Gas Supply
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 (see stellar classification), 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.
For still other types of X-ray pulsars, the companion star is a Be star that rotates very rapidly and apparently sheds a disk of gas around its equator. The orbits of the neutron star with these companions are usually large and very elliptical in shape. When the neutron star passes nearby or through the Be circumstellar disk, it will capture material and temporarily become an X-ray pulsar. The circumstellar disk around the Be star expands and contracts for unknown reasons, so these are transient X-ray pulsars that are observed only intermittently, often with months to years between episodes of observable X-ray pulsation.
Read more about this topic: X-ray Pulsar
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