Gravitational waves from neutron star instabilities: What do we actually know?. Nils Andersson Department of Mathematics University of Southampton IAP Paris January 2006. binary merger:
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What do we actually know?
Department of Mathematics
University of Southampton
IAP Paris January 2006
the inspiral chirp provides a clean GW signal carrying information about the system, while the merger phase probes strong field gravity
supernova core collapse:
the birth of a NS may lead to a GW burst
crustal asymmetries lead to GWs at twice the spin frequency
free precession is the most general motion of a solid body (cf. Earth’s “Chandler wobble”)
fast spinning NS may suffer both dynamical bar-mode and secular instabilities(r-modes?)
phase transitions (?):
eg. quark deconfinement as NS spins down; may lead to energy being releasedHow can NS radiate?
In principle, neutron stars are cosmic laboratories of extreme physics. They are expected to be important GW sources, and we hope to be able to probe matter at supranuclear densities.
For rapidly (differentially!) rotating stars with
the “bar mode” grows on a dynamical timescale.
(differential rotation law)
An oscillation is unstable if the star thinks it has “negative energy”.
A “neutral” mode of oscillation signals the onset of CFS instability.
The modes that are currently thought to be the most important are the “acoustic” f-modes, and the “Coriolis driven” r-modes.The CFS instability
Chandrasekhar 1969: Gravitational waves lead to a secular instability
Friedman & Schutz 1978: The instability is generic: for any W modes with sufficiently large m are unstable.
? “negative energy”.The r-modes
The r-modes belong to a large class of “inertial” modes, which are driven unstable by the emission of gravitational waves at all rates of rotation!
The l=m=2 r-mode grows on a timescale of a few tens of seconds, and leads to a stronger instability than the f-modes.
Instability window depends on uncertain core-physics.
Need to account for “exotic” states of matter :
Suppressed by superfluidity
Thought to stabilise the f-modes completely!
runaway “negative energy”.
What is the saturates mechanism and what is the maximum amplitude?
r-modes to small amplitudes, but GWs scenarios are still viable?
Will the mode survive saturation?
What is the backreaction on the star?
Saturation results need to be confirmed by alternative methods, and stratification, superfluidity, magnetic fields etcetera should be included.
A number of other instabilities may affect the dynamical evolution of neutron stars. Whether these instabilities are relevant as GW sources is not clear, but they may play a role, eg. by altering the other scenarios…