Artist's representation of an X-ray nova. The compact object on the right – a neutron star or a black hole – accretes gas from a normal companion star. The gas swirls in a disk around the compact object at very high velocity close to the speed of light from where it emits X-rays. Credit: ESA.
An X-ray nova is a short-lived X-ray source that appears suddenly in the sky and dramatically increases in strength over a period of a few days and then decreases, with an overall lifetime of a few months; it may have an optical counterpart. Unlike a conventional nova, in which the compact component is a white dwarf, an X-ray nova may be caused by material falling onto a neutron star or a black hole.