1,467 research outputs found
Random template banks and relaxed lattice coverings
Template-based searches for gravitational waves are often limited by the
computational cost associated with searching large parameter spaces. The study
of efficient template banks, in the sense of using the smallest number of
templates, is therefore of great practical interest. The "traditional" approach
to template-bank construction requires every point in parameter space to be
covered by at least one template, which rapidly becomes inefficient at higher
dimensions. Here we study an alternative approach, where any point in parameter
space is covered only with a given probability < 1. We find that by giving up
complete coverage in this way, large reductions in the number of templates are
possible, especially at higher dimensions. The prime examples studied here are
"random template banks", in which templates are placed randomly with uniform
probability over the parameter space. In addition to its obvious simplicity,
this method turns out to be surprisingly efficient. We analyze the statistical
properties of such random template banks, and compare their efficiency to
traditional lattice coverings. We further study "relaxed" lattice coverings
(using Zn and An* lattices), which similarly cover any signal location only
with probability < 1. The relaxed An* lattice is found to yield the most
efficient template banks at low dimensions (n < 10), while random template
banks increasingly outperform any other method at higher dimensions.Comment: 13 pages, 10 figures, submitted to PR
Mineralogy of Interplanetary Dust Particles from the Comet Giacobini-Zinner Dust Stream Collections
The Draconoid meteor shower, originating from comet 21P/Giacobini-Zinner, is a low-velocity Earth-crossing dust stream that had a peak anticipated flux on Oct. 8, 2012. In response to this prediction, NASA performed dedicated stratospheric dust collections to target interplanetary dust particles (IDPs) from this comet stream on Oct 15-17, 2012 [3]. Twelve dust particles from this targeted collection were allocated to our coordinated analysis team for studies of noble gas (Univ. Minnesota, Minnesota State Univ.), SXRF and Fe-XANES (SSL Berkeley) and mineralogy/isotopes (JSC). Here we report a mineralogical study of 3 IDPs from the Draconoid collection.
The Radiative Efficiency of a Radiatively Inefficient Accretion Flow
A recent joint XMM-Newton/Nuclear Spectroscopic Telescope Array (NuSTAR)
observation of the accreting neutron star Cen X-4 () revealed a hard power-law component
(-) with a relatively low cut-off energy (~10 keV),
suggesting bremsstrahlung emission. The physical requirements for
bremsstrahlung combined with other observed properties of Cen X-4 suggest the
emission comes from a boundary layer rather than the accretion flow. The
accretion flow itself is thus undetected (with an upper limit of ). A deep search for coherent pulsations (which
would indicate a strong magnetic field) places a 6 per cent upper limit on the
fractional amplitude of pulsations, suggesting the flow is not magnetically
regulated. Considering the expected energy balance between the accretion flow
and the boundary layer for different values of the neutron star parameters
(size, magnetic field, and spin) we use the upper limit on to
set an upper limit of for the intrinsic radiative
efficiency of the accretion flow for the most likely model of a fast-spinning,
non-magnetic neutron star. The non-detection of the accretion flow provides the
first direct evidence that this flow is indeed 'radiatively inefficient', i.e.
most of the gravitational potential energy lost by the flow before it hits the
star is not emitted as radiation.Comment: 15 pages, 3 figures - minor modifications to match published versio
Measuring a cosmological distance-redshift relationship using only gravitational wave observations of binary neutron star coalescences
Detection of gravitational waves from the inspiral phase of binary neutron
star coalescence will allow us to measure the effects of the tidal coupling in
such systems. These effects will be measurable using 3rd generation
gravitational wave detectors, e.g. the Einstein Telescope, which will be
capable of detecting inspiralling binary neutron star systems out to redshift
z=4. Tidal effects provide additional contributions to the phase evolution of
the gravitational wave signal that break a degeneracy between the system's mass
parameters and redshift and thereby allow the simultaneous measurement of both
the effective distance and the redshift for individual sources. Using the
population of O(10^3-10^7) detectable binary neutron star systems predicted for
the Einstein Telescope the luminosity distance--redshift relation can be probed
independently of the cosmological distance ladder and independently of
electromagnetic observations. We present the results of a Fisher information
analysis applied to waveforms assuming a subset of possible neutron star
equations of state. We conclude that for our range of representative neutron
star equations of state the redshift of such systems can be determined to an
accuracy of 8-40% for z<1 and 9-65% for 1<z<4.Comment: 5 pages, 1 figure, submitted to Phys. Rev. Let
Structure and Innervation of the Extrahepatic Biliary System in the Australian Possum, Trichosurus Vulpecula
The morphology, microanatomy and innervation of the biliary tree of the Australian possum,
Trichosurus vulpecula, was examined. The gross morphology of the gallbladder, hepatic and cystic
ducts, and the course of the common bile duct, conforms to those of other species. The sphincter of
Oddi has an extraduodenal segment that extends 15mm from the duodenal wall; within this segment the
pancreatic and common bile ducts are ensheathed together by sphincter muscle. Their lumens unite to
form a common channel within the terminal intraduodenal segment
Robertson-Walker fluid sources endowed with rotation characterised by quadratic terms in angular velocity parameter
Einstein's equations for a Robertson-Walker fluid source endowed with
rotation Einstein's equations for a Robertson-Walker fluid source endowed with
rotation are presented upto and including quadratic terms in angular velocity
parameter. A family of analytic solutions are obtained for the case in which
the source angular velocity is purely time-dependent. A subclass of solutions
is presented which merge smoothly to homogeneous rotating and non-rotating
central sources. The particular solution for dust endowed with rotation is
presented. In all cases explicit expressions, depending sinusoidally on polar
angle, are given for the density and internal supporting pressure of the
rotating source. In addition to the non-zero axial velocity of the fluid
particles it is shown that there is also a radial component of velocity which
vanishes only at the poles. The velocity four-vector has a zero component
between poles
Bench Crater Meteorite: Hydrated Asteroidal Material Delivered to the Moon
D/H measurements from the lunar regolith agglutinates [8] indicate mixing between a low D/H solar implanted component and additional higher D/H sources (e.g., meteoritic/ cometary/volcanic gases). We have determined the range and average D/H ratio of Bench Crater meteorite, which is the first direct D/H analysis of meteoritic material delivered to the lunar surface. This result provides an important ground truth for future investigations of lunar water resources by missions to the Moon
The very faint X-ray binary IGR J17062-6143: a truncated disc, no pulsations, and a possible outflow
We present a comprehensive X-ray study of the neutron star low-mass X-ray binary IGR J17062-6143, which has been accreting at low luminosities since its discovery in 2006. Analysing NuSTAR, XMM–Newton, and Swift observations, we investigate the very faint nature of this source through three approaches: modelling the relativistic reflection spectrum to constrain the accretion geometry, performing high-resolution X-ray spectroscopy to search for an outflow, and searching for the recently reported millisecond X-ray pulsations. We find a strongly truncated accretion disc at
77+22−18
gravitational radii (∼164 km) assuming a high inclination, although a low inclination and a disc extending to the neutron star cannot be excluded. The high-resolution spectroscopy reveals evidence for oxygen-rich circumbinary material, possibly resulting from a blueshifted, collisionally ionized outflow. Finally, we do not detect any pulsations. We discuss these results in the broader context of possible explanations for the persistent faint nature of weakly accreting neutron stars. The results are consistent with both an ultra-compact binary orbit and a magnetically truncated accretion flow, although both cannot be unambiguously inferred. We also discuss the nature of the donor star and conclude that it is likely a CO or O–Ne–Mg white dwarf, consistent with recent multiwavelength modelling
A Bayesian parameter estimation approach to pulsar time-of-arrival analysis
The increasing sensitivities of pulsar timing arrays to ultra-low frequency
(nHz) gravitational waves promises to achieve direct gravitational wave
detection within the next 5-10 years. While there are many parallel efforts
being made in the improvement of telescope sensitivity, the detection of stable
millisecond pulsars and the improvement of the timing software, there are
reasons to believe that the methods used to accurately determine the
time-of-arrival (TOA) of pulses from radio pulsars can be improved upon. More
specifically, the determination of the uncertainties on these TOAs, which
strongly affect the ability to detect GWs through pulsar timing, may be
unreliable. We propose two Bayesian methods for the generation of pulsar TOAs
starting from pulsar "search-mode" data and pre-folded data. These methods are
applied to simulated toy-model examples and in this initial work we focus on
the issue of uncertainties in the folding period. The final results of our
analysis are expressed in the form of posterior probability distributions on
the signal parameters (including the TOA) from a single observation.Comment: 16 pages, 4 figure
Building a stochastic template bank for detecting massive black hole binaries
Coalescence of two massive black holes is the strongest and most promising
source for LISA. In fact, gravitational signal from the end of inspiral and
merger will be detectable throughout the Universe. In this article we describe
the first step in the two-step hierarchical search for gravitational wave
signal from the inspiraling massive BH binaries. It is based on the routinely
used in the ground base gravitational wave astronomy method of filtering the
data through the bank of templates. However we use a novel Monte-Carlo based
(stochastic) method to lay a grid in the parameter space, and we use the
likelihood maximized analytically over some parameters, known as F-statistic,
as a detection statistic. We build a coarse template bank to detect
gravitational wave signals and to make preliminary parameter estimation. The
best candidates will be followed up using Metropolis-Hasting stochastic search
to refine the parameter estimation. We demonstrate the performance of the
method by applying it to the Mock LISA data challenge 1B (training data set).Comment: revtex4, 8 figure
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