Comets are cold bodies consisting of frozen gases and boulders.
X-rays are normally emitted only from very hot gas. Since then, X-rays from other comets have been detected, but their cause was a mystery. They show that the radiation comes from highly ionized atoms, e.g. oxygen atoms which have lost six electrons. Such ions are present in the solar wind. These ions remove electrons from the atoms in the gaseous envelope of the comet when the latter collide with one another.