Astronomers have detected a jet at the “wrong” neutron star

Magnetic lines and matter moving around a neutron star, in the artist’s view
ICRAR / Universiteit van Amsterdam

Astronomers have detected a jet emanating from a neutron star with an extremely strong magnetic field, according to the journal Nature . Previously it was believed that such stars are not able to create narrow-flow plasma flows. The discovery forces scientists to reconsider the modern theories of jet formation.

When a star with a mass several times larger than a solar one approaches the final stage of evolution, a supernova explosion occurs, as a result of which a neutron star is formed. It has an extremely high density, a strong magnetic field and a small radius – about 10 kilometers. Like black holes, neutron stars are able to emit jets – narrow-beam currents of particles dispersed to very high velocities. Previously, only objects with a magnetic field induction above 10 12 gauss (for comparison, for the Earth this value is about one) were considered to be an exception , but now scientists have discovered that even such neutron stars can form jets.

Astronomers led by Van den Eijnden from the University of Amsterdam observed the Swift J0243.6 + 6124 (Sw J0243), the first ultra-bright X-ray pulsar found in the Milky Way. It was opened in 2017 by the space observatory Swift (hence the name). The object is a slowly rotating neutron star that pulls the material of another companion star, probably much larger than our Sun.

During observations at the VLA telescope, the researchers found that during pulsations, not only the X-ray but also the radio emission comes from the object. In addition, the brightness of the system in the radio range began to decrease when the x-ray emission of the neutron star reached its maximum and began to decrease. Typically, this behavior is typical for systems in which there is a jet.

Standard theories suggest that the flow of particles dispersed to high velocities is triggered by a magnetic field in the inner parts of the accretion disk. However, if the magnetic field of the star is too strong, it will suppress the creation of the jet. Nevertheless, the discovery of astronomers suggests that, probably, other mechanisms are possible. Thus, scientists assume that plasma flows can be formed due to the rotation of a neutron star, and not to the influence of a magnetic field ( the Blanford-Znaek process ). This theory assumes that for slowly rotating pulsars the jet will be weaker – this is exactly what is observed in the Swift J0243 system.

Astronomers believe that pulsar Sw J0243 can be a representative of a whole class of objects whose radio emission was too weak to notice it with the help of old instruments. With the help of VLA, it is possible to find other similar objects, and then scientists will be able to unambiguously determine how the jets form in neutron stars.

This year, astronomers for the first time managed to fully follow the way a black hole tears a star and forms a jet from its substance, and also discerns the  structure of the jet near a supermassive black hole.

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