X-ray pulsars or accretion-powered pulsars are a class of astronomical objects that are X-ray sources displaying strict periodic variations in X-ray intensity. The X-ray periods range from as little as a fraction of a second to as much as several minutes.
Characteristics
An X-ray pulsar is a type of binary star system consisting of a typical star (stellar companion) in orbit around a magnetized neutron star. The magnetic field strength at the surface of the neutron star is typically about 10<sup>8</sup> tesla, over a trillion times stronger than the strength of the magnetic field measured at the surface of the Earth (60 μT).
Gas is accreted from the stellar companion and is channeled by the neutron star's magnetic field on to the magnetic poles producing two or more localized X-ray hot spots, similar to the two auroral zones on Earth, but far hotter. At these hotspots the infalling gas can reach half the speed of light before it impacts the neutron star surface. So much gravitational potential energy is released by the infalling gas, that the hotspots, which are estimated to be about one square kilometer in area, can be ten thousand times, or more, as luminous as the Sun.
Temperatures of millions of degrees are produced so the hotspots emit mostly X-rays. As the neutron star rotates, pulses of X-rays are observed as the hotspots move in and out of view if the magnetic axis is tilted with respect to the spin axis.
Spin behaviors
Radio pulsars (rotation-powered pulsars) and X-ray pulsars exhibit very different spin behaviors and have different mechanisms producing their characteristic pulses although it is accepted that both kinds of pulsar are manifestations of a rotating magnetized neutron star. The rotation cycle of the neutron star in both cases is identified with the pulse period.
The major differences are that radio pulsars have periods on the order of milliseconds to seconds, and all radio pulsars are losing angular momentum and slowing down. In contrast, the X-ray pulsars exhibit a variety of spin behaviors. Some X-ray pulsars are observed to be continuously spinning faster and faster or slower and slower (with occasional reversals in these trends) while others show either little change in pulse period or display erratic spin-down and spin-up behavior.