52,383 research outputs found

    Resolving the discrepancy between X-ray and gravitational lensing mass measurements for clusters of galaxies

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    We present a detailed comparison of mass measurements for clusters of galaxies using ASCA and ROSAT X-ray data and constraints from strong and weak gravitational lensing. Our results for a sample of 13 clusters provide a consistent description of the distribution of gravitating matter in these systems. For the 6 cooling-flow clusters in the sample, which are the more dynamically-relaxed systems, the X-ray and strong gravitational lensing mass measurements show excellent agreement. The core radii for the mass distributions are small, with a mean value of ~ 50 h50‚ąí1h_{50}^{-1} kpc. For the non-cooling flow clusters, the masses determined from the strong lensing data exceed the X-ray values by factors of 2‚ąí42-4. However, significant offsets between the X-ray and lensing centres are observed, indicating that the X-ray and strong-lensing data are probing different lines of sight through the clusters. These offsets, and the generally complex dynamical states of the clusters inferred from their X-ray morphologies, lensing data and galaxy distributions, suggest that the gravitational potentials in the central regions of the non-cooling flow systems are evolving rapidly, and that the assumption of hydrostatic equilibrium involved in the X-ray mass measurements is likely to have broken down. The discrepancies between the X-ray and strong lensing mass measurements may be reconciled if the dynamical activity has caused the X-ray analyses to overestimate the core radii of the dominant mass clumps in these clusters. On larger spatial scales, comparisons of the X-ray mass results with measurements from weak gravitational lensing show excellent agreement for both cooling-flow and non-cooling flow clusters. (ABRIDGED)Comment: MNRAS in press. 18 pages including 4 figures in MNRAS LaTex styl

    The relationship between cooling flows and metallicity measurements for X-ray luminous clusters

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    We explore the relationship between the metallicity of the intracluster gas in clusters of galaxies, determined by X-ray spectroscopy, and the presence of cooling flows. Using ASCA spectra and ROSAT images, we demonstrate a clear segregation between the metallicities of clusters with and without cooling flows. On average, cooling-flow clusters have an emission-weighted metallicity a factor ~ 1.8 times higher than that of non-cooling flow systems. We suggest this to be due to the presence of metallicity gradients in the cooling flow clusters, coupled with the sharply peaked X-ray surface brightness profiles of these systems. Non-cooling flow clusters have much flatter X-ray surface brightness distributions and are thought to have undergone recent merger events which may have mixed the central high-metallicity gas with the surrounding less metal-rich material. We find no evidence for evolution in the emission-weighted metallicities of clusters within z~0.3.Comment: Submitted to MNRAS letters (December 1997). 6 pages, 2 figures in MNRAS LaTex style. Minor revision

    A decision model applied to alcohol effects on driver signal light behavior

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    A decision model including perceptual noise or inconsistency is developed from expected value theory to explain driver stop and go decisions at signaled intersections. The model is applied to behavior in a car simulation and instrumented vehicle. Objective and subjective changes in driver decision making were measured with changes in blood alcohol concentration (BAC). Treatment levels averaged 0.00, 0.10 and 0.14 BAC for a total of 26 male subjects. Data were taken for drivers approaching signal lights at three timing configurations. The correlation between model predictions and behavior was highly significant. In contrast to previous research, analysis indicates that increased BAC results in increased perceptual inconsistency, which is the primary cause of increased risk taking at low probability of success signal lights

    Modeling and control study of the NASA 0.3-meter transonic cryogenic tunnel for use with sulfur hexafluoride medium

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    The NASA Langley 0.3-m Transonic Cryogenic Tunnel is to be modified to operate with sulfur hexafluoride gas while retaining its present capability to operate with nitrogen. The modified tunnel will provide high Reynolds number flow on aerodynamic models with two different test gases. The document details a study of the SF6 tunnel performance boundaries, thermodynamic modeling of the tunnel process, nonlinear dynamical simulation of math model to yield tunnel responses, the closed loop control requirements, control laws, and mechanization of the control laws on the microprocessor based controller

    The NASA Langley Research Center 0.3-meter transonic cryogenic tunnel microcomputer controller source code

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    The 0.3 m Transonic Cryogenic Tunnel (TCT) microcomputer based controller has been operating for several thousand hours in a safe and efficient manner. A complete listing is provided of the source codes for the tunnel controller and tunnel simulator. Included also is a listing of all the variables used in these programs. Several changes made to the controller are described. These changes are to improve the controller ease of use and safety

    The NASA Langley Research Center 0.3-meter transonic cryogenic tunnel T-P/Re-M controller manual

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    A new microcomputer based controller for the 0.3-m Transonic Cryogenic Tunnel (TCT) has been commissioned in 1988 and has reliably operated for more than a year. The tunnel stagnation pressure, gas stagnation temperature, tunnel wall structural temperature and flow Mach number are precisely controlled by the new controller in a stable manner. The tunnel control hardware, software, and the flow chart to assist in calibration of the sensors, actuators, and the controller real time functions are described. The software installation details are also presented. The report serves as the maintenance and trouble shooting manual for the 0.3-m TCT controller

    Microcomputer based controller for the Langley 0.3-meter Transonic Cryogenic Tunnel

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    Flow control of the Langley 0.3-meter Transonic Cryogenic Tunnel (TCT) is a multivariable nonlinear control problem. Globally stable control laws were generated to hold tunnel conditions in the presence of geometrical disturbances in the test section and precisely control the tunnel states for small and large set point changes. The control laws are mechanized as four inner control loops for tunnel pressure, temperature, fan speed, and liquid nitrogen supply pressure, and two outer loops for Mach number and Reynolds number. These integrated control laws have been mechanized on a 16-bit microcomputer working on DOS. This document details the model of the 0.3-m TCT, control laws, microcomputer realization, and its performance. The tunnel closed loop responses to small and large set point changes were presented. The controller incorporates safe thermal management of the tunnel cooldown based on thermal restrictions. The controller was shown to provide control of temperature to + or - 0.2K, pressure to + or - 0.07 psia, and Mach number to + or - 0.002 of a given set point during aerodynamic data acquisition in the presence of intrusive geometrical changes like flexwall movement, angle-of-attack changes, and drag rake traverse. The controller also provides a new feature of Reynolds number control. The controller provides a safe, reliable, and economical control of the 0.3-m TCT

    Chandra observations of the galaxy cluster Abell 1835

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    We present the analysis of 30 ksec of Chandra observations of the galaxy cluster Abell 1835. Overall, the X-ray image shows a relaxed morphology, although we detect substructure in in the inner 30 kpc radius. Spectral analysis shows a steep drop in the X-ray gas temperature from ~12 keV in the outer regions of the cluster to ~4 keV in the core. The Chandra data provide tight constraints on the gravitational potential of the cluster which can be parameterized by a Navarro, Frenk & White (1997) model. The X-ray data allow us to measure the X-ray gas mass fraction as a function of radius, leading to a determination of the cosmic matter density of \Omega_m=0.40+-0.09 h_50^-0.5. The projected mass within a radius of ~150 kpc implied by the presence of gravitationally lensed arcs in the cluster is in good agreement with the mass models preferred by the Chandra data. We find a radiative cooling time of the X-ray gas in the centre of Abell 1835 of about 3x10^8 yr. Cooling flow model fits to the Chandra spectrum and a deprojection analysis of the Chandra image both indicate the presence of a young cooling flow (~6x10^8 yr) with an integrated mass deposition rate of 230^+80_-50 M_o yr^-1 within a radius of 30 kpc. We discuss the implications of our results in the light of recent RGS observations of Abell 1835 with XMM-Newton.Comment: 15 pages, 15 figures, accepted by MNRA

    Cosmological constraints from the X-ray gas mass fraction in relaxed lensing clusters observed with Chandra

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    We present precise measurements of the X-ray gas mass fraction for a sample of luminous, relatively relaxed clusters of galaxies observed with the Chandra Observatory, for which independent confirmation of the mass results is available from gravitational lensing studies. Parameterizing the total (luminous plus dark matter) mass profiles using the model of Navarro, Frenk & White (1997), we show that the X-ray gas mass fractions in the clusters asymptote towards an approximately constant value at a radius r_2500, where the mean interior density is 2500 times the critical density of the Universe at the redshifts of the clusters. Combining the Chandra results on the X-ray gas mass fraction and its apparent redshift dependence with recent measurements of the mean baryonic matter density in the Universe and the Hubble Constant determined from the Hubble Key Project, we obtain a tight constraint on the mean total matter density of the Universe, Omega_m = 0.30^{+0.04}_{-0.03}, and measure a positive cosmological constant, Omega_Lambda = 0.95^{+0.48}_{-0.72}. Our results are in good agreement with recent, independent findings based on analyses of anisotropies in the cosmic microwave background radiation, the properties of distant supernovae, and the large-scale distribution of galaxies.Comment: Accepted for publication in MNRAS Letters (6 pages, 3 figures
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