359 research outputs found
Zoom and whirl: Eccentric equatorial orbits around spinning black holes and their evolution under gravitational radiation reaction
We study eccentric equatorial orbits of a test-body around a Kerr black hole under the influence of gravitational radiation reaction. We have adopted a well established two-step approach: assuming that the particle is moving along a geodesic (justifiable as long as the orbital evolution is adiabatic) we calculate numerically the fluxes of energy and angular momentum radiated to infinity and to the black hole horizon, via the Teukolsky-Sasaki-Nakamura formalism. We can then infer the rate of change of orbital energy and angular momentum and thus the evolution of the orbit. The orbits are fully described by a semilatus rectum p and an eccentricity e. We find that while, during the inspiral, e decreases until shortly before the orbit reaches the separatrix of stable bound orbits [which is defined by p(s)(e)], in many astrophysically relevant cases the eccentricity will still be significant in the last stages of the inspiral. In addition, when a critical value p(crit)(e) is reached, the eccentricity begins to increase as a result of continued radiation induced inspiral. The two values p(s), p(crit) (for given e) move closer to each other, in coordinate terms, as the black hole spin is increased, as they do also for fixed spin and increasing eccentricity. Of particular interest are moderate and high eccentricity orbits around rapidly spinning black holes, with p(e)approximate top(s)(e). We call these "zoom-whirl" orbits, because of their characteristic behavior involving several revolutions around the central body near periastron. Gravitational waveforms produced by such orbits are calculated and shown to have a very particular signature. Such signals may well prove of considerable astrophysical importance for the future Laser Interferometer Space Antenna detector
How well can ultracompact bodies imitate black hole ringdowns?
The ongoing observations of merging black holes by the instruments of the
fledging gravitational wave astronomy has opened the way for testing the
general relativistic Kerr black hole metric and, at the same time, for probing
the existence of more speculative horizonless ultracompact objects. In this
paper we quantify the difference that these two classes of objects may exhibit
in the post-merger ringdown signal. By considering rotating systems in general
relativity and assuming an eikonal limit and a third-order Hartle-Thorne slow
rotation approximation, we provide the first calculation of the early ringdown
frequency and damping time as a function of the body's multipolar structure.
Using the example of a gravastar, we show that the main ringdown signal may
differ as much as a few percent with respect to that of a Kerr black hole, a
deviation that could be probed by near future Advanced LIGO/Virgo searches.Comment: 6 pages, 1 figure, some additional discussion in the text and some
modifications in the figure to indicate the accuracy of the approach.
Accepted for publication as a Rapid Communication in Physical Review
Persistent crust-core spin lag in neutron stars
It is commonly believed that the magnetic field threading a neutron star
provides the ultimate mechanism (on top of fluid viscosity) for enforcing
long-term corotation between the slowly spun down solid crust and the liquid
core. We show that this argument fails for axisymmetric magnetic fields with
closed field lines in the core, the commonly used `twisted torus' field being
the most prominent example. The failure of such magnetic fields to enforce
global crust-core corotation leads to the development of a persistent spin lag
between the core region occupied by the closed field lines and the rest of the
crust and core. We discuss the repercussions of this spin lag for the evolution
of the magnetic field, suggesting that, in order for a neutron star to settle
to a stable state of crust-core corotation, the bulk of the toroidal field
component should be deposited into the crust soon after the neutron star's
birth.Comment: 17 pages, 1 figure; v2: minor corrections, matches the version to
appear in MNRA
Three evolutionary paths for magnetar oscillations
Quasi-periodic oscillations have been seen in the light curves following
several magnetar giant flares. These oscillations are of great interest as they
probably provide our first ever view of the normal modes of oscillation of
neutron stars. The state-of-the-art lies in the study of the oscillations of
elastic-magnetic stellar models, mainly with a view to relating the observed
frequencies to the structure and composition of the star itself. We advance
this programme by considering several new physical mechanisms that are likely
to be important for magnetar oscillations. These relate to the
superfluid/superconducting nature of the stellar interior, and the damping of
the modes, both through internal dissipation mechanisms and the launching of
waves into the magnetosphere. We make simple order-of-magnitude estimates to
show that both the frequencies and the damping time of magnetar oscillations
can evolve in time, identifying three distinct `pathways' that can be followed,
depending upon the initial magnitude of the mode excitation. These results are
interesting as they show that the information buried in magnetar QPOs may be
even richer than previously thought, and motivate more careful examination of
magnetar light curves, to search for signatures of the different types of
evolution that we have identified.Comment: To appear in MNRAS. This version reflects changes made in response to
referee's comments, mainly extra discussion in Section 2.
Superfluid instability of r-modes in "differentially rotating" neutron stars
Superfluid hydrodynamics affects the spin-evolution of mature neutron stars,
and may be key to explaining timing irregularities such as pulsar glitches.
However, most models for this phenomenon exclude the global instability
required to trigger the event. In this paper we discuss a mechanism that may
fill this gap. We establish that small scale inertial r-modes become unstable
in a superfluid neutron star that exhibits a rotational lag, expected to build
up due to vortex pinning as the star spins down. Somewhat counterintuitively,
this instability arises due to the (under normal circumstances dissipative)
vortex-mediated mutual friction. We explore the nature of the superfluid
instability for a simple incompressible model, allowing for entrainment
coupling between the two fluid components. Our results recover a previously
discussed dynamical instability in systems where the two components are
strongly coupled. In addition, we demonstrate for the first time that the
system is secularly unstable (with a growth time that scales with the mutual
friction) throughout much of parameter space. Interestingly, large scale
r-modes are also affected by this new aspect of the instability. We analyse the
damping effect of shear viscosity, which should be particularly efficient at
small scales, arguing that it will not be sufficient to completely suppress the
instability in astrophysical systems.Comment: RevTex, 11 figure
Lagrangian perturbation theory for a superfluid immersed in an elastic neutron star crust
The inner crust of mature neutron stars, where an elastic lattice of
neutron-rich nuclei coexists with a neutron superfluid, impacts on a range of
astrophysical phenomena. The presence of the superfluid is key to our
understanding of pulsar glitches, and is expected to affect the thermal
conductivity and hence the evolution of the surface temperature. The coupling
between crust and superfluid must also be accounted for in studies of neutron
star dynamics, discussions of global oscillations and associated instabilities.
In this paper we develop Lagrangian perturbation theory for this problem,
paying attention to key issues like superfluid entrainment, potential vortex
pinning, dissipative mutual friction and the star's magnetic field. We also
discuss the nature of the core-crust interface. The results provide a
theoretical foundation for a range of interesting astrophysical applications.Comment: 13 pages, no figures, to appear in MNRA
- âŚ