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A team of Dutch and English astronomers, including Jakob van den Eijnden of the University of Amsterdam, has discovered why the neutron star Rapid Burster exhibits such irregular bursts. The gas the neutron star attracts from its companion star is first blocked by the neutron star’s magnetic field. Only when too much gas has accreted does this magnetic field yield, allowing the gas to flow to the neutron star and resulting in a burst.

Jakob van den Eijnden and fellow astronomers investigated the origin of so-called type II X-ray bursts in the Rapid Burster. The Rapid Burster is a neutron star discovered in 1976 that attracts gas from an orbiting star. What is special about the Rapid Burster is the fact it exhibits type II bursts in addition to the more common type I bursts. Type II bursts are considerably more erratic and intense because they occur as a series of shocks. Until now the reasons for a type II burst occurring in the first place were not understood.

Possible explanation

The team investigated the possible explanation that the neuron star’s magnetic field blocks the gas, causing it to accrete. A burst would only follow once so much gas had accreted that the magnetic field was no longer strong enough to block it, in which case the gas would flow towards the neutron star in a single shock. The astronomers thought that if the magnetic field is in fact blocking the gas, it should be possible to observe a gaseous disc orbiting the neutron star at some distance.

Observations made with the X-ray satellites NuSTAR, XMM-Newton and Swift allowed them to infer there must be a gap measuring about ninety kilometres between the neutron star and the gaseous cloud. This seems an easily bridged gap, but a neutron star only takes up ten kilometres and the magnetic field is extremely strong. Consequently, the researchers suspect it is the magnetic field that explains the type II bursts.

Irregular burst of gas bij neutron star Rapid Burster. Image: J. Poutanen

Lead author

The findings will be published in a forthcoming issue of Monthly Notices of the Royal Astronomical Society. Doctoral researcher Jakob van den Eijnden is the article’s lead author: ‘The fact I already have such neat results is thanks in part to one of my predecessors, Tullio Bagnoli, having submitted a request for observations with no fewer than three X-ray satellites. His requests were granted, and so I got off to a flying start.’

In subsequent studies the astronomers want to investigate what happens to the gaseous disc at the time of a burst. They also want to measure the neutron star’s rotational speed and find out more about the companion star orbiting the neutron star.