The Experts below are selected from a list of 46125 Experts worldwide ranked by ideXlab platform
Wangjianping - One of the best experts on this subject based on the ideXlab platform.
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Multihop Range-Free Localization in Anisotropic Wireless Sensor Networks
IEEE Transactions on Mobile Computing, 2010Co-Authors: Xiaoqingjun, Xiaobin, Caojiannong, WangjianpingAbstract:This paper focuses on multihop range-free localization in anisotropic wireless sensor networks. In anisotropic networks, Geometric Distance between a pair of sensor nodes is not always proportional...
A. L. Argon - One of the best experts on this subject based on the ideXlab platform.
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toward a new Geometric Distance to the active galaxy ngc 4258 iii final results and the hubble constant
The Astrophysical Journal, 2013Co-Authors: J M Moran, L. J. Greenhill, M J Reid, E M L Humphreys, A. L. ArgonAbstract:We report a new Geometric maser Distance estimate to the active galaxy NGC 4258. The data for the new model are maser line-of-sight (LOS) velocities and sky positions from 18 epochs of very long baseline interferometry observations, and LOS accelerations measured from a 10 yr monitoring program of the 22 GHz maser emission of NGC 4258. The new model includes both disk warping and confocal elliptical maser orbits with differential precession. The Distance to NGC 4258 is 7.60 ± 0.17 ± 0.15 Mpc, a 3% uncertainty including formal fitting and systematic terms. The resulting Hubble constant, based on the use of the Cepheid variables in NGC 4258 to recalibrate the Cepheid Distance scale, is H0 = 72.0 ± 3.0 km s −1 Mpc −1 .
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toward a new Geometric Distance to the active galaxy ngc 4258 iii final results and the hubble constant
arXiv: Cosmology and Nongalactic Astrophysics, 2013Co-Authors: Liz Humphreys, J M Moran, L. J. Greenhill, M J Reid, A. L. ArgonAbstract:We report a new Geometric maser Distance estimate to the active galaxy NGC 4258. The data for the new model are maser line-of-sight velocities and sky positions from 18 epochs of Very Long Baseline Interferometry observations, and line-of-sight accelerations measured from a 10-year monitoring program of the 22 GHz maser emission of NGC 4258. The new model includes both disk warping and confocal elliptical maser orbits with differential precession. The Distance to NGC 4258 is 7.60 +/- 0.17 +/- 0.15 Mpc, a 3% uncertainty including formal fitting and systematic terms. The resulting Hubble Constant, based on the use of the Cepheid Variables in NGC 4258 to recalibrate the Cepheid Distance scale (Riess et al. 2011), is H_0 = 72.0 +/- 3.0 km/s/Mpc.
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toward a new Geometric Distance to the active galaxy ngc 4258 ii centripetal accelerations and investigation of spiral structure
The Astrophysical Journal, 2008Co-Authors: E M L Humphreys, J M Moran, L. J. Greenhill, M J Reid, A. L. ArgonAbstract:We report measurements of centripetal accelerations of maser spectral components of NGC 4258 for 51 epochs spanning 1994 to 2004. This is the second paper of a series, in which the goal is the determination of a new Geometric maser Distance to NGC 4258, accurate to possibly ~3%. We measure accelerations using a formal analysis method that involves simultaneous decomposition of maser spectra for all epochs into multiple, Gaussian components. Components are coupled between epochs by linear drifts (accelerations) from their centroid velocities at a reference epoch. For high-velocity emission, accelerations lie in the range –0.7 to +0.7 km s−1 yr−1, indicating an origin within 13° of the disk midline (the perpendicular to the line of sight [LOS] to the black hole). Comparison of the projected positions of high-velocity emission in VLBI images with those derived from acceleration data provides evidence that masers trace real gas dynamics. High-velocity emission accelerations do not support a model of trailing shocks associated with spiral arms in the disk. However, we find strengthened evidence for spatial periodicity in high-velocity emission, of wavelength 0.75 mas. This supports suggestions of spiral structure due to density waves in the nuclear accretion disk of an active galaxy. Accelerations of low-velocity (systemic) emission lie in the range 7.7 to 8.9 km s−1 yr−1, consistent with emission originating from a concavity where the thin, warped disk is tangent to the LOS. A trend in accelerations of low-velocity emission, as a function of Doppler velocity, may be associated with disk geometry and orientation or with the presence of spiral structure.
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toward a new Geometric Distance to the active galaxy ngc 4258 i vlbi monitoring of water maser emission
The Astrophysical Journal, 2007Co-Authors: A. L. Argon, L. J. Greenhill, J M Moran, M J Reid, E M L HumphreysAbstract:We report a 3 yr, 18 epoch, VLBI monitoring study of H2O masers in the subparsec, warped, accretion disk within the NGC 4258 AGN. Our immediate goals are to trace the geometry of the underlying disk, track rotation via measurement of proper motion, and ascertain the radii of masers for which centripetal acceleration may be measured separately. The monitoring includes ~4 times as many epochs, ~3 times denser sampling, and tighter control over sources of systematic error than earlier VLBI investigations. Coverage of a ~2400 km s-1 bandwidth has also enabled mapping of molecular material ~30% closer to the black hole than accomplished previously, which will strengthen Geometric and dynamical disk models. Through repeated observation we have also measured for the first time a 5 μas (1 σ) thickness of the maser medium. Assuming that this corresponds to the thickness of the accretion disk, hydrostatic equilibrium requires a disk plane temperature of ≈600 K. Our long-term goal is a Geometric Distance to NGC 4258 that is accurate to ~3%, a ~2 times improvement over the current best estimate. A Geometric estimate of Distance can be compared to Distances obtained from analysis of Cepheid light curves, with the intent to recalibrate the extragalactic Distance scale with reduced systematic uncertainties. This is the first paper in a series. We present here VLBI observations, data reduction, and temporal and spatial characteristics of the maser emission. Later papers will report estimation of orbital acceleration and proper motion, modeling of disk 3D geometry and dynamics, and estimation of a "maser Distance." Estimation of a "Cepheid Distance" is presented in a parallel paper series.
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toward a new Geometric Distance to the active galaxy ngc4258 i vlbi monitoring of water maser emission
arXiv: Astrophysics, 2007Co-Authors: A. L. Argon, L. J. Greenhill, J M Moran, M J Reid, E M L HumphreysAbstract:We report a three year, 18 epoch, VLBI monitoring study of H2O masers in the sub-parsec, warped, accretion disk within the NGC4258 AGN. Our immediate goals are to trace the geometry of the underlying disk, track rotation via measurement of proper motion, and ascertain the radii of masers for which centripetal acceleration may be measured separately. The monitoring includes ~ 4 times as many epochs, ~ 3 times denser sampling, and tighter control over sources of systematic error than earlier VLBI investigations. Coverage of a ~ 2400 km/s bandwidth has also enabled mapping of molecular material ~ 30% closer to the black hole than accomplished previously, which will strengthen Geometric and dynamical disk models. Through repeated observation we have also measured for the first time a 5 microarsecond (1 sigma) thickness of the maser medium. Assuming this corresponds to the thickness of the accretion disk, hydrostatic equilibrium requires a disk plane temperature of ~ 600 K. Our long-term goal is a Geometric Distance to NGC4258 that is accurate to ~ 3%, a ~ 2 times improvement over the current best estimate. A Geometric estimate of Distance can be compared to Distances obtained from analysis of Cepheid light curves, with the intent to recalibrate the extragalactic Distance scale with reduced systematic uncertainties. This is the first paper in a series. We present here VLBI observations, data reduction, and temporal and spatial characteristics of the maser emission. Later papers will report estimation of orbital acceleration and proper motion, modeling of disk 3-D geometry and dynamics, and estimation of a "maser Distance." Estimation of a "Cepheid Distance" is presented in a parallel paper series.
Jean Ponce - One of the best experts on this subject based on the ideXlab platform.
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Using Geometric Distance fits for 3-D object modeling and recognition
IEEE Transactions on Pattern Analysis and Machine Intelligence, 1994Co-Authors: S. Sullivan, L. Sandford, Jean PonceAbstract:Addresses the problems of automatically constructing algebraic surface models from sets of 2D and 3D images and using these models in pose computation, motion and deformation estimation, and object recognition. We propose using a combination of constrained optimization and nonlinear least-squares estimation techniques to minimize the mean-squared Geometric Distance between a set of points or rays and a parameterized surface. In modeling tasks, the unknown parameters are the surface coefficients, while in pose and deformation estimation tasks they represent the transformation which maps the observer's coordinate system onto the modeled surface's own coordinate system. We have applied this approach to a variety of real range, computerized tomography and video images. >
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CVPR - On using Geometric Distance fits to estimate 3D object shape, pose, and deformation from range, CT, and video images
Proceedings of IEEE Conference on Computer Vision and Pattern Recognition, 1Co-Authors: S. Sullivan, L. Sandford, Jean PonceAbstract:The problems of automatically constructing algebraic surface models from sets of 3D and 2D images and using these models in pose computation, motion and deformation estimation, and object recognition are addressed. It is proposed that a combination of constrained optimization and nonlinear least-squares estimation techniques be used to minimize the mean-squared Geometric Distance between a set of points or rays and a parameterized surface. In modeling tasks, the unknown parameters are the surface coefficients, while in pose and deformation estimation tasks they represent the transformation mapping the observer's coordinate system onto the modeled surface's own coordinate system. This approach is applied to a variety of real range, computerized tomography (CT), and video images. >
William B. Sparks - One of the best experts on this subject based on the ideXlab platform.
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the long period galactic cepheid rs puppis iii a Geometric Distance from hst polarimetric imaging of its light echoes
arXiv: Solar and Stellar Astrophysics, 2014Co-Authors: Pierre Kervella, Howard E. Bond, William B. Sparks, Antoine Mérand, Laszlo Szabados, Misty Cracraft, J Breitfelder, A GallenneAbstract:As one of the most luminous Cepheids in the Milky Way, the 41.5-day RS Puppis is an analog of the long-period Cepheids used to measure extragalactic Distances. An accurate Distance to this star would therefore help anchor the zero-point of the bright end of the period-luminosity relation. But, at a Distance of about 2 kpc, RS Pup is too far away for measuring a direct trigonometric parallax with a precision of a few percent with existing instrumentation. RS Pup is unique in being surrounded by a reflection nebula, whose brightness varies as pulses of light from the Cepheid propagate outwards. We present new polarimetric imaging of the nebula obtained with HST/ACS. The derived map of the degree of linear polarization pL allows us to reconstruct the three-dimensional structure of the dust distribution. To retrieve the scattering angle from the pL value, we consider two different polarization models, one based on a Milky Way dust mixture and one assuming Rayleigh scattering. Considering the derived dust distribution in the nebula, we adjust a model of the phase lag of the photometric variations over selected nebular features to retrieve the Distance of RS Pup. We obtain a Distance of 1910 +/- 80 pc (4.2%), corresponding to a parallax of 0.524 +/- 0.022 mas. The agreement between the two polarization models we considered is good, but the final uncertainty is dominated by systematics in the adopted model parameters. The Distance we obtain is consistent with existing measurements from the literature, but light echoes provide a Distance estimate that is not subject to the same systematic uncertainties as other estimators (e.g. the Baade-Wesselink technique). RS Pup therefore provides an important fiducial for the calibration of systematic uncertainties of the long-period Cepheid Distance scale.
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On Geometric Distance determination to the Cepheid RS Puppis from its light echoes
Astronomy & Astrophysics, 2008Co-Authors: Howard E. Bond, William B. SparksAbstract:Context. The luminous Galactic Cepheid RS Puppis is unique in being surrounded by a dust nebula illuminated by the variable light of the Cepheid. In a recent paper in this journal, Kervella et al. (2008) report a very precise Geometric Distance to RS Pup, based on measured phase lags of the light variations of individual knots in the reflection nebula. Aims. In this commentary, we examine the validity of the Distance measurement, as well as the reality of the spatial structure of the nebula determined by Feast (2008) based upon the phase lags of the knots. Methods. Kervella et al. assumed that the illuminated dust knots lie, on average, in the plane of the sky (otherwise it is not possible to derive a Geometric Distance from direct imaging of light echoes). We consider the biasing introduced by the high efficiency of forward scattering. Results. We conclude that most of the knots are in fact likely to lie in front of the plane of the sky, thus invalidating the Kervella et al. result. We also show that the flat equatorial disk structure determined by Feast is unlikely; instead, the morphology of the nebula is more probably bipolar, with a significant tilt of its axis with respect to the plane of the sky. Conclusions. Although the Kervella et al. Distance result is invalidated, we show that high-resolution polarimetric imaging has the potential to yield a valid Geometric Distance to this important Cepheid.
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V838 MONOCEROTIS: A Geometric Distance FROM HUBBLE SPACE TELESCOPE POLARIMETRIC IMAGING OF ITS LIGHT ECHO*
The Astronomical Journal, 2008Co-Authors: William B. Sparks, Howard E. Bond, Misty Cracraft, Zolt Levay, Lisa A. Crause, Michael A. Dopita, Arne Henden, Ulisse Munari, Nino Panagia, Sumner StarrfieldAbstract:Following the outburst of the unusual variable star V838 Monocerotis in 2002, a spectacular light echo appeared. A light echo provides the possibility of direct Geometric Distance determination, because it should contain a ring of highly linearly polarized light at a linear radius of ct, where t is the time since the outburst. We present imaging polarimetry of the V838 Mon light echo, obtained in 2002 and 2005 with the Advanced Camera for Surveys on board the Hubble Space Telescope, which confirms the presence of the highly polarized ring. Based on detailed modeling that takes into account the outburst light curve, the paraboloidal echo geometry, and the physics of dust scattering and polarization, we find a Distance of 6.1 ± 0.6 kpc. The error is dominated by the systematic uncertainty in the scattering angle of maximum linear polarization, taken to be θmax = 90° ± 5°. The polarimetric Distance agrees remarkably well with a Distance of 6.2 ± 1.2 kpc obtained from the entirely independent method of main-sequence fitting to a sparse star cluster associated with V838 Mon. At this Distance, V838 Mon at maximum light had MV −9.8, making it temporarily one of the most luminous stars in the Local Group. Our validation of the polarimetric method offers promise for measurement of extragalactic Distances using supernova light echoes.
Xiaoqingjun - One of the best experts on this subject based on the ideXlab platform.
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Multihop Range-Free Localization in Anisotropic Wireless Sensor Networks
IEEE Transactions on Mobile Computing, 2010Co-Authors: Xiaoqingjun, Xiaobin, Caojiannong, WangjianpingAbstract:This paper focuses on multihop range-free localization in anisotropic wireless sensor networks. In anisotropic networks, Geometric Distance between a pair of sensor nodes is not always proportional...