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 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 (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 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.
== 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.
== 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.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 any
nearby nebulae that surround them.
In contrast, X-ray pulsars are members of
binary star systems and accrete matter from
either stellar winds or accretion disks.
The accreted matter transfers angular momentum
to (or from) the neutron star causing the
spin rate to increase or decrease at rates
that are often hundreds of times faster than
the typical spin down rate in radio pulsars.
Exactly why the X-ray pulsars show such varied
spin behavior is still not clearly understood.
== Observations ==
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.
== See also ==
Neutron star
Pulsar
Radio pulsar
Anomalous X-ray pulsar
Magnetar
Millisecond pulsar
Pulsar planets
List of X-ray pulsars
== References ==
== External links ==
BATSE Pulsar Studies
Cain/Gay - Astronomy Cast.
Pulsars - Nov 2009
