X-ray pulsar
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X-ray pulsars are a type of astronomical objects which are X-ray sources with a strict periodic variations in intensity. The periods range from as little as a fraction of a second, to as much as several minutes.
X-ray pulsars behave differently from radio pulsars, although it is accepted that both kinds are manifestations of a rotating magnetised neutron star, with the rotation cycle of the neutron star in both cases being identified with the pulse period. The major differences are that radio pulsars have periods in the order of milliseconds, and all radio pulsars are losing angular momentum and slowing down, while several of the X-ray pulsars have been shown to be continuously spinning faster, while others show either little change in period or display erratic spin-down and spin-up behaviour.
The explanation of this difference can be found in the physical nature of the two pulsar classes. Over 99% of radio pulsars are single objects that radiate away their rotational energy in the form of relativistic particles and magnetic dipole radiation, lighting up the nebulae that surround them. In contrast, X-ray pulsars are members of so called "close binary star systems", in which two stars interact via a flow of matter. This matter flow is directed from the larger companion star towards the smaller, degenerate neutron star. The X-ray pulses are thought to be caused by the magnetic field of the neutron star channeling the accreting gas from its companion star, thus producing localized X-ray hot spots that move in and out of view as the neutron star spins. The spin-up (and spin-down) behaviour is thought to be caused by a transfer of angular momentum from the accreting matter as it spirals its way from the surface of the larger companion star onto the surface of the neutron star, like water down a plug hole.
The first X-ray pulsar to be discovered was Cen X-3.