Neutron star spin up is the name given to the increase in rotational speed over time first noted in
Cen X-3
Centaurus X-3 (4U 1118-60) is an X-ray pulsar with a period of 4.84 seconds. It was the first X-ray pulsar to be discovered, and the third X-ray source to be discovered in the constellation Centaurus. The system consists of a neutron star ...
and
Her X-1 but now observed in other
X-ray pulsar
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 m ...
s. In the case of Cen X-3, the pulse period is decreasing over a timescale of 3400 years (defined as
, where
is the rotation period and
is the rate of change in the rotation period).
Ever since the detection of the first
millisecond pulsar
A millisecond pulsar (MSP) is a pulsar with a rotational period less than about 10 milliseconds. Millisecond pulsars have been detected in radio, X-ray, and gamma ray portions of the electromagnetic spectrum. The leading theory for the origin of ...
(MSP), it has been theorized that MSPs are
neutron star
A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. w ...
s that have been spun up by
accretion in a close
binary system
A binary system is a system of two astronomical bodies which are close enough that their gravitational attraction causes them to orbit each other around a barycenter ''(also see animated examples)''. More restrictive definitions require that th ...
.
The change in rotational period of the neutron star comes from the transition region between the magnetosphere and the
plasma flow from the
companion star
A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in w ...
. In this context the magnetosphere is defined as the region of space surrounding the neutron star, in which the
magnetic field determines the motion of the plasma. Inside the magnetic field, the plasma will eventually co-rotate with the neutron star while in the transition region,
angular momentum
In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...
from the
accretion disk will be transferred via the magnetic field to the neutron star, leading to the spin-up.
See also
Stellar spin-down
References
Neutron stars
Stellar evolution
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