A glitch is a sudden increase (around 1 part in 10
6) in the rotational
frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
of a
rotation-powered pulsar
A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...
, which usually decreases steadily due to braking provided by the emission of radiation and high-energy particles. It is unknown whether they are related to the
timing noise
A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...
which all pulsars exhibit. Following a glitch is a period of gradual recovery where the observed
periodicity
Periodicity or periodic may refer to:
Mathematics
* Bott periodicity theorem, addresses Bott periodicity: a modulo-8 recurrence relation in the homotopy groups of classical groups
* Periodic function, a function whose output contains values tha ...
slows to a period close to that observed before the glitch. These gradual recovery periods have been observed to last from days to years. Currently, only multiple glitches of the
Crab
Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all the ...
and
Vela pulsars have been observed and studied extensively.
Cause
While the exact cause of glitches is unknown, they are thought to be caused by an internal process within the pulsar. This differs from the steady decrease in the star's rotational frequency which is caused by external processes. Although the details of the glitch process are unknown, it is thought that the resulting increase in the pulsar's rotational frequency is caused by a brief coupling of the pulsar's faster-spinning
superfluid
Superfluidity is the characteristic property of a fluid with zero viscosity which therefore flows without any loss of kinetic energy. When stirred, a superfluid forms vortices that continue to rotate indefinitely. Superfluidity occurs in two ...
core to the crust, which are usually decoupled. This brief coupling transfers angular momentum from core to the surface, which causes a decrease in the measured period. It is thought that the coupling could be caused by a breaking of the pulsar's magnetic dipole, which would apply a torque to the crust, causing a brief coupling between the two parts.
Implications
Assuming that the mechanism described above is correct, observed pulsar glitches set a limit on the
moment of inertia
The moment of inertia, otherwise known as the mass moment of inertia, angular mass, second moment of mass, or most accurately, rotational inertia, of a rigid body is a quantity that determines the torque needed for a desired angular acceler ...
of the pulsar being observed and, thus, the mass-radius relation possible in dense nuclear matter. From extrapolating from a linear fit to the angular momentum transfer implied by the glitches observed in the Vela and Crab Pulsars, a so-called ''causality limit'' can be placed on the mass-radius relation of approximately
.
See also
*
Anti-glitch
References
*
* https://web.archive.org/web/20051018233130/http://www.saao.ac.za/~wgssa/as4/urama.html http://www.saao.ac.za/~wgssa/as4/urama.html
*
{{Neutron star
Rotation-powered pulsars