9 research outputs found
Measuring the angular momentum distribution in core-collapse supernova progenitors with gravitational waves
Stability of general-relativistic accretion disks
Self-gravitating relativistic disks around black holes can form as transient
structures in a number of astrophysical scenarios such as binary neutron star
and black hole-neutron star coalescences, as well as the core-collapse of
massive stars. We explore the stability of such disks against runaway and
non-axisymmetric instabilities using three-dimensional hydrodynamics
simulations in full general relativity using the THOR code. We model the disk
matter using the ideal fluid approximation with a -law equation of
state with . We explore three disk models around non-rotating black
holes with disk-to-black hole mass ratios of 0.24, 0.17 and 0.11. Due to metric
blending in our initial data, all of our initial models contain an initial
axisymmetric perturbation which induces radial disk oscillations. Despite these
oscillations, our models do not develop the runaway instability during the
first several orbital periods. Instead, all of the models develop unstable
non-axisymmetric modes on a dynamical timescale. We observe two distinct types
of instabilities: the Papaloizou-Pringle and the so-called intermediate type
instabilities. The development of the non-axisymmetric mode with azimuthal
number m = 1 is accompanied by an outspiraling motion of the black hole, which
significantly amplifies the growth rate of the m = 1 mode in some cases.
Overall, our simulations show that the properties of the unstable
non-axisymmetric modes in our disk models are qualitatively similar to those in
Newtonian theory.Comment: 30 pages, 21 figure
The Magnetosphere of Oscillating Neutron Stars in General Relativity
Just as a rotating magnetised neutron star has material pulled away from its
surface to populate a magnetosphere, a similar process can occur as a result of
neutron-star pulsations rather than rotation. This is of interest in connection
with the overall study of neutron star oscillation modes but with a particular
focus on the situation for magnetars. Following a previous Newtonian analysis
of the production of a force-free magnetosphere in this way Timokhin et al.
(2000), we present here a corresponding general-relativistic analysis. We give
a derivation of the general relativistic Maxwell equations for small-amplitude
arbitrary oscillations of a non-rotating neutron star with a generic magnetic
field and show that these can be solved analytically under the assumption of
low current density in the magnetosphere. We apply our formalism to toroidal
oscillations of a neutron star with a dipole magnetic field and find that the
low current density approximation is valid for at least half of the oscillation
modes, similarly to the Newtonian case. Using an improved formula for the
determination of the last closed field line, we calculate the energy losses
resulting from toroidal stellar oscillations for all of the modes for which the
size of the polar cap is small. We find that general relativistic effects lead
to shrinking of the size of the polar cap and an increase in the energy density
of the outflowing plasma. These effects act in opposite directions but the net
result is that the energy loss from the neutron star is significantly smaller
than suggested by the Newtonian treatment.Comment: Accepted for publication in MNRAS, 21 pages, 4 figure