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 thermo-nuclear 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 BeppoSAX/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.