Powerful jet discovered coming from 'wrong' kind of star

Astronomers had thought the star was incapable of launching such a jet

Astronomers have discovered a fast-moving jet of material which they believe to have been propelled outward from a type of neutron star.

Observed using the National Science Foundation's Karl G. Jansky Very Large Array (VLA), the finding is peculiar because it was previously thought that the star was incapable of launching such a jet.

As a result, the scientists said, they may have to fundamentally revise their ideas about how such jets originate as it's the "wrong" kind of star.

Neutron stars are super-dense objects, the remnants of massive stars that exploded as supernovas. When in binary pairs with "normal" stars, their powerful gravity can pull material away from their companions.

That material forms a disk, called an accretion disk, rotating around the neutron star. Jets of material are propelled at nearly the speed of light, perpendicular to the disk.

"We've seen jets coming from all types of neutron stars that are pulling material from their companions, with a single exception. Never before have we seen a jet coming from a neutron star with a very strong magnetic field," said Jakob van den Eijnden of the University of Amsterdam.

"That led to a theory that strong magnetic fields prevent jets from forming."

However, the new discovery contradicts that theory.

^{Narrated animation describing the findings and what they mean (English)from ICRARon Vimeo}

The object the scientists studied is called Swift J0243.6+6124 (Sw J0243), and was originally discovered on 3 October 2017, by NASA's orbiting Neil Gehrels Swift Observatory, when the object emitted a burst of X-rays.

The object is a slowly-spinning neutron star pulling material from a companion star that is likely significantly bigger than the Sun. The VLA observations began a week after the Swift discovery and continued until January this year.

Both the fact that the object's emission at X-ray and radio wavelengths weakened together over time and the characteristics of the radio emission itself convinced the astronomers that they were seeing radio waves produced by a jet.

"This combination is what we see in other jet-producing systems. Alternative mechanisms just don't explain it," van den Eijnden said.

Common theories for jet formation in systems like Sw J0243 say the jets are launched by magnetic field lines anchored in the inner parts of the accretion disks.

But in this scenario, if the neutron star has a very strong magnetic field, that field is overpowering and prevents the jet from forming.

"Our clear discovery of a jet in Sw J0243 disproves that longstanding idea," van den Eijnden added.