3,111 research outputs found
Optimizing Pulsar Timing Arrays to Maximize Gravitational Wave Single Source Detection: a First Cut
Pulsar Timing Arrays (PTAs) use high accuracy timing of a collection of low
timing noise pulsars to search for gravitational waves in the microhertz to
nanohertz frequency band. The sensitivity of such a PTA depends on (a) the
direction of the gravitational wave source, (b) the timing accuracy of the
pulsars in the array and (c) how the available observing time is allocated
among those pulsars. Here, we present a simple way to calculate the sensitivity
of the PTA as a function of direction of a single GW source, based only on the
location and root-mean-square residual of the pulsars in the array. We use this
calculation to suggest future strategies for the current North American
Nanohertz Observatory for Gravitational Waves (NANOGrav) PTA in its goal of
detecting single GW sources. We also investigate the affects of an additional
pulsar on the array sensitivity, with the goal of suggesting where PTA pulsar
searches might be best directed. We demonstrate that, in the case of single GW
sources, if we are interested in maximizing the volume of space to which PTAs
are sensitive, there exists a slight advantage to finding a new pulsar near
where the array is already most sensitive. Further, the study suggests that
more observing time should be dedicated to the already low noise pulsars in
order to have the greatest positive effect on the PTA sensitivity. We have made
a web-based sensitivity mapping tool available at http://gwastro.psu.edu/ptasm.Comment: 14 pages, 3 figures, accepted by Ap
Relativistic r-modes in Slowly Rotating Neutron Stars: Numerical Analysis in the Cowling Approximation
We investigate the properties of relativistic -modes of slowly rotating
neutron stars by using a relativistic version of the Cowling approximation. In
our formalism, we take into account the influence of the Coriolis like force on
the stellar oscillations, but ignore the effects of the centrifugal like force.
For three neutron star models, we calculated the fundamental -modes with
and 3. We found that the oscillation frequency of the
fundamental -mode is in a good approximation given by , where is defined in the corotating frame at the
spatial infinity, and is the angular frequency of rotation of the
star. The proportional coefficient is only weakly dependent on
, but it strongly depends on the relativistic parameter ,
where and are the mass and the radius of the star. All the fundamental
-modes with computed in this study are discrete modes with distinct
regular eigenfunctions, and they all fall in the continuous part of the
frequency spectrum associated with Kojima's equation (Kojima 1998). These
relativistic -modes are obtained by including the effects of rotation higher
than the first order of so that the buoyant force plays a role, the
situation of which is quite similar to that for the Newtonian -modes.Comment: 22 pages, 8 figures, accepted for publication in Ap
Binary inspiral, gravitational radiation, and cosmology
Observations of binary inspiral in a single interferometric gravitational
wave detector can be cataloged according to signal-to-noise ratio and
chirp mass . The distribution of events in a catalog composed of
observations with greater than a threshold depends on the
Hubble expansion, deceleration parameter, and cosmological constant, as well as
the distribution of component masses in binary systems and evolutionary
effects. In this paper I find general expressions, valid in any homogeneous and
isotropic cosmological model, for the distribution with and of
cataloged events; I also evaluate these distributions explicitly for relevant
matter-dominated Friedmann-Robertson-Walker models and simple models of the
neutron star mass distribution. In matter dominated Friedmann-Robertson-Walker
cosmological models advanced LIGO detectors will observe binary neutron star
inspiral events with from distances not exceeding approximately
, corresponding to redshifts of (0.26) for
(), at an estimated rate of 1 per week. As the binary system mass
increases so does the distance it can be seen, up to a limit: in a matter
dominated Einstein-deSitter cosmological model with () that limit
is approximately (1.7) for binaries consisting of two
black holes. Cosmological tests based on catalogs of the
kind discussed here depend on the distribution of cataloged events with
and . The distributions found here will play a pivotal role in testing
cosmological models against our own universe and in constructing templates for
the detection of cosmological inspiraling binary neutron stars and black holes.Comment: REVTeX, 38 pages, 9 (encapsulated) postscript figures, uses epsf.st
A First Comparison of SLOPE and Other LIGO Burst Event Trigger Generators
A number of different methods have been proposed to identify unanticipated
burst sources of gravitational waves in data arising from LIGO and other
gravitational wave detectors. When confronted with such a wide variety of
methods one is moved to ask if they are all necessary, i.e. given detector data
that is assumed to have no gravitational wave signals present, do they
generally identify the same events with the same efficiency, or do they each
'see' different things in the detector? Here we consider three different
methods, which have been used within the LIGO Scientific Collaboration as part
of its search for unanticipated gravitational wave bursts. We find that each of
these three different methods developed for identifying candidate gravitational
wave burst sources are, in fact, attuned to significantly different features in
detector data, suggesting that they may provide largely independent lists of
candidate gravitational wave burst events.Comment: 10 Pages, 5 Figures, Presented at the 10th Gravitational Wave Data
Analysis Workshop (GWDAW-10), 14-17 December 2005 at the University of Texas,
Brownsvill
Crustal Oscillations of Slowly Rotating Relativistic Stars
We study low-amplitude crustal oscillations of slowly rotating relativistic
stars consisting of a central fluid core and an outer thin solid crust. We
estimate the effect of rotation on the torsional toroidal modes and on the
interfacial and shear spheroidal modes. The results compared against the
Newtonian ones for wide range of neutron star models and equations of state.Comment: 15 page
The Liquid-Gas Phase Transitions in a Multicomponent Nuclear System with Coulomb and Surface Effects
The liquid-gas phase transition is studied in a multi-component nuclear
system using a local Skyrme interaction with Coulomb and surface effects. Some
features are qualitatively the same as the results of Muller and Serot which
uses relativistic mean field without Coulomb and surface effects. Surface
tension brings the coexistance binodal surface to lower pressure. The Coulomb
interaction makes the binodal surface smaller and cause another pair of binodal
points at low pressure and large proton fraction with less protons in liquid
phase and more protons in gas phase.Comment: 20 pages including 7 postscript figure
Identification of a Small Regulatory RNA UspS Associated with the Universal Stress Protein in Lactobacillus Species
The gut microbiome is a complex habitat with many bacterial species, each playing crucial roles in regulating various physiological processes in the body. As the use of probiotics to combat human disease continues to increase, it is important to understand the mechanisms by which probiotic bacteria regulate their interactions with other bacteria and their host. Our exploration of the physiological functions of probiotic bacteria hopes to elucidate the role of small regulatory RNA (sRNA) in regulating gene expression within the microbiome. The goal of this project was to characterize the structure and function of the sRNA, UspS, which is found in probiotic, lactic acid bacteria. In Lactobacillus, UspS is closely associated with a downstream universal stress protein and contains an orphaned Lacto-usp RNA motif of unknown function. Computational methods have been used to study the UspS sRNA sequences from two Lactobacillus species in order to predict the secondary structures, generate 3D models, and search for potential mRNA interactions. Comparative sequence alignments and covariance analysis within the secondary structures predict a pseudoknot structure. The UspS sequence was isolated from two Lactobacillus species and sRNAs were synthesized by in vitro transcription with a T7 RNA polymerase. In preliminary studies, differential scanning fluorimetry of the UspS sRNA was able to confirm the presence of stable secondary structures. Future work will be focused on the structure of the pseudoknot region of UspS and its role in regulating the expression of the downstream universal stress protein
Aperture synthesis for gravitational-wave data analysis: Deterministic Sources
Gravitational wave detectors now under construction are sensitive to the
phase of the incident gravitational waves. Correspondingly, the signals from
the different detectors can be combined, in the analysis, to simulate a single
detector of greater amplitude and directional sensitivity: in short, aperture
synthesis. Here we consider the problem of aperture synthesis in the special
case of a search for a source whose waveform is known in detail: \textit{e.g.,}
compact binary inspiral. We derive the likelihood function for joint output of
several detectors as a function of the parameters that describe the signal and
find the optimal matched filter for the detection of the known signal. Our
results allow for the presence of noise that is correlated between the several
detectors. While their derivation is specialized to the case of Gaussian noise
we show that the results obtained are, in fact, appropriate in a well-defined,
information-theoretic sense even when the noise is non-Gaussian in character.
The analysis described here stands in distinction to ``coincidence
analyses'', wherein the data from each of several detectors is studied in
isolation to produce a list of candidate events, which are then compared to
search for coincidences that might indicate common origin in a gravitational
wave signal. We compare these two analyses --- optimal filtering and
coincidence --- in a series of numerical examples, showing that the optimal
filtering analysis always yields a greater detection efficiency for given false
alarm rate, even when the detector noise is strongly non-Gaussian.Comment: 39 pages, 4 figures, submitted to Phys. Rev.
Unstable Nonradial Oscillations on Helium Burning Neutron Stars
Material accreted onto a neutron star can stably burn in steady state only
when the accretion rate is high (typically super-Eddington) or if a large flux
from the neutron star crust permeates the outer atmosphere. For such situations
we have analyzed the stability of nonradial oscillations, finding one unstable
mode for pure helium accretion. This is a shallow surface wave which resides in
the helium atmosphere above the heavier ashes of the ocean. It is excited by
the increase in the nuclear reaction rate during the oscillations, and it grows
on the timescale of a second. For a slowly rotating star, this mode has a
frequency of approximately 20-30 Hz (for l=1), and we calculate the full
spectrum that a rapidly rotating (>>30 Hz) neutron star would support. The
short period X-ray binary 4U 1820--30 is accreting helium rich material and is
the system most likely to show this unstable mode,especially when it is not
exhibiting X-ray bursts. Our discovery of an unstable mode in a thermally
stable atmosphere shows that nonradial perturbations have a different stability
criterion than the spherically symmetric thermal perturbations that generate
type I X-ray bursts.Comment: Accepted for publication in Astrophysical Journal, 22 pages, 14
figure
Black Hole Spectroscopy: Testing General Relativity through Gravitational Wave Observations
Assuming that general relativity is the correct theory of gravity in the
strong field limit, can gravitational wave observations distinguish between
black hole and other compact object sources? Alternatively, can gravitational
wave observations provide a test of one of the fundamental predictions of
general relativity? Here we describe a definitive test of the hypothesis that
observations of damped, sinusoidal gravitational waves originated from a black
hole or, alternatively, that nature respects the general relativistic no-hair
theorem. For astrophysical black holes, which have a negligible charge-to-mass
ratio, the black hole quasi-normal mode spectrum is characterized entirely by
the black hole mass and angular momentum and is unique to black holes. In a
different theory of gravity, or if the observed radiation arises from a
different source (e.g., a neutron star, strange matter or boson star), the
spectrum will be inconsistent with that predicted for general relativistic
black holes. We give a statistical characterization of the consistency between
the noisy observation and the theoretical predictions of general relativity,
together with a numerical example.Comment: 19 pages, 7 figure
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