say that neutron stars have three distinct layers. In a press release, co-author Paul Lasky, also from the Monash School of Physics and Astronomy, said, “One of these components, a soup with super-liquid neutrons in the crust's inner layer, first moves outward and hits the star's outer crust and causes it to spin up. But then a second soup with super liquid that moves in the core catches up to the first one, which causes the star to spin again. "
They call this phenomenon an excess. According to the authors, other researchers have predicted this in studies, but it has not been observed.
"This excess has been predicted a few times in the literature, but it is the first real time it has been identified in observations, Sa Lasky.
Student co-author Dr Vanessa Graber of McGill University was one of the researchers who predicted this overshoot, and she talked about it in her 2018 paper "Rapid crust coupling and glitch rise in superfluid neutron stars."
But during the live observation by Vela 2016, the rotating neutron star showed some other odd behavior: before the glitch, it actually slowed down. This is something that has never been observed before.
"Immediately before the glitch, we noticed that the star appears to slow its rotational speed before spinning again," said Dr Ashton. "We actually have no idea why this is, and this is the first time it has ever seen."
The artist's illustration of a rotating neutron star, the remnants of a super nova explosion. Credit: NASA, Caltech-JPL
"It may be related to the cause of the error, but we're honestly not sure," Ashton said.
This study is a new puzzle piece with regard to neutron stars. They call the slower preceding the spin-up an "anti-glitch." Anti-glitch is followed by "overshoot" as predicted by co-author Graber and others. Then the relaxation is down to the actual glitch speed. This three-step sequence has not been observed in its entirety before. The authors think this three-step model for glitches is an important discovery.
At the end of their paper they say, “During the 2016 glitch, Vela first spun down. A few seconds later, it spun up quickly, before it finally spun down with an exponential relaxation time of? 60 s. This model is significantly preferred over a simple step glitch, or one with just a single spin-up event. "
It is the observation of anti-glitch that is the key. If astronomers can observe other pulsars that behave like this, they can test predictions against them.
The artist's illustration of a neutron star, a small remnant that remains after its predecessor star explodes. Here, the 12 mile (20 kilometer) sphere is compared to the size of Hanover, Germany. Credit: NASA's Goddard Space Flight Center
But for the moment, there is only one observed instance of anti-glitch. Without more evidence, researchers are limited to models. As the authors state in their conclusion, "Analyzes such as those presented here assess only the relative evidence of models." Also "The best fitting models tested here do not explain all the features of the data."
The authors suspect that their analysis will ignite more observation and study of neutron stars and their glitches and inspire some new theories.