489 research outputs found
Time-domain modelling of Extreme-Mass-Ratio Inspirals for the Laser Interferometer Space Antenna
When a stellar-mass compact object is captured by a supermassive black hole
located in a galactic centre, the system losses energy and angular momentum by
the emission of gravitational waves. Subsequently, the stellar compact object
evolves inspiraling until plunging onto the massive black hole. These EMRI
systems are expected to be one of the main sources of gravitational waves for
the future space-based Laser Interferometer Space Antenna (LISA). However, the
detection of EMRI signals will require of very accurate theoretical templates
taking into account the gravitational self-force, which is the responsible of
the stellar-compact object inspiral. Due to its potential applicability on
EMRIs, the obtention of an efficient method to compute the scalar self-force
acting on a point-like particle orbiting around a massive black hole is being
object of increasing interest. We present here a review of our time-domain
numerical technique to compute the self-force acting on a point-like particle
and we show its suitability to deal with both circular and eccentric orbits.Comment: 4 pages, 2 figures, JPCS latex style. Submitted to JPCS (special
issue for the proceedings of the Spanish Relativity Meeting (ERE2010)
Are Time-Domain Self-Force Calculations Contaminated by Jost Solutions?
The calculation of the self force in the modeling of the gravitational-wave
emission from extreme-mass-ratio binaries is a challenging task. Here we
address the question of the possible emergence of a persistent spurious
solution in time-domain schemes, referred to as a {\em Jost junk solution} in
the literature, that may contaminate self force calculations. Previous studies
suggested that Jost solutions are due to the use of zero initial data, which is
inconsistent with the singular sources associated with the small object,
described as a point mass. However, in this work we show that the specific
origin is an inconsistency in the translation of the singular sources into jump
conditions. More importantly, we identify the correct implementation of the
sources at late times as the sufficient condition guaranteeing the absence of
Jost junk solutions.Comment: RevTeX. 5 pages, 2 figures. Version updated to match the contents of
the published articl
The Evolution of Cluster Substructure with Redshift
Using Chandra archival data, we quantify the evolution of cluster morphology
with redshift. To quantify cluster morphology, we use the power ratio method
developed by Buote and Tsai (1995). Power ratios are constructed from moments
of the two-dimensional gravitational potential and are, therefore, related to a
cluster's dynamical state. Our sample will include 40 clusters from the Chandra
archive with redshifts between 0.11 and 0.89. These clusters were selected from
two fairly complete flux-limited X-ray surveys (the ROSAT Bright Cluster Sample
and the Einstein Medium Sensitivity Survey), and additional high-redshift
clusters were selected from recent ROSAT flux-limited surveys. Here we present
preliminary results from the first 28 clusters in this sample. Of these, 16
have redshifts below 0.5, and 12 have redshifts above 0.5.Comment: 5 pages, 1 figure, corrected a reference, to appear in the proceeding
of Multiwavelength Cosmology, ed. M. Plioni
Gravitational wave parameter estimation with compressed likelihood evaluations
One of the main bottlenecks in gravitational wave (GW) astronomy is the high cost of performing parameter estimation and GW searches on the fly. We propose a novel technique based on reduced order quadratures (ROQs), an application and data-specific quadrature rule, to perform fast and accurate likelihood evaluations. These are the dominant cost in Markov chain Monte Carlo algorithms, which are widely employed in parameter estimation studies, and so ROQs offer a new way to accelerate GW parameter estimation. We illustrate our approach using a four-dimensional GW burst model embedded in noise. We build an ROQ for this model and perform four-dimensional Markov chain Monte Carlo searches with both the standard and ROQ rules, showing that, for this model, the ROQ approach is around 25 times faster than the standard approach with essentially no loss of accuracy. The speed-up from using ROQs is expected to increase for more complex GW signal models and therefore has significant potential to accelerate parameter estimation of GW sources such as compact binary coalescences
The Evolution of Structure in X-ray Clusters of Galaxies
Using Chandra archival data, we quantify the evolution of cluster morphology
with redshift. Clusters form and grow through mergers with other clusters and
groups, and the amount of substructure in clusters in the present epoch and how
quickly it evolves with redshift depend on the underlying cosmology. Our sample
includes 40 X-ray selected, luminous clusters from the Chandra archive, and we
quantify cluster morphology using the power ratio method (Buote & Tsai 1995).
The power ratios are constructed from the moments of the X-ray surface
brightness and are related to a cluster's dynamical state. We find that, as
expected qualitatively from hierarchical models of structure formation,
high-redshift clusters have more substructure and are dynamically more active
than low-redshift clusters. Specifically, the clusters with z>0.5 have
significantly higher average third and fourth order power ratios than the lower
redshift clusters. Of the power ratios, is the most unambiguous
indicator of an asymmetric cluster structure, and the difference in
between the two samples remains significant even when the effects of noise and
other systematics are considered. After correcting for noise, we apply a linear
fit to versus redshift and find that the slope is greater than zero
at better than 99% confidence. This observation of structure evolution
indicates that dynamical state may be an important systematic effect in cluster
studies seeking to constrain cosmology, and when calibrated against numerical
simulations, structure evolution will itself provide interesting bounds on
cosmological models.Comment: 42 pages, 6 figures, ApJ accepted. For a version of the paper
containing an appendix with images of all of the clusters, see
http://www.ociw.edu/~tesla/structure.ps.g
Theory and modeling of the magnetic field measurement in LISA PathFinder
The magnetic diagnostics subsystem of the LISA Technology Package (LTP) on
board the LISA PathFinder (LPF) spacecraft includes a set of four tri-axial
fluxgate magnetometers, intended to measure with high precision the magnetic
field at their respective positions. However, their readouts do not provide a
direct measurement of the magnetic field at the positions of the test masses,
and hence an interpolation method must be designed and implemented to obtain
the values of the magnetic field at these positions. However, such
interpolation process faces serious difficulties. Indeed, the size of the
interpolation region is excessive for a linear interpolation to be reliable
while, on the other hand, the number of magnetometer channels does not provide
sufficient data to go beyond the linear approximation. We describe an
alternative method to address this issue, by means of neural network
algorithms. The key point in this approach is the ability of neural networks to
learn from suitable training data representing the behavior of the magnetic
field. Despite the relatively large distance between the test masses and the
magnetometers, and the insufficient number of data channels, we find that our
artificial neural network algorithm is able to reduce the estimation errors of
the field and gradient down to levels below 10%, a quite satisfactory result.
Learning efficiency can be best improved by making use of data obtained in
on-ground measurements prior to mission launch in all relevant satellite
locations and in real operation conditions. Reliable information on that
appears to be essential for a meaningful assessment of magnetic noise in the
LTP.Comment: 10 pages, 8 figures, 2 tables, submitted to Physical Review
Spectral Line Imaging Observations of 1E0102.2-7219
E0102-72 is the second brightest X-ray source in the Small Magellanic Cloud
and the brightest supernova remnant in the SMC. We observed this SNR for ~140
ksec with the High Energy Transmission Gratings (HETG) aboard the Chandra X-ray
Observatory. The small angular size and high surface brightness make this an
excellent target for HETG and we resolve the remnant into individual lines. We
observe fluxes from several lines which include O VIII Ly, Ly,
and O VII along with several lines from Ne X, Ne IX and Mg XII. These line
ratios provide powerful constraints on the electron temperature and the
ionization age of the remnant.Comment: To appear in "Young Supernova Remnants" (11th Annual Astrophysics
Conference in Maryland), S. S. Holt & U. Hwang (eds), AIP, New York (2001
Disk Dominated States of 4U 1957+11: Chandra, XMM, and RXTE Observations of Ostensibly the Most Rapidly Spinning Galactic Black Hole
We present simultaneous Chandra-HETG and RXTE observations of a moderate flux
`soft state' of the black hole candidate 4U1957+11. These spectra, having a
minimally discernible hard X-ray excess, are an excellent test of modern disk
atmosphere models that include the effects of black hole spin. The HETG data
show that the soft disk spectrum is only very mildly absorbed with N_H =1-2 X
10^{21} cm^-2. These data additionally reveal 13.449 A NeIX absorption
consistent with the warm/hot phase of the interstellar medium. The fitted disk
model implies a highly inclined disk around a low mass black hole rapidly
rotating with normalized spin a*~1. We show, however, that pure Schwarzschild
black hole models describe the data extremely well, albeit with large disk
atmosphere ``color-correction'' factors. Standard color-correction factors can
be attained if one additionally incorporates mild Comptonization. We find that
the Chandra observations do not uniquely determine spin. Similarly, XMM/RXTE
observations, taken only six weeks later, are equally unconstraining. This lack
of constraint is partly driven by the unknown mass and unknown distance of
4U1957+11; however, it is also driven by the limited bandpass of Chandra and
XMM. We therefore present a series of 48 RXTE observations taken over the span
of several years and at different brightness/hardness levels. These data prefer
a spin of a*~1, even when including a mild Comptonization component; however,
they also show evolution of the disk atmosphere color-correction factors. If
the rapid spin models with standard atmosphere color-correction factors of
h_d=1.7 are to be believed, then the RXTE observations predict that 4U1957+11
can range from a 3 M_sun black hole at 10 kpc with a*~0.83 to a 16 M_sun black
hole at 22 kpc with a* ~ 1, with the latter being statistically preferred.Comment: 16 pages (emulateapj style). Accepted for Publication in the
Astrophysical Journa
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