Mon. Not. R. Astron. Soc. 466, L98–L102 (2017)

Neutron star binaries are known for their X-ray bursts. Most of the time these are well-behaved, regular flashes, or type-I bursts. But there are a couple of anomalies. One is the binary system known as Rapid Burster, which also displays sudden, erratic flashes: type-II bursts. Rapid Burster was discovered in the 1970s, but the origin of its type-II bursts is still poorly understood. But now, using simultaneous observations from three space X-ray telescopes to study Rapid Burster, Jakob van den Eijnden and colleagues have likely found the culprit.

The neutron star continuously feeds on the gas from its accretion disk, burping out type-I flashes. But as van den Eijnden et al. discovered, there is a hole in the middle of Rapid Burster's accretion disk. The gap, with a radius of 87 km, is created by the star's extremely strong magnetic field and keeps the gas at bay. So even though both the star and its disk are spinning, the gas accumulates at the inner edge. Then, when it has gained sufficient speed, the gas suddenly breaks through into the star, creating a spectacular burst of X-rays.