An unexpected discovery of a neutron star forces us to rethink the radio

Just a little 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 more massive and rapidly rotating star.

It is from here that we have detected radio jets, materials moving at almost the speed of light and emitting radio waves, with the details published today. Nature.

But the discovery was something that was not predicted by the current theory. This neutron star has a very strong magnetic field, but neutron star jets have only been observed in systems with magnetic fields about 1,000 times weaker.

Neutron stars are dense stellar bodies, with about one and a half times the mass of the sun compressed in a sphere of ten kilometers in diameter.

With huge densities (similar to those of an atomic nucleus), it is the densest objects that can defend themselves against their own gravity. If they were denser, they would collapse to form a black hole.

A quick discovery

This particular binary system, known as Swift J0243.6 + 6124, was first discovered on October 3, 2017 by NASA's Neil Gehrels Swift observatory. This satellite, called Swift, constantly sweeps the sky in search of new light sources of X-ray emission.

After a new explosion of X-rays was detected at the location of this binary system, astronomers around the world trained their telescopes at the source to try to determine what was producing them.

It turned out that the strong gravity of the neutron star in this system captured materials ejected by the rapid rotation of the other star. For many years, this gas had accumulated in a disk of matter swirling around the neutron star.

When enough material had accumulated, everything began to move at a time. We all know a weight shot from the top of a hill that is gaining speed as it falls. The physics behind this daily phenomenon is the release of gravitational energy, which is converted into motion energy.

In the same way, the gravitational energy of the mass was released when it went to the neutron star. This energy was first converted into motion, then X-ray, which the Swift satellite detected.

Closer inspection

Our team, led by doctoral student Jakob van den Eijnden of the University of Amsterdam, also detected radio waves from the source, using the Karl G Jansky Very Large Array observatory in New Mexico.

The brightness of the radio broadcast tracked the X-ray brightness from the source as the gust rose and faded in a few months. The behavior of the radio show led us to conclude that it came from jets.

Jets are clusters of closely focused matter and energy that move outward at a speed close to that of light. They take away some of the gravitational energy released as the material falls to a central object, such as a black hole or a neutron star.

The jets deposit this energy in the environment, often at great distances from the launching point.

In neutron stars and black holes that are only a few times more massive than the Sun, this energy can be transported many light years away. For the supermassive black holes that are at the center of the galaxies, the jets can take away energy at hundreds of years of light from the center of the galaxy.

The first jet plane was discovered 100 years ago by astronomer Heber Curtis, who noticed a "curious straight ray" associated with the nearby M87 galaxy. Since the dawn of radio and X-ray astronomy in the middle of the last century, the jets have been the subject of numerous studies.

They are produced whenever the material falls on a central and dense object, ranging from stars in formation to white dwarfs, neutron stars and black holes. The only exception was neutron stars with strong magnetic fields – about a trillion times stronger than those of the Sun.

Against the theory

Despite decades of observations, no jet has been detected in these systems. This led to the belief that strong magnetic fields prevented the throwing of jets.

Our detection of the jets of a neutron star with a strong magnetic field refuted the idea that had been maintained over the last decades. But this requires a reconsideration of our theories of how the jets are produced.

There are two main theories explaining how throws are thrown. If a magnetic field passes through the event horizon of a rotating black hole, the rotation energy of the hole can be extracted to feed the streams.

But as neutron stars have no horizon of events, it is thought that their jets are launched from rotating magnetic fields in the inner part of the gas disk that surrounds them. Particles can be projected along the magnetic field lines in the same way that a pearl moves outward on a wire as you turn over your head.

If the magnetic field of a neutron star is strong enough, it should prevent the material disk from getting close enough to the neutron star for this second mechanism to work. So we need another explanation.

Recent theoretical work has suggested that under certain circumstances it would be possible to launch jets from the extraction of rotational energy from the neutron star.

In our case, this could have been made possible by the high rate of falling of the material. This would also explain why the jets we saw were about 100 times weaker than those seen in other neutron stars with weaker magnetic fields.

Whatever the explanation, our result is an excellent example of how science works, with theories being 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 the impact of magnetic fields on jet throwing, which helps us understand this key feedback mechanism in the universe.

This article is republished from The Conversation under a Creative Commons license. Read the original article.