The observers' view of (very) long X-ray bursts: they are super!

In many X-ray point sources on the sky, the X-ray emission arises because hydrogen and/or helium is accreted onto a neutron star from a nearby donor star. When this matter settles on the neutron star surface, it will undergo nuclear fusion. For a large range of physical parameters the fusion is unstable. The resulting thermonuclear explosions last from seconds to minutes. They are observed as short flares in X-rays and are called 'type I X-ray bursts'. Recently, hours-long X-ray flares have been found in seven X-ray burst sources with the Beppo-SAX/WFC, RXTE/ASM and RXTE/PCA. They have similar properties to the usual X-ray bursts, except they last for two or three orders of magnitude longer (hence they are referred to as 'superbursts'). This can not be understood in the context of the standard nuclear-fusion picture mentioned above. Instead, the superbursts are thought to be related to the unstable burning of the leftovers from the hydrogen and/or helium fusion. I will discuss the observational properties of these superbursts.