X-ray Pulsar

X-ray 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.

An X-ray pulsar consists of a magnetized neutron star in orbit with a normal stellar companion and is a type of binary star system. The magnetic-field strength at the surface of the neutron star is typically about 108 Tesla, over a trillion times stronger than the strength of the magnetic field measured at the surface of the Earth.

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 about one square kilometer in area, can be ten thousand times, or more, as luminous than 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.

The gas that supplies the X-ray pulsar can reach the neutron star by a variety of ways that depend on the size and shape of the neutron star’s orbital path and the nature of the companion star.

Some companion stars of X-ray pulsars are very massive young stars, usually OB supergiants, that emit a radiation driven stellar wind from their surface. The neutron star is immersed in the wind and continuously captures gas that flows nearby. Vela X-1 is an example of this kind of system.

In other systems, the neutron star orbits so closely to its companion that its strong gravitational force can pull material from the companion’s atmosphere into an orbit around itself, a mass transfer process known as Roche lobe overflow. The captured material forms a gaseous accretion disc and spirals inwards to ultimately fall onto the neutron star as in the binary system Cen X-3.

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.

X-ray pulsars are observed using X-ray telescopes that are satellites in low Earth orbit although some observations have been made, mostly in the early years of X-ray astronomy, using detectors carried by balloons or sounding rockets. The first X-ray pulsar to be discovered was Centaurus X-3, in 1971 with the Uhuru X-ray satellite.

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