A team of astronomers led by Jakob van den Eijnden of the University of Amsterdam was the first ever to use the Very Large Array (VLA) to observe a strongly magnetised neutron star producing jets. Their results are being published online by Nature this week.
Jets are energy-rich plasma streams which are blown into space at high velocity in the direct vicinity of gas absorbing accretors, black holes or neutron stars. It turns out that during the eruption, radio emission behaves in the exact same manner as it does in jets known from other sources, such as black holes or neutron stars with a weak magnetic field.
Never observed before
Last year, the team already published two articles that gave credence to the possibility that neutron stars with jets exist, despite the fact that this is ruled out by present theory. Jets have been known to exist for dozens of years. However, up until now, they had never been observed for neutron stars with strong magnetic fields. General opinion held that the strong magnetic field prevented the formation of plasma streams because the gas was prevented from getting close enough to the neutron star.
Jets are important because they spew vast amounts of material into space, which can have a major impact on the immediate environment. In addition, jets may play a significant role in the evolution of X-ray binary stars. As such, it is crucial to understand the way in which jets are produced and the exact amount of material they blow into space.
PhD candidate Van den Eijnden and colleagues at the Anton Pannekoek Institute for Astronomy (UvA) studying Swift J0243, discovered in October 2017, with the VLA radio telescope in New Mexico and the Swift Observatory. ’The radio spectrum of Swift J0243 is the same as the spectrum found in jets from other sources and changes in the same manner’, according to Van den Eijnden. 'In addition, the radio luminosity follows the luminosity of the incoming gas as is seen elsewhere. So, for the first time ever, we have really observed a jet emitted by a neutron star with a strong magnetic field.'
This means that existing theory, which rules this out, can be ignored. The results are indeed nicely in line with a recently developed model. This predicts the formation of jets in a very different manner, where the magnetic field is not a problem. The discovery opens an entirely new field of research. 'For example, we can now test whether the rotational speed of an object determines the strength of the jets. Many jet models predict this, but up until now there was no strong proof for it', says second author Nathalie Degenaar (UvA).
Van den Eijnden recently won a UvA anniversary grant which he can use for a working visit to the United States. He wants to spend the second half of his PhD research continuing to detect jets of strongly magnetic neutron stars. 'We want to learn how jets work: although we may have disproved current theories, this doesn't mean that we understand how things do work straight away.'
An evolving jet from a strongly-magnetised accreting X-ray pulsar. J. van den Eijnden, N. Degenaar, T.D. Russell, R. Wijnands, J.C.A. Miller-Jones, G.R. Sivakoff, J.V. Hernandez-Santisteban. Online in Nature, 27 september 2018.