7,750 research outputs found
Towards adiabatic waveforms for inspiral into Kerr black holes: I. A new model of the source for the time domain perturbation equation
We revisit the problem of the emission of gravitational waves from a test
mass orbiting and thus perturbing a Kerr black hole. The source term of the
Teukolsky perturbation equation contains a Dirac delta function which
represents a point particle. We present a technique to effectively model the
delta function and its derivatives using as few as four points on a numerical
grid. The source term is then incorporated into a code that evolves the
Teukolsky equation in the time domain as a (2+1) dimensional PDE. The waveforms
and energy fluxes are extracted far from the black hole. Our comparisons with
earlier work show an order of magnitude gain in performance (speed) and
numerical errors less than 1% for a large fraction of parameter space. As a
first application of this code, we analyze the effect of finite extraction
radius on the energy fluxes. This paper is the first in a series whose goal is
to develop adiabatic waveforms describing the inspiral of a small compact body
into a massive Kerr black hole.Comment: 21 pages, 6 figures, accepted by PRD. This version removes the
appendix; that content will be subsumed into future wor
Report on the first round of the Mock LISA Data Challenges
The Mock LISA Data Challenges (MLDCs) have the dual purpose of fostering the development of LISA data analysis tools and capabilities, and demonstrating the technical readiness already achieved by the gravitational-wave community in distilling a rich science payoff from the LISA data output. The first round of MLDCs has just been completed: nine challenges consisting of data sets containing simulated gravitational-wave signals produced either by galactic binaries or massive black hole binaries embedded in simulated LISA instrumental noise were released in June 2006 with deadline for submission of results at the beginning of December 2006. Ten groups have participated in this first round of challenges. All of the challenges had at least one entry which successfully characterized the signal to better than 95% when assessed via a correlation with phasing ambiguities accounted for. Here, we describe the challenges, summarize the results and provide a first critical assessment of the entries
A Chandra X-ray Study of Cygnus A - II. The Nucleus
We report Chandra ACIS and quasi-simultaneous RXTE observations of the
nearby, powerful radio galaxy Cygnus A, with the present paper focusing on the
properties of the active nucleus. In the Chandra observation, the hard (> a few
keV) X-ray emission is spatially unresolved with a size \approxlt 1 arcsec (1.5
kpc, H_0 = 50 km s^-1 Mpc^-1) and coincides with the radio and near infrared
nuclei. In contrast, the soft (< 2 keV) emission exhibits a bi-polar nebulosity
that aligns with the optical bi-polar continuum and emission-line structures
and approximately with the radio jet. In particular, the soft X-ray emission
corresponds very well with the [O III] \lambda 5007 and H\alpha + [N II]
\lambda\lambda 6548, 6583 nebulosity imaged with HST. At the location of the
nucleus there is only weak soft X-ray emission, an effect that may be intrinsic
or result from a dust lane that crosses the nucleus perpendicular to the source
axis. The spectra of the various X-ray components have been obtained by
simultaneous fits to the 6 detectors. The compact nucleus is detected to 100
keV and is well described by a heavily absorbed power law spectrum with
\Gamma_h = 1.52^{+0.12}_{-0.12} (similar to other narrow line radio galaxies)
and equivalent hydrogen column N_H (nuc) = 2.0^{+0.1}_{-0.2} \times 10^{23}
cm^-2.
(Abstract truncated).Comment: To be published in the Astrophysical Journal, v564 January 1, 2002
issue; 34 pages, 11 figures (1 color
Extracting galactic binary signals from the first round of Mock LISA Data Challenges
We report on the performance of an end-to-end Bayesian analysis pipeline for
detecting and characterizing galactic binary signals in simulated LISA data.
Our principal analysis tool is the Blocked-Annealed Metropolis Hasting (BAM)
algorithm, which has been optimized to search for tens of thousands of
overlapping signals across the LISA band. The BAM algorithm employs Bayesian
model selection to determine the number of resolvable sources, and provides
posterior distribution functions for all the model parameters. The BAM
algorithm performed almost flawlessly on all the Round 1 Mock LISA Data
Challenge data sets, including those with many highly overlapping sources. The
only misses were later traced to a coding error that affected high frequency
sources. In addition to the BAM algorithm we also successfully tested a Genetic
Algorithm (GA), but only on data sets with isolated signals as the GA has yet
to be optimized to handle large numbers of overlapping signals.Comment: 13 pages, 4 figures, submitted to Proceedings of GWDAW-11 (Berlin,
Dec. '06
ASCA and ROSAT observations of nearby cluster cooling flows
We present a detailed analysis of the X-ray properties of the cooling flows
in a sample of nearby, X-ray bright clusters of galaxies using high-quality
ASCA spectra and ROSAT X-ray images. We demonstrate the need for multiphase
models to consistently explain the spectral and imaging X-ray data for the
clusters. The mass deposition rates of the cooling flows, independently
determined from the ASCA spectra and ROSAT images, exhibit reasonable
agreement. We confirm the presence of intrinsic X-ray absorption in the
clusters using a variety of spectral models. We also report detections of
extended m infrared emission, spatially coincident with the cooling
flows, in several of the systems studied. The observed infrared fluxes and flux
limits are in good agreement with the predicted values due to reprocessed X-ray
emission from the cooling flows. We present precise measurements of the
abundances of iron, magnesium, silicon and sulphur in the central regions of
the Virgo and Centaurus clusters. Our results firmly favour models in which a
high mass fraction (70-80 per cent) of the iron in the X-ray gas in these
regions is due to Type Ia supernovae. Finally, we present a series of methods
which may be used to measure the ages of cooling flows from the X-ray data. The
results for the present sample of clusters indicate ages of between 2.5 and 7
Gyr. If the ages of cooling flows are primarily set by subcluster merger
events, then our results suggest that in the largest clusters, mergers with
subclusters with masses of approximately 30 per cent of the final cluster mass
are likely to disrupt cooling flows.Comment: Final version. MNRAS, in press. 36 pages, 9 figs, 14 tables in MNRAS
LaTex styl
Searching for Massive Black Hole Binaries in the first Mock LISA Data Challenge
The Mock LISA Data Challenge is a worldwide effort to solve the LISA data
analysis problem. We present here our results for the Massive Black Hole Binary
(BBH) section of Round 1. Our results cover Challenge 1.2.1, where the
coalescence of the binary is seen, and Challenge 1.2.2, where the coalescence
occurs after the simulated observational period. The data stream is composed of
Gaussian instrumental noise plus an unknown BBH waveform. Our search algorithm
is based on a variant of the Markov Chain Monte Carlo method that uses
Metropolis-Hastings sampling and thermostated frequency annealing. We present
results from the training data sets and the blind data sets. We demonstrate
that our algorithm is able to rapidly locate the sources, accurately recover
the source parameters, and provide error estimates for the recovered
parameters.Comment: 11 pages, 6 figures, Submitted to CQG proceedings of GWDAW 11, AEI,
Germany, Dec 200
A Multi-Cloud Warm-Absorber Model for NGC 4051
A multi-cloud model is presented which explains the soft X-ray excess in NGC
4051 and, consistently, the optical line spectrum and the SED of the continuum.
The clouds are heated and ionized by the photoionizing flux from the active
center and by shocks. Diffuse radiation, partly absorbed throughout the clouds,
nicely fits the bump in the soft X-ray domain, while bremsstrahlung radiation
from the gaseous clouds contribute to the fit of the continuum SED. Debris of
high density fragmented clouds are necessary to explain the absorption oxygen
throats observed at 0.87 keV and 0.74 keV. The debris are heated by shocks of
about 200-300 km/s. Low velocity (100 km/s)-density (100 cm-3) clouds
contribute to the line and continuum spectra, as well as high velocity (1000
km/s)-density (8000 cm-3) clouds which are revealed by the FWHM of the line
profiles. The SED in the IR is explained by reradiation of dust, however, the
dust-to-gas ratio is not particularly high. Radio emission is well fitted by
synchrotron radiation created at the shock front by Fermi mechanism.Comment: 19 pages + 3 figures PostScrip
A Cloudy/Xspec Interface
We discuss new functionality of the spectral simulation code CLOUDY which
allows the user to calculate grids with one or more initial parameters varied
and formats the predicted spectra in the standard FITS format. These files can
then be imported into the x-ray spectral analysis software XSPEC and used as
theoretical models for observations. We present and verify a test case.
Finally, we consider a few observations and discuss our results.Comment: 13 pages, 1 table, 4 figures, accepted for publication in PAS
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