15,334 research outputs found
Relativistic Two-stream Instability
We study the (local) propagation of plane waves in a relativistic,
non-dissipative, two-fluid system, allowing for a relative velocity in the
"background" configuration. The main aim is to analyze relativistic two-stream
instability. This instability requires a relative flow -- either across an
interface or when two or more fluids interpenetrate -- and can be triggered,
for example, when one-dimensional plane-waves appear to be left-moving with
respect to one fluid, but right-moving with respect to another. The dispersion
relation of the two-fluid system is studied for different two-fluid equations
of state: (i) the "free" (where there is no direct coupling between the fluid
densities), (ii) coupled, and (iii) entrained (where the fluid momenta are
linear combinations of the velocities) cases are considered in a
frame-independent fashion (eg. no restriction to the rest-frame of either
fluid). As a by-product of our analysis we determine the necessary conditions
for a two-fluid system to be causal and absolutely stable and establish a new
constraint on the entrainment.Comment: 15 pages, 2 eps-figure
Seismology of adolescent neutron stars: Accounting for thermal effects and crust elasticity
We study the oscillations of relativistic stars, incorporating key physics
associated with internal composition, thermal gradients and crust elasticity.
Our aim is to develop a formalism which is able to account for the
state-of-the-art understanding of the complex physics associated with these
systems. As a first step, we build models using a modern equation of state
including composition gradients and density discontinuities associated with
internal phase-transitions (like the crust-core transition and the point where
muons first appear in the core). In order to understand the nature of the
oscillation spectrum, we carry out cooling simulations to provide realistic
snapshots of the temperature distribution in the interior as the star evolves
through adolescence. The associated thermal pressure is incorporated in the
perturbation analysis, and we discuss the presence of -modes arising as a
result of thermal effects. We also consider interface modes due to
phase-transitions and the gradual formation of the star's crust and the
emergence of a set of shear modes.Comment: 27 pages, 14 figure
Buoyancy and g-modes in young superfluid neutron stars
We consider the local dynamics of a realistic neutron star core, including
composition gradients, superfluidity and thermal effects. The main focus is on
the gravity g-modes, which are supported by composition stratification and
thermal gradients. We derive the equations that govern this problem in full
detail, paying particular attention to the input that needs to be provided
through the equation of state and distinguishing between normal and superfluid
regions. The analysis highlights a number of key issues that should be kept in
mind whenever equation of state data is compiled from nuclear physics for use
in neutron star calculations. We provide explicit results for a particular
stellar model and a specific nucleonic equation of state, making use of cooling
simulations to show how the local wave spectrum evolves as the star ages. Our
results show that the composition gradient is effectively dominated by the
muons whenever they are present. When the star cools below the superfluid
transition, the support for g-modes at lower densities (where there are no
muons) is entirely thermal. We confirm the recent suggestion that the g-modes
in this region may be unstable, but our results indicate that this instability
will be weak and would only be present for a brief period of the star's life.
Our analysis accounts for the presence of thermal excitations encoded in
entrainment between the entropy and the superfluid component. Finally, we
discuss the complete spectrum, including the normal sound waves and, in
superfluid regions, the second sound.Comment: 29 pages, 9 figures, submitted to MNRA
Pulsar spin-down: the glitch-dominated rotation of PSR J0537-6910
The young, fast-spinning, X-ray pulsar J0537-6910 displays an extreme glitch
activity, with large spin-ups interrupting its decelerating rotation every ~100
days. We present nearly 13 years of timing data from this pulsar, obtained with
the {\it Rossi X-ray Timing Explorer}. We discovered 22 new glitches and
performed a consistent analysis of all 45 glitches detected in the complete
data span. Our results corroborate the previously reported strong correlation
between glitch spin-up size and the time to the next glitch, a relation that
has not been observed so far in any other pulsar. The spin evolution is
dominated by the glitches, which occur at a rate ~3.5 per year, and the
post-glitch recoveries, which prevail the entire inter-glitch intervals. This
distinctive behaviour provides invaluable insights into the physics of
glitches. The observations can be explained with a multi-component model which
accounts for the dynamics of the neutron superfluid present in the crust and
core of neutron stars. We place limits on the moment of inertia of the
component responsible for the spin-up and, ignoring differential rotation, the
velocity difference it can sustain with the crust. Contrary to its rapid
decrease between glitches, the spin-down rate increased over the 13 years, and
we find the long-term braking index , the only negative
braking index seen in a young pulsar. We briefly discuss the plausible
interpretations of this result, which is in stark contrast to the predictions
of standard models of pulsar spin-down.Comment: Minor changes to match the MNRAS accepted versio
Implications of an r-mode in XTE J1751-305: Mass, radius and spin evolution
Recently Strohmayer and Mahmoodifar presented evidence for a coherent
oscillation in the X-ray light curve of the accreting millisecond pulsar XTE
J1751-305, using data taken by RXTE during the 2002 outburst of this source.
They noted that a possible explanation includes the excitation of a non-radial
oscillation mode of the neutron star, either in the form of a g-mode or an
r-mode. The r-mode interpretation has connections with proposed spin-evolution
scenarios for systems such as XTE J1751-305. Here we examine in detail this
interesting possible interpretation. Using the ratio of the observed
oscillation frequency to the star's spin frequency, we derive an approximate
neutron star mass-radius relation which yields reasonable values for the mass
over the range of expected stellar radius (as constrained by observations of
radius-expansion burst sources). However, we argue that the large mode
amplitude suggested by the Strohmayer and Mahmoodifar analysis would inevitably
lead to a large spin-down of the star, inconsistent with its observed spin
evolution, regardless of whether the r-mode itself is in a stable or unstable
regime. We therefore conclude that the r-mode interpretation of the observed
oscillation is not consistent with our current understanding of neutron star
dynamics and must be considered unlikely. Finally we note that, subject to the
availability of a sufficiently accurate timing model, a direct
gravitational-wave search may be able to confirm or reject an r-mode
interpretation unambiguously, should such an event, with a similar inferred
mode amplitude, recur during the Advanced detector era.Comment: 8 pages, 3 figures; submitted to MNRA
Verifying black hole orbits with gravitational spectroscopy
Gravitational waves from test masses bound to geodesic orbits of rotating
black holes are simulated, using Teukolsky's black hole perturbation formalism,
for about ten thousand generic orbital configurations. Each binary radiates
power exclusively in modes with frequencies that are
integer-linear-combinations of the orbit's three fundamental frequencies. The
following general spectral properties are found with a survey of orbits: (i)
99% of the radiated power is typically carried by a few hundred modes, and at
most by about a thousand modes, (ii) the dominant frequencies can be grouped
into a small number of families defined by fixing two of the three integer
frequency multipliers, and (iii) the specifics of these trends can be
qualitatively inferred from the geometry of the orbit under consideration.
Detections using triperiodic analytic templates modeled on these general
properties would constitute a verification of radiation from an adiabatic
sequence of black hole orbits and would recover the evolution of the
fundamental orbital frequencies. In an analogy with ordinary spectroscopy, this
would compare to observing the Bohr model's atomic hydrogen spectrum without
being able to rule out alternative atomic theories or nuclei. The suitability
of such a detection technique is demonstrated using snapshots computed at
12-hour intervals throughout the last three years before merger of a kludged
inspiral. Because of circularization, the number of excited modes decreases as
the binary evolves. A hypothetical detection algorithm that tracks mode
families dominating the first 12 hours of the inspiral would capture 98% of the
total power over the remaining three years.Comment: 18 pages, expanded section on detection algorithms and made minor
edits. Final published versio
R-mode oscillations and rocket effect in rotating superfluid neutron stars. I. Formalism
We derive the hydrodynamical equations of r-mode oscillations in neutron
stars in presence of a novel damping mechanism related to particle number
changing processes. The change in the number densities of the various species
leads to new dissipative terms in the equations which are responsible of the
{\it rocket effect}. We employ a two-fluid model, with one fluid consisting of
the charged components, while the second fluid consists of superfluid neutrons.
We consider two different kind of r-mode oscillations, one associated with
comoving displacements, and the second one associated with countermoving, out
of phase, displacements.Comment: 10 page
The enigmatic spin evolution of PSR J0537-6910: r-modes, gravitational waves and the case for continued timing
We discuss the unique spin evolution of the young X-ray pulsar PSR
J0537-6910, a system in which the regular spin down is interrupted by glitches
every few months. Drawing on the complete timing data from the Rossi X-ray
Timing Explorer (RXTE, from 1999-2011), we argue that a trend in the
inter-glitch behaviour points to an effective braking index close to ,
much larger than expected. This value is interesting because it would accord
with the neutron star spinning down due to gravitational waves from an unstable
r-mode. We discuss to what extent this, admittedly speculative, scenario may be
consistent and if the associated gravitational-wave signal would be within
reach of ground based detectors. Our estimates suggest that one may, indeed, be
able to use future observations to test the idea. Further precision timing
would help enhance the achievable sensitivity and we advocate a joint observing
campaign between the Neutron Star Interior Composition ExploreR (NICER) and the
LIGO-Virgo network.Comment: 10 pages, 4 figures, emulate ApJ forma
Gravitational-wave astronomy: the high-frequency window
This contribution is divided in two parts. The first part provides a
text-book level introduction to gravitational radiation. The key concepts
required for a discussion of gravitational-wave physics are introduced. In
particular, the quadrupole formula is applied to the anticipated
``bread-and-butter'' source for detectors like LIGO, GEO600, EGO and TAMA300:
inspiralling compact binaries. The second part provides a brief review of high
frequency gravitational waves. In the frequency range above (say) 100Hz,
gravitational collapse, rotational instabilities and oscillations of the
remnant compact objects are potentially important sources of gravitational
waves. Significant and unique information concerning the various stages of
collapse, the evolution of protoneutron stars and the details of the
supranuclear equation of state of such objects can be drawn from careful study
of the gravitational-wave signal. As the amount of exciting physics one may be
able to study via the detections of gravitational waves from these sources is
truly inspiring, there is strong motivation for the development of future
generations of ground based detectors sensitive in the range from hundreds of
Hz to several kHz.Comment: 21 pages, 5 figures, Lectures presented at the 2nd Aegean Summer
School on the Early Universe, Syros, Greece, September 200
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