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At least 613 records · Page 34
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NASA NTRS1994

The mass and dynamics of cD clusters with cooling flows. 1: ROSAT observations of A 496

As part of a program to determine the mass distribution of cD galaxy clusters with cooling flows, we obtained a ROSAT image of the cluster A 496. The image reveals sharply peaked emission centered on the cD galaxy. Both the peaked cooling flow emission and the more extended emission filling the cluster are centered on the cD galaxy to within 15 sec . The surface brightness profile is consistent with previous Einstein observations. We measure spatially resolved spectra for the X-ray emission, and find a significant decline in temperature in the innermost 2 min to 4 min. We also find a gradient in absorption due to cold neutral gas, with an excess above the neutral hydrogen column due to our own galaxy in the inner 4 min. The excess absorption, however, is far below previously reported values. The surface brightness profile and the spatially resolved temperature profile are indicative of a cooling flow in the cluster. Cooling flow models fit to the X-ray spectra in the innermost 2 min yield a mass flow rate of 59 solar mass yr(exp -1). The spatially resolved temperature and surface brightness profiles are used to derive the mass distribution of the cluster both in the hot, X-ray emitting plasma and in the unseen dark matter that binds the cluster. To a radius of 1.0 Mpc we find a total cluster mass of 3.44 x 10(exp 14) solar mass ; the X-ray emitting gas mass of 0.75 x 10(exp 14) solar mass to this radius comprises 16 percent of the total cluster mass.

Kriss, Gerard A.↗
NASA NTRS1994

Investigation of the stability and 1.0 MeV proton radiation resistance of commercially produced hydrogenated amorphous silicon alloy solar cells

The radiation resistance of commercial solar cells fabricated from hydrogenated amorphous silicon alloys is reported. A number of different device structures were irradiated with 1.0 MeV protons. The cells were annealing at 200 C. The annealing time was dependent on proton fluence. Annealing devices for one hour restores cell parameters or fluences below 1(exp 14) cm(exp -2); fluences above 1(exp 14) cm(exp -2) require longer annealing times. A parametric fitting model was used to characterize current mechanisms observed in dark I-V measurements. The current mechanisms were explored with irradiation fluence, and voltage and light soaking times. The thermal generation current density and quality factor increased with proton fluence. Device simulation shows the degradation in cell characteristics may be explained by the reduction of the electric field in the intrinsic layer.

Lord, Kenneth R., II↗
NASA NTRS1995

Exploratory flow visualization investigation of mast-mounted sights in presence of a rotor

A flow visualization investigation with a laser light sheet system was conducted on a 27-percent-scale AH-64 attack helicopter model fitted with two mast-mounted sights in the langley 14- by 22-foot subsonic tunnel. The investigation was conducted to identify aerodynamic phenomena that may have contributed to adverse vibration encountered during full-scale flight of the AH-64D apache/longbow helicopter with an asymmetric mast-mounted sight. Symmetric and asymmetric mast-mounted sights oriented at several skew angles were tested at simulated forward and rearward flight speeds of 30 and 45 knots. A laser light sheet system was used to visualize the flow in planes parallel to and perpendicular to the free-stream flow. Analysis of these flow visualization data identified frequencies of flow patterns in the wake shed from the sight, the streamline angle at the sight, and the location where the shed wake crossed the rotor plane. Differences in wake structure were observed between the sight configurations and various skew angles. Analysis of lateral light sheet plane data implied significant vortex structure in the wake of the asymmetric mast-mounted sight in the configuration that produced maximum in-flight vibration. The data showed no significant vortex structure in the wake of the asymmetric and symmetric configurations that produced no increase in in-flight adverse vibration.

Ghee, Terence A.↗
NASA NTRS1994

A new transient pulsar in the Small Magellanic Cloud with an unusual x-ray spectrum

This article reports the discovery of a luminous (3.5 x 10(exp 37) ergs/sec over the 0.2 to 2 keV band) transient X-ray pulsar in the Small Magellanic Cloud (SMC) with an extremely soft component to its X-ray spectrum. This is the first time that a spectrum of this type has been seen in this class of X-ray source. The pulse period is 2.7632 s, and the pulse modulation appears to vary with energy from nearly unpulsed in the low-energy band of the ROSAT Position Sensitive Proportional Counter (PSPC) (0.07 to 0.4 keV) to about 50% in the high-energy band (1.0 to 2.4 keV). The object, RX J0059.2-7138, also shows flickering variability in its X-ray emission on timescales of 50 to 100s. The pulse-phase-averaged PSPC X-ray spectrum can be well described by a two-component source model seen through an absorbing column density of approximately 10(exp 21) atoms cm(exp -2). One spectral component is a power law with photon index 2.4. The other component is significantly softer and can be described by either a steeply falling power law or a blackbody with a temperature KT(sub BB) approximately 35 eV. Ths component is transient, but evidently upulsed, and, for the blackbody model fits, requires a large bolometric luminosity: near, or even several times greater than, the Eddington luminosity for a 1.4 solar mass object. When these characteristics of its soft emission are considered, RX J0059.2-7138 appears quite similar to other X-ray sources in the magellanic Clouds, such as CAL 83, CAL 87, and RX J0527.8-6954, which show only extreme ultrasoft (EUS) X-ray spectra. The discovery of RX J0059.2-7138, a probably high-mass X-ray binary, clearly indicates that EUS spectra may arise from accretion-powered neutron-star X-ray sources. This result lends support to the idea that some of the 'pure' EUS sources may be shrouded low-mass X-ray binaries rather than accreting white dwarfs.

Hughes, John P.↗
NASA NTRS1994

Spectral evolution of a subclass of gamma-ray bursts observed by batse

Among the gamma-ray bursts (GRBs) observed by the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory we define a subclass of bursts based on similar morphology: a sharp rise followed by a longer decay time. About 7% of all the gamma-ray bursts observed by BATSE fall into this subclass. We study the spectral evolution of these bursts by fitting models to time-segmented burst spectra and find no clear distinction between the spectral evolutionary properties of this subclass and those of other bursts. Further, we study the high time resolution spectral evolution of this subclass of GRBs using their spectral hardness ratios. A majority of the bursts show hardness ratio leading the counting rate and also display a continuous hard to soft evolution. The time lag between the counting rate and the hardness ratio is found to be directly correlated with the rise time of the counting rate profile. We also find, for the first time, evidence for spectral variation in a timescale of 64 ms.

Bhat, P. N.↗
NASA NTRS1994

Hubble Space Telescope Medium Deep Survey. 2: Deconvolution of Wide Field Camera field galaxy images in the 13 hour + 43 deg field

We present isophotal profiles of six faint field galaxies from some of the first deep images taken for the Hubble Space Telescope (HST) Medium Deep Survey (MDS). These have redshifts in the range z = 0.126 to 0.402. The images were taken with the Wide Field Camera (WFC) in `parallel mode' and deconvolved with the Lucy method using as the point-spread function nearby stars in the image stack. The WFC deconvolutions have a dynamic range of 16 to 20 dB (4 to 5 mag) and an effective resolution approximately less than 0.2 sec (FWHM). The multiorbit HST images allow us to trace the morphology, light profiles, and color gradients of faint field galaxies down to V approximately equal to 22 to 23 mag at sub-kpc resolution, since the redshift range covered is z = 0.1 to 0.4. The goals of the MDS are to study the sub-kpc scale morphology, light profiles, and color gradients for a large samole of faint field galaxies down to V approximately equal to 23 mag, and to trace the fraction of early to late-type galaxies as function of cosmic time. In this paper we study the brighter MDS galaxies in the 13 hour + 43 deg MDS field in detail, and investigate to what extent model fits with pure exponential disks or a(exp 1/4) bulges are justified at V approximately less than 22 mag. Four of the six field galaxies have light profiles that indicate (small) inner bulges following r(exp 1/4) laws down to 0.2 sec resolution, plus a dominant surrounding exponential disk with little or no color gradients. Two occur in a group at z = 0.401, two are barred spiral galaxies at z = 0.179 and z = 0.302, and two are rather subluminous (and edge-on) disk galaxies at z = 0.126 and z = 0.179. Our deep MDS images can detect galaxies down to V, I approximately less than 25 to 26 mag, and demonstrate the impressive potential of HST--even with its pre-refurbished optics--to resolve morphological details in galaxies at cosmologically significant distances (v approximately less than 23 mag). Since the median redshift of these galaxies is approximately less than 0.4, the HST resolution allows us to study sub kpc size scales at the galaxy, which cannot be done with stable images over wide fields from the best ground-based sites.

Windhorst, R. A.↗
NASA NTRS1994

The evolution of X-ray clusters in a cold plus hot dark matter universe

We present the first self-consistently computed results on the evolution of X-ray properties of galaxy clusters in a cold + hot dark matter (CHDM) model. We have performed a hydrodynamic plus N-body simulation for the COBE-compatible CHDM model with standard mass components: Omega(sub hot) = 0.3, Omega (sub cold) = 0.6 and Omega(sub baryon) = 0.1 (h = 0.5). In contrast with the CDM model, which fails to reproduce the observed temperature distribution function dN/dT (Bryan et al. 1994b), the CHDM model fits the observational dN/dT quite well. Our results on X-ray luminosity are less firm but even more intriguing. We find that the resulting X-ray luminosity functions at redshifts z = 0.0, 0.2, 0.4, 0.7 are well fit by observations, where they overlap. The fact that both temperatures and luminosities provide a reasonable fit to the available observational data indicates that, unless we are missing some essential physics, there is neither room nor need for a large fraction of gas in rich clusters: 10% (or less) in baryons is sufficient to explain their X-ray properties. We also see a tight correlation between X-ray luminosity and gas temperature.

Bryan, Greg L.↗
NASA NTRS1993

Probing Venus's cloud structure with Galileo NIMS

The spectral image cubes obtained by the Near-Infrared Mapping Spectrometer (NIMS) on Galileo as it flew by Venus have been analyzed to constrain the vertical structure of the clouds, the nature of the aerosol particles, and the location and particle properties of the opacity variations responsible for high-contrast features observed in the near-infrared windows at 1.7 and 2.3 micrometers. A radiative transfer program was used to simulate mid-latitude curves of limb darkening at 3.7 micrometers. Best-fit models to these curves demonstrate that the upper clouds are dominated by mode 2 particles (r-bar = 1.0 micrometers), with a contribution of approximately 15% of opacity from mode 1 particles (r-bar = 0.3 micrometers). The low-latitude upper cloud is well represented by a dual scale-height model, with a particle scale height of approximately 1 km from an altitude of 61-63 km, and a scale height of approximately 6 km above this, up to the level where tau = 1 at approximately 71 km. This model also successfully simulates limb-darkening curves at 11.5 micrometers from the Pioneer Venus Orbiter Infrared Radiometer. Successful simulations of correlation plots of 1.7 vs 2.3 micrometers intensities reveal that mode 3 particles (r-bar = 3.65 micrometers) represent the dominant source of opacity in the lower and middle clouds, and that variation in total cloud opacity reflects chiefly the addition and removal of mode 3 particles near the cloud base. We find that the full spectrum of brightnesses at 1.7 and 2.3 micrometers implies that the total cloud optical depth varies from approximately 25 to approximately 40.

Grinspoon, D. H.↗
NASA NTRS1995

Long-term total solar irradiance variability during sunspot cycle 22

Total solar irradiance measurements from the 1984-1993 Earth Radiation Budget Satellite (ERBS) active cavity radiometer and 1978-1993 Nimbus 7 transfer cavity radiometer spacecraft experiments are analyzed to detect the presence of 11-, 22-, and 80-year irradiance variability components. The analyses confirmed the existence of a significant 11-year irradiance variability component, associated with solar magnetic activity and the sunspot cycle. The analyses also suggest the presence of a 22- or 80-year variability component. The earlier Nimbus 7 and Solar Maximum Mission (SMM) spacecraft irradiance measurements decreased approximately 1.2 and 1.3 W/sq m, respectively, between 1980 and 1986. The Nimbus 7 values increased 1.2 W/sq m between 1986 and 1989. The ERBS irradiance measurements increased 1.3 W/sq m during 1986-1989, and then decreased 0.4 W/sq m (at an annual rate of 0.14 W/sq. m/yr) during 1990-1993. Considering the correlations between ERBS, Nimbus 7, and SMM irradiance trends and solar magnetic activity, the total solar irradiance should decrease to minimum levels by 1997 as solar activity decreases to minimum levels, and then increase to maximum levels by the year 2000 as solar activity rises. The ERBS measurements yielded 165.4 +/- 0.7 W/sq m as the mean irradiance value with measurement accuracies and precisions of 0.2% and 0.02%, respectively. The ERBS mean irradiance value is within 0.2% of the 1367.4, 1365.9, and 1366.9 W/sq m mean values for the SMM, Upper Atmosphere Research Satellite (UARS), and Space Shuttle Atmospheric Laboratory for Applications and Science (ATLAS 1) Solar Constant (SOLCON) active cavity radiometer spacecraft experiments, respectively. The Nimbus 7 measurements yielded 1372.1 W/sq m as the mean value with a measurement accuracy of 0.5%. Empirical irradiance model fits, based upon 10.7 -cm solar radio flux (F10) and photometric sunspot index (PSI), were used to assess the quality of the ERBS, Numbus 7, SMM, and the UARS irradiance data sets and to identify irradiance variability trends which may be caused by drifts or shifts in the spacecraft sensor responses. Comparisons among the fits and measured irradiances indicate that the Nimbus 7 radiometer response shifted by a total of 0.8 W/sq m between September 1989 and April 1990 and that the ERBS and UARS radiometers each drifted approximately 0.5 W/sq m during the first 5 months in orbit.

Lee, Robert B., III↗
NASA NTRS1996

Dust in the small Magellanic cloud. 1: Interstellar polarization and extinction data

The typical extinction curve for the Small Magellanic Cloud (SMC), in contrast to that for the Galaxy, has no bump at 2175 A and has a steeper rise into the far ultraviolet. For the Galaxy the interpretation of the extinction and, therefore, the dust content of the interstellar medium has been greatly assisted by measurements of the wavelength dependence of the polarization. For the SMC no such measurements existed. Therefore, to further elucidate the dust properties in the SMC we have for the first time measured linear polarization with five colors in the optical region of the spectrum for a sample of reddened stars. For two of these stars, for which there were no existing UV spectrophotometric measurements, but for which we measured a relatively large polarization, we have also obtained data from the International Ultraviolet Explorer (IUE) in order to study the extinction. We also attempt to correlate the SMC extinction and polarization data. The main results are: the wavelength of maximum polarization, lambda(sub max), in the SMC is typically smaller than that in the Galaxy; however, AZC 456, which shows the UV extinction bump, has a lambda(sub max) typical of that in the Galaxy, but its polarization curve is narrower and its bump is shifted to shorter wavelengths as compared to the Galaxy; and from an analysis of both the extinction and polarization data it appears that the SMC has typically smaller grains than those in the Galaxy. The absence of the extinction bump in the SMC has generally been thought to imply a lower carbon abundance in the SMC compared to the Galaxy. We interpret our results to mean that te size distribution of the interstellar grains, and not only the carbon abundance, is different in the SMC as compared to the Galaxy. In Paper 2 we present dust model fits to these observations.

Magalhaes, A. M.↗
NASA NTRS1996

Isopycnic Phases and Structures in H2O/CO2/Ethoxylated Alcohol Surfactant Mixtures

Ternary mixtures of H2O and CO2 with ethoxylated alcohol (C(i)E(j)) surfactants can form three coexisting liquid phases at conditions where two of the phases have the same density (isopycnic phases). Isopycnic phase behavior has been observed for mixtures containing the surfactants C8E5, C10E6, and C12E6, but not for those mixtures containing either C4E1 or CgE3. Pressure-temperature (PT) projections for this isopycnic three-phase equilibrium were determined for H2O/CO2/C8E5 and H2O/CO2/C10E6 mixtures at temperatures from approximately 25 to 33 C and pressures between 90 and 350 bar. As a preliminary to measuring the microstructure in isopycnic three component mixtures, phase behavior and small angle neutron scattering (SANS) experiments were performed on mixtures of D2O/CO2/ n-hexaethyleneglycol monododecyl ether (C12E6) as a function of temperature (25-31 C), pressure (63.1-90.7 bar), and CO2 composition (0-3.9 wt%). Parameters extracted from model fits of the SANS spectra indicate that, while micellar structure remains essentially unchanged, critical concentration fluctuations increase as the phase boundary and plait point are approached.

Paulaitis, Michael E.↗
NASA NTRS1997

Coordinated XTE/EUVE Observations of Algol

We performed a coordinated RXTE observation of the prototypical eclipsing binary Algol in conjunction with EUVE and ASCA observations. Observing the x-ray spectrum of Algol with ASCA, EUVE, and RXTE provides critical, near-simultaneous constraints on the distribution of the hottest temperature plasma in the Agol system: this is required to unambiguously determine the coronal Fe abundance. After some initial shuffling of observatory schedules with the help of mission planners, the coordinated multiwavelength observation was successfully performed in February, 1996. Analysis of the data, including development of new, IDL-based global model fitting routines, was accomplished from mid-1996 to mid-1997. The results indicate that the initial estimates of [Fe/H] approximately 0.3 dex (Stern et al.) are confirmed, and the Algol high temperature plasma thus has an Fe deficiency relative to the (Anders and Grevesse) solar abundance.

Stern, Robert A.↗
NASA NTRS1997

Large Volcanic Rises on Venus

Large volcanic rises on Venus have been interpreted as hotspots, or the surface manifestation of mantle upwelling, on the basis of their broad topographic rises, abundant volcanism, and large positive gravity anomalies. Hotspots offer an important opportunity to study the behavior of the lithosphere in response to mantle forces. In addition to the four previously known hotspots, Atla, Bell, Beta, and western Eistla Regiones, five new probable hotspots, Dione, central Eistla, eastern Eistla, Imdr, and Themis, have been identified in the Magellan radar, gravity and topography data. These nine regions exhibit a wider range of volcano-tectonic characteristics than previously recognized for venusian hotspots, and have been classified as rift-dominated (Atla, Beta), coronae-dominated (central and eastern Eistla, Themis), or volcano-dominated (Bell, Dione, western Eistla, Imdr). The apparent depths of compensation for these regions ranges from 65 to 260 km. New estimates of the elastic thickness, using the 90 deg and order spherical harmonic field, are 15-40 km at Bell Regio, and 25 km at western Eistla Regio. Phillips et al. find a value of 30 km at Atla Regio. Numerous models of lithospheric and mantle behavior have been proposed to interpret the gravity and topography signature of the hotspots, with most studies focusing on Atla or Beta Regiones. Convective models with Earth-like parameters result in estimates of the thickness of the thermal lithosphere of approximately 100 km. Models of stagnant lid convection or thermal thinning infer the thickness of the thermal lithosphere to be 300 km or more. Without additional constraints, any of the model fits are equally valid. The thinner thermal lithosphere estimates are most consistent with the volcanic and tectonic characteristics of the hotspots. Estimates of the thermal gradient based on estimates of the elastic thickness also support a relatively thin lithosphere (Phillips et al.). The advantage of larger estimates of the thermal lithospheric thickness is that they provide an explanation for the apparently modest levels of geologic activity on Venus over the last half billion years.

Smrekar, Suzanne E.↗
NASA NTRS1994

Radiation Resistance Studies of Amorphous Silicon Alloy Photovoltaic Materials

The radiation resistance of commercial solar cells fabricated from hydrogenated amorphous silicon alloys was investigated. A number of different device structures were irradiated with 1.0 MeV protons. The cells were insensitive to proton fluences below 1E12 sq cm. The parameters of the irradiated cells were restored with annealing at 200 C. The annealing time was dependent on proton fluence. Annealing devices for one hour restores cell parameters for fluences below lE14 sq cm require longer annealing times. A parametric fitting model was used to characterize current mechanisms observed in dark I-V measurements. The current mechanisms were explored with irradiation fluence, and voltage and light soaking times. The thermal generation current density and quality factor increased with proton fluence. Device simulation shows the degradation in cell characteristics may be explained by the reduction of the electric field in the intrinsic layer.

Woodyard, James R.↗
NASA NTRS1998

Derivation of multi-dimensional ellipsoidal convex model for experimental data

This paper deals with determination of the best ellipsoidal model fitting the available limited experimental data. The problem is defined as that of finding the minimum volume ellipsoid containing all experimental data. A general transformation matrix for the rotation of N-dimensional coordinate system is first obtained by the Gramm-Schmidt orthogonalization procedure. The use of this matrix makes it possible to search in all possible directions to find an ellipsoid with a minimum volume. The general procedure is illustrated by examples in which the real data is utilized. An invariance property of the response with uncertain parameters of different physical nature is also discussed.

Zhu, L. P.↗
NASA NTRS1998

Low Luminosity States of the Black Hole Candidate GX 339-4: ASCA and Simultaneous Radio/RXTE Observations - 1

We discuss a series of observations of the black hole candidate GX 339-4 in low luminosity, spectrally hard states. We present spectral analysis of three separate archival Advanced Satellite for Cosmology and Astrophysics (ASCA) data sets and eight separate Rossi X-ray Timing Explorer (RXTE) data sets. Three of the RXTE observations were strictly simultaneous with 843 Mega Hertz and 8.3-9.1 Giga Hertz radio observations. All of these observations have (3-9 keV) flux approximately less than 10(exp-9) ergs s(exp-1) CM(exp -2). The ASCA data show evidence for an approximately 6.4 keV Fe line with equivalent width approximately 40 eV, as well as evidence for a soft excess that is well-modeled by a power law plus a multicolor blackbody spectrum with peak temperature approximately equals 150-200 eV. The RXTE data sets also show evidence of an Fe line with equivalent widths approximately equal to 20-1OO eV. Reflection models show a hardening of the RXTE spectra with decreasing X-ray flux; however, these models do not exhibit evidence of a correlation between the photon index of the incident power law flux and the solid angle subtended by the reflector. 'Sphere+disk' Comptonization models and Advection Dominated Accretion Flow (ADAF) models also provide reasonable descriptions of the RXTE data. The former models yield coronal temperatures in the range 20-50 keV and optical depths of r approximately equal to 3. The model fits to the X-ray data, however, do not simultaneously explain the observed radio properties. The most likely source of the radio flux is synchrotron emission from an extended outflow of extent greater than O(10 (exp7) GM/c2).

Wilms, Joern↗
NASA NTRS2000

The Hot Coronae of Pre-Main Sequence Stars

This grant supported the analysis of two RXTE observations of low mass pre-main sequence stars. The data were obtained without problem, and both sources were clearly detected. I analyzed the data at GSFC in July 1999 (it was necessary to defer the analysis because the sources are weak, and the background model was not adequate until recently). The data are good and, although there is still some tweaking of the background subtraction and model fitting remaining to be done, we expect to work up the data for publication this spring.

Walter, Fredrick M.↗
NASA NTRS2000

Imaging the Sunyaev-Zeldovich Effect in the High Redshift Galaxy Cluster MS1137+66

We present interferometric measurements of the Sunyaev-Zelldovich Effect (SZE) in MS1137+66, a distant galaxy cluster at a redshift of 0.78. The data were obtained in 1997 and 1998 at the Berkeley-Illinois-Maryland millimeter array using sensitive 28.5 GHz receivers optimized for imaging of the SZE, with a total on-source integration time of 87.8 hours. We discuss constraints derived from spherical "beta" model fits to the SZE data, place an upper limit on the strength of any possible radio point sources in the field, and compare the results with the x-ray data published by Donahue et al. in 1999.

Joy, M. K.↗