are a class of binary stars which are luminous in X-rays
. They contain a neutron star
and a low-mass companion star.
The companion fills its Roche lobe
and therefore the neutron star is accreting matter from it. The inflowing gas forms an accretion disk
around the neutron star. Sometimes X-ray bursters show a sudden increase in their X-ray luminosity, called X-ray burst
. All properties of the X-ray bursts can be explained assuming that they result from nuclear fusion
on the surface of a neutron star
. The accumulated hydrogen-rich gas forms a layer on the surface. This layer ignites spontaneously as the pressure at the bottom of the layer is high enough.
Luminous X-ray bursts can be considered standard candles, as the mass of neutron star determines the luminosity of the burst. Therefore, comparing the observed X-ray flux to the predicted value, relatively accurate distances can be determined. Observations of X-ray bursts allow also the determination of the radius of the neutron star.