Slightly to the left of the leftmost part of the "W" in the constellation, Cassiopeia is a binary system of a neutron star in a 27-day orbit with a massive, fast-rotating star.
It is from here that we have detected radio waves – material that moves close to the speed of light and emits radio waves – with details that are Nature.
But the find was something that was not predicted by current theory. This particular neutron star has a very strong magnetic field, but rays of neutron stars were previously observed only in systems with magnetic fields that were about 1,000 times weaker.
Neutron stars are dense stellar bodies, with about one and a half times the mass of the sun pressed into a sphere only ten kilometers wide.
With enormous densities (comparable to those of an atomic nucleus) they are the densest objects that can support themselves against their own gravity. If they were closer, they would collapse to form a black hole.
A quick discovery
This specific binary system, known as Swift J0243.6 + 6124, was first discovered on October 3, 2017 by NASA's Neil Gehrels Swift Observatory. This satellite, known as Swift, constantly scans the air in search of new, clear sources of X-rays.
After a clear new X-ray of the location of this binary system was detected, astronomers from all over the world trained their telescopes at the source to try to determine what they produced.
It turned out that the strong gravity of the neutron star in this system was the recording of material that was thrown away by the rapid rotation of the other star. For many years this gas was piled up in a disc of matter that swirled around the neutron star.
When enough dust had been collected, it all started going in at the same time. We all know a weight that is thrown from the top of a hill and that the speed picks up when it falls. The physics behind this everyday phenomenon is the release of gravitational energy, which is converted into the energy of movement.
In exactly the same way, the gravitational energy of the mass was released when it fell to the neutron star. That energy was initially converted into motion and ultimately into X-rays, which the Swift satellite discovered.
Our team, led by PhD student Jakob van den Eijnden of the University of Amsterdam, also discovered radio waves from the source using the Karl G Jansky Very Large Array observatory in New Mexico.
The brightness of the radio broadcast followed the brightness of the source's x-rays when the burst rose and then faded over a period of several months. The behavior of the radio emission led us to the conclusion that it emanated.
Jets are narrowly directed rays of matter and energy that extend outward close to the speed of light. They remove part of the gravitational energy that is released when matter falls in the direction of a central object, such as a black hole or neutron star.
The rays pour this energy into the environment, often at very great distances from the starting point.
In neutron stars and black holes that are only a few times bigger than the sun, this energy can be transported many light years further. For supermassive black holes in the centers of galaxies, the rays can carry energy to hundreds of thousands of light years from the center of the galaxy.
The first jet plane was discovered 100 years ago by astronomer Heber Curtis, who noted a "curious straight jet" in connection with the nearby galaxy M87. Since the beginning of radio and X-ray astronomy in the middle of the last century jet fighters have been extensively studied.
They are produced when matter falls on a dense central object, from newly formed stars to white dwarfs, neutron stars and black holes. The only exception were neutron stars with strong magnetic fields – about a trillion times stronger than those of the sun.
Against the theory
Despite decades of observations, no rays were detected in these systems. This led to the suggestion that strong magnetic fields prevented jets from being launched.
Our detection of rays from a neutron star with a strong magnetic field refuted the idea that had existed in recent decades. But it requires a re-examination of our theories about how jets are produced.
There are two main theories that explain how jets are launched. If a magnetic field connects the event horizon of a rotating black hole, the rotational energy of the hole can be extracted to feed the rays.
But because neutron stars have no horizon, their rays are instead thought to be launched from rotating magnetic fields in the inner part of the gas disk around them. Particles can be thrown out along magnetic field lines in the same way as a bead moves outwards over a wire that swirls around your head.
If the magnetic field of a neutron star is sufficiently strong, it should prevent the disk of matter getting close enough to the neutron star to make this second mechanism work. We therefore need another explanation.
Recent theoretical work has suggested that under certain circumstances it might be possible to launch jets from the extraction of the rotational energy of the neutron star.
In our case, this could have been made possible by the high rate at which matter fell within. It would also explain why the jets we saw were about 100 times weaker than seen in other neutron stars with weaker magnetic fields.
Whatever the explanation, our result is a good example of how science works, with theories that are developed, tested against observations and revised in the light of new experimental results.
It also provides us with a new class of sources to test how magnetic fields affect jets launch, helping us understand this key feedback mechanism in the universe.
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