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At least 631 records · Page 35
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NASA NTRS1985

Chromosphere flare models

Further calculated results based on the F1 and F2 chromospheric models of Machado et al. (1980) are presented in addition to results from a model with enhanced temperatures relative to the weak-flare model F1 in the upper photosphere and low chromosphere, and from a model with enhanced temperatures relative to the strong flare model F2 in the upper chromosphere. The coupled equations of statistical equilibrium and radiative transfer for H, H(-), He I-II, C I-IV, Si I-II, Mg I-II, Fe, Al, O I-II, Na, and Ca II are solved, and the overall absorption and emission of radiation by lines throughout the spectrum are determined by means of a reduced set of opacities taken from a compilation of over 10 million lines. Semiempirical models show that the white light flare continuum may arise by extreme chromospheric overheating, as well as by an enhancement of the minimum temperature region.

Avrett, E. H.↗
NASA NTRS1991

SN 1991T - Gamma-Ray Observatory's first supernova?

Consideration is given to the explosion of the Type Ia supernova SN 1991T in the nearby galaxy NGC 4527 detected in gamma-ray lines by the recently launched GRO. The dominant gamma-line and continuum features of the new 'delayed detonation' model FDEFA1 are calculated and compared to those for standard deflagration models W7 and cdtg7. It is shown that there are many useful hard photon discriminants of the Type Ia explosion mechanism that can, in principle, be detected by the OSSE and COMPTEL instruments on the GRO. Either SN 1991T, if bright enough, or one of the several Type Ia supernovae expected to be within the GRO's range during its active life, may make it possible to settle the detonation/deflagration debate, verify the generic thermonuclear white dwarf model of Type Ia explosions, and calibrate the Type Ia B(max)/847 keV line flux ratio.

Burrows, Adam↗
NASA NTRS1992

Anisotropic line emission and the geometry of the broad-line region in active galactic nuclei

Attention is given to an escape probability-based formalism, including the interaction with a background opacity, which is used to predict line-beaming factors for broad-line region (BLR) clouds of AGN. Several computational details necessary to model gas exposed to an intense nonthermal continuum are outlined. It is found that all hydrogen lines are strongly inwardly beamed, and that C IV 1549 is moderately beamed. The implications of this for the geometry of the emission-line regions of AGN are discussed. Beaming is found to cause the source-cloud separation to be overestimated by variability studies (so that the radiation field at the BLR is even more intense than previously thought); it can even cause spherical geometries to have little instantaneous response to continuum changes.

Ferland, G. J.↗
NASA NTRS1992

Design sensitivity analysis and optimization tool (DSO) for sizing design applications

The DSO tool, a structural design software system that provides the designer with a graphics-based menu-driven design environment to perform easy design optimization for general applications, is presented. Three design stages, preprocessing, design sensitivity analysis, and postprocessing, are implemented in the DSO to allow the designer to carry out the design process systematically. A framework, including data base, user interface, foundation class, and remote module, has been designed and implemented to facilitate software development for the DSO. A number of dedicated commercial software/packages have been integrated in the DSO to support the design procedures. Instead of parameterizing an FEM, design parameters are defined on a geometric model associated with physical quantities, and the continuum design sensitivity analysis theory is implemented to compute design sensitivity coefficients using postprocessing data from the analysis codes. A tracked vehicle road wheel is given as a sizing design application to demonstrate the DSO's easy and convenient design optimization process.

Chang, Kuang-Hua↗
NASA NTRS1994

GHRS observations of cool, low-gravity stars. 1: The far-ultraviolet spectrum of alpha Orionis (M2 Iab)

We present far-UV (1200-1930 A) observations of the prototypical red supergiant star alpha Ori, obtained with the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST). The observations, obtained in both low- (G140L) and medium- (G160/200M) resolution modes, unamibiguously confirm that the UV 'continuum' tentatively seen with (IUE) is in fact a true continuum and is not due to a blend of numerous faint emission features or scattering inside the IUE spectrograph. This continuum appears to originate in the chromospheric of the star at temperatures ranging from 3000-5000 K, and we argue that it is not related to previously reported putative companions or to bright spots on the stellar disk. Its stellar origin is further confirmed by overlying atomic and molecular absorptions from the chromosphere and circumstellar shell. The dominant structure in this spectral region is due to nine strong, broad absorption bands of the fourth-positive A-X system of CO, superposed on this continuum in the 1300-1600 A region. Modeling of this CO absorption indicates that it originates in the circumstellar shell in material characterized by T = 500 K, N(CO) = 1.0 x 10(exp 18) per sq cm, and V(sub turb) = 5.0 km per sec. The numerous chromospheric emission features are attributed mostly to fluorescent lines of Fe II and Cr II (both pumped by Lyman Alpha) and S I lines, plus a few lines of O I, C I, and Si II. The O I and C I UV 2 multiplets are very deficient in flux, compared to both the flux observed in lines originating from common upper levels but with markedly weaker intrinsic strength (i.e., O I UV 146 and C I UV 32) and to the UV 2 line fluxes seen in other cool, less luminous stars. This deficiency appears to be caused by strong self-absorption of these resonance lines in the circumstellar shell and/or upper chromosphere of alpha Ori. Atomic absorption features, primarily due to C I and Fe II are clearly seen in the G160M spectrum centered near 1655 A. These Fe II features are formed at temperatures that can occur only in the chromosphere of the star and are clearly not photospheric or circumstellar in origin.

Carpenter, Kenneth G.↗
NASA NTRS1993

Multiwavelength monitoring of active galactic nuclei

Recent multiwavelength monitoring of active galactic nuclei (AGN), particularly with the IUE satellite, has produced extraordinay advances in our understanding of the energy-generation mechanism(s) in the central engine and of the structure of the surrounding material. Examples discussed here include both ordinary AGN and blazars (the collective name for highly variable, radio-loud AGN like BL Lac objects and Optically Violently Variable quasars). In the last decade, efforts to obtain single-epoch multiwavelength spectra led to fundamentally new models for the structure of AGN, involving accretion disks for AGN and relativistic jets for blazars. Recent extensions of multiwavelength spectroscopy into the temporal domain have shown that while these general pictures may be correct, the details were probably wrong. Campaigns to monitor Seyfert 1 galaxies like NGC 4151, NGC 5548 and Fairall 9 at infrared, optical, ultraviolet and X-ray wavelengths indicate that broad-emission line regions are stratified by ionization, density, and velocity; argue against a standard thin accretion disk model; and suggest that X-rays represent primary rather than reprocessed radiation. For blazars, years of radio monitoring indicated emission from an inhomogeneous synchrotron-emitting plasma, which could also produce at least some of the shorter-wavelength emission. The recent month-long campaign to observe the BL Lac object PKS 2155-304 has revealed remarkably rapid variability that extends from the infrared through the X-ray with similar amplitude and little or no discernible lag. This lends strong support to relativistic jet models and rules out the proposed accretion disk model for the ultraviolet-X-ray continuum.

Urry, C. M.↗
NASA NTRS1995

Radio properties of narrow-lined Seyfert 1 galaxies

We have observed seven narrow-linedd Seyfert 1 (NLS1) galaxies and one high-ionization Seyfert 2 galaxy with the Very Large Array (VLA). Combining these observations with published data, we summarize the radio properties of the NLS1 galaxies for which spectropolarimetry was reported by Goodrich. Fifteen of these 17 objects now have published radio observations of high sensitivity, and only nine of those have been detected. For a Hubble parameter of 75 km/s/Mpc, the 6 cm radio powers range from 10(exp 20) to 10(exp 23) W/Hz, within the range previously found for other types of Seyfert galaxy. The median radio size of the nine VLA-detected galaxies is no larger than 300 pc, similar to the median size found by Ulvestad & Wilson for a distance-limited sample of Seyfert galaxies. Of the six NLS1 galaxies known to have significant intrinsic optical polarization, three have measurable radio axes. Two of those three galaxies have radio major axes close to 90 deg from their polarization position angles, while the third has an inner radio axis that may be nearly parallel to the polarization position angle. The former relationship is expected for a Seyfert 1 in a unified model of Seyfert galaxies, assuming no intrinsic continuum polarization.

Ulvestad, James S.↗
NASA NTRS2004

Design of a Far-Infrared Spectrometer for Atmospheric Thermal Emission Measurements

Global measurements of far infrared emission from the upper troposphere are required to test models of cloud radiative forcing, water vapor continuum emission, and cooling rates. Spectra with adequate resolution can also be used for retrieving atmospheric temperature and humidity profiles, and yet there are few spectrally resolved measurements of outgoing longwave flux at wavelengths longer than 16 m. It has been difficult to make measurements in the far infrared due to the need for liquid-helium cooled detectors and large optics to achieve adequate sensitivity and bandwidth. We review design considerations for infrared Fourier transform spectrometers, including the dependence of system performance on basic system parameters, and discuss the prospects for achieving useful sensitivity from a satellite platform with a lightweight spectrometer using uncooled detectors.

Johnson, David G.↗
NASA NTRS2008

Chandra X-ray Spectroscopy of the Focused Wind In the Cygnus X-1 System I. The Non-Dip Spectrum in the Low/Hard State

We present analyses of a 50 ks observation of the supergiant X-ray binary system CygnusX-1/HDE226868 taken with the Chandra High Energy Transmission Grating Spectrometer (HETGS). CygX-1 was in its spectrally hard state and the observation was performed during superior conjunction of the black hole, allowing for the spectroscopic analysis of the accreted stellar wind along the line of sight. A significant part of the observation covers X-ray dips as commonly observed for CygX-1 at this orbital phase, however, here we only analyze the high count rate non-dip spectrum. The full 0.5-10 keV continuum can be described by a single model consisting of a disk, a narrow and a relativistically broadened Fe K line, and a power law component, which is consistent with simultaneous RXTE broad band data. We detect absorption edges from overabundant neutral O, Ne and Fe, and absorption line series from highly ionized ions and infer column densities and Doppler shifts. With emission lines of He-like Mg XI, we detect two plasma components with velocities and densities consistent with the base of the spherical wind and a focused wind. A simple simulation of the photoionization zone suggests that large parts of the spherical wind outside of the focused stream are completely ionized, which is consistent with the low velocities (<200 km/s) observed in the absorption lines, as the position of absorbers in a spherical wind at low projected velocity is well constrained. Our observations provide input for models that couple the wind activity of HDE 226868 to the properties of the accretion flow onto the black hole.

Hanke, Manfred↗
NASA NTRS2011

Using Models for How Energetic Electrons Heat the Atmosphere During Flares

Using models for how energetic electrons heat the atmosphere during flares, we simulate the radiative-hydrodynamic response of the lower solar atmosphere to flare heating. The simulations account for much of the non-LTE, optically thick radiative transfer that occurs in the chromosphere. Our models predict an increase in white light continuum during the flare on the order of 20%, but this is highly sensitive to the electron beam flux used in the simulation. We find that a majority of the white light continuum originates in the chromosphere as a result of Balmer and Paschen recombinations, but a significant portion also forms in the photosphere which has been heated by radiative backwarming.

Allred, Joel↗
NASA NTRS2014

Spectral and Timing Nature of the Symbiotic X-Ray Binary 4U 1954+319: The Slowest Rotating Neutron Star in AN X-Ray Binary System

The symbiotic X-ray binary (SyXB) 4U 1954+319 is a rare system hosting a peculiar neutron star (NS) and an M-type optical companion. Its approx. 5.4 hr NS spin period is the longest among all known accretion-powered pulsars and exhibited large (is approx. 7%) fluctuations over 8 yr. A spin trend transition was detected with Swift/BAT around an X-ray brightening in 2012. The source was in quiescent and bright states before and after this outburst based on 60 ks Suzaku observations in 2011 and 2012. The observed continuum is well described by a Comptonized model with the addition of a narrow 6.4 keV Fe-K alpha line during the outburst. Spectral similarities to slowly rotating pulsars in high-mass X-ray binaries, its high pulsed fraction (approx. 60%-80%), and the location in the Corbet diagram favor high B-field (approx. greater than 10(exp12) G) over a weak field as in low-mass X-ray binaries. The observed low X-ray luminosity (10(exp33)-10(exp35) erg s(exp−1)), probable wide orbit, and a slow stellar wind of this SyXB make quasi-spherical accretion in the subsonic settling regime a plausible model. Assuming a approx. 10(exp13) G NS, this scheme can explain the approx. 5.4 hr equilibrium rotation without employing the magnetar-like field (approx. 10(exp16) G) required in the disk accretion case. The timescales of multiple irregular flares (approx. 50 s) can also be attributed to the free-fall time from the Alfv´en shell for a approx. 10(exp13) G field. A physical interpretation of SyXBs beyond the canonical binary classifications is discussed.

accretion disks aEuro" binaries: symbiotic aEuro" ↗
NASA NTRS2013

Suzaku View of the Neutron Star in the Dipping Source 4U 1822-37

The dipping X-ray source 4U 1822−37 was observed by Suzaku on 2006 Octrober 20 for a net exposure of 37 ks. The source was detected with the XIS at a 1-10 keV flux of 5.5 ×10(exp −10) erg per square centimeter per second, and with the HXD (HXD-PIN) at a 10-50 keV flux of 8.9 ×10(exp −10) erg per square centimeter per second. With HXD-PIN, the pulsation was detected at a barycentric period of 0.592437 seconds, and its change rate was reconfirmed as −2.43 × 10(exp −12) seconds per second. The 1-50 keV spectra of 4U 1822-37 were found to be very similar to those of Her X- 1 in the slopes, cutoff and iron lines. Three iron lines (Fe Kalpha, Fe XXV, and Fe XXVI) were detected, on top of a 1-50 keV continuum that is described by an NPEX model plus a soft blackbody. In addition, a cyclotron resonance scattering feature was detected significantly ( greater than 99% confidence), at an energy of 33+/-2 keV with a depth of 0.4(sup +0.6)/(sub −0.3). Therefore, the neutron star in this source is concluded to have a strong magnetic field of 2.8 × 10(exp 12) G. Further assuming that the source has a relatively high intrinsic luminosity of several times 10(exp 37) erg per second, its spectral and timing properties are consistently explained.

Stars: magnetic field↗
NASA NTRS2013

Measurements of Cyclotron Features and Pulse Periods in the High-Mass X-Ray Binaries 4U 1538-522 and 4U 1907+09 with the International Gamma-Ray Astrophysics Laboratory

We present a spectral and timing analysis of International Gamma-Ray Astrophysics Laboratory (INTEGRAL) observations of two high-mass X-ray binaries, 4U 1538−522 and 4U 1907+09. Our timing measurements for 4U 1538−522 find the pulse period to have exhibited a spin-up trend until approximately 2009, after which there is evidence for a torque reversal, with the source beginning to spin down to the most recently measured period of 525.407 plus or minus 0.001 seconds. The most recent INTEGRAL observations of 4U 1907+09 are not found to yield statistically significant pulse periods due to the significantly lower flux from the source compared with 4U 1538−522. A spectral model consisting of a power-law continuum with an exponential cutoff and modified by two cyclotron resonance scattering features is found to fit both sources well, with the cyclotron scattering features detected at approximately 22 and approximately 49 kiloelectronvolts for 4U 1538−522 and at approximately 18 and approximately 36 kiloelectronvolts for 4U 1907+09. The spectral parameters of 4U 1538−522 are generally not found to vary significantly with flux and there is little to no variation across the torque reversal. Examining our results in conjunction with previous work, we find no evidence for a correlation between cyclotron line energy and luminosity for 4U 1538−522. 4U 1907+09 shows evidence for a positive correlation between cyclotron line energy and luminosity, which would make it the fourth, and lowest luminosity, cyclotron line source to exhibit this relationship.

stars: magnetic field↗
NASA NTRS2018

In-Flight Calibration of Hitomi Soft X-Ray Spectrometer (3) Effective Area

We present the result of the in-flight calibration of the effective area of the Soft X-ray Spectrometer (SXS) on board the Hitomi X-ray satellite using an observation of the Crab nebula. We corrected for artifacts when observing high count rate sources with the X-ray microcalorimeter. We then constructed a spectrum in the 0.5–20 keV band, which we modeled with a single power-law continuum attenuated by interstellar extinction. We evaluated the systematic uncertainty of the spectral parameters by various calibration items. In the 2–12 keV band, the SXS result is consistent with the literature values in flux (2.20 ± 0.08 × 10(exp −8) erg s(exp-1) cm(exp −2) with a 1 σ statistical uncertainty) but is softer in the power-law index (2.19 ± 0.11). The discrepancy is attributable to the systematic uncertainty of about +6%/−7% and +2%/−5% respectively for the flux and the power-law index. The softer spectrum is affected primarily by the systematic uncertainty of the Dewar gate valve transmission and the event screening.

Tsujimoto, Masahiro↗
NASA NTRS2015

The Ly(alpha) Line Profiles of Ultraluminous Infrared Galaxies: Fast Winds and Lyman Continuum Leakage

We present new Hubble Space Telescope Cosmic Origins Spectrograph far-ultraviolet (far-UV) spectroscopy and Keck Echellete optical spectroscopy of 11 ultraluminous infrared galaxies (ULIRGs), a rare population of local galaxies experiencing massive gas inflows, extreme starbursts, and prominent outflows. We detect Ly(alpha) emission from eight ULIRGs and the companion to IRAS09583+4714. In contrast to the P Cygni profiles often seen in galaxy spectra, the Ly(alpha) profiles exhibit prominent, blueshifted emission out to Doppler shifts exceeding −1000 km/s in three H II-dominated and two AGN-dominated ULIRGs. To better understand the role of resonance scattering in shaping the Ly(alpha) line profiles, we directly compare them to non-resonant emission lines in optical spectra. We find that the line wings are already present in the intrinsic nebular spectra, and scattering merely enhances the wings relative to the line core. The Ly(alpha) attenuation (as measured in the COS aperture) ranges from that of the far-UV continuum to over 100 times more. A simple radiative transfer model suggests the Ly(alpha) photons escape through cavities which have low column densities of neutral hydrogen and become optically thin to the Lyman continuum in the most advanced mergers. We show that the properties of the highly blueshifted line wings on the Ly(alpha) and optical emission-line profiles are consistent with emission from clumps of gas condensing out of a fast, hot wind. The luminosity of the Ly(alpha) emission increases nonlinearly with the ULIRG bolometric luminosity and represents about 0.1-1% of the radiative cooling from the hot winds in the H II-dominated ULIRGs.

Infrared↗
NASA NTRS1994

Rapid X-ray spectral variability in NGC 3227

We present preliminary results of an ASCA observation of the Seyfert 1 galaxy, NGC 3227. The source exhibits rapid X-ray amplitude and spectral variability, the flux below 2 keV varying by a factor of approximately 3 in approximately 10,000 s while the flux in the 2-10 keV band varies by a factor of approximately 2 in the same interval. The spectrum below approximately 1 keV shows complex structure compared to a simple power-law model. We argue that the simplest interpretation of the spectrum is in terms of a power-law continuum modified by absorption in a photoionized medium. Simple, static, ionized absorber models yield an ionization parameter of approximately 0.05 and column density approximately 3.6 x 10(exp 21)/sq cm. However, the data strongly indicate that the situation is much more complex than this. If the spectral variability is caused by a changing ionization state of the absorber, then both the ionization state and column density are required to decrease as the intrinsic source luminosity increases. This does not have a simple physical interpretation. On the other hand, the data are also consistent with the spectral variability being due to changes in the intrinsic power-law index with little change in the ionization state of the absorber. This case could correspond to an absorber which is always in some average ionization state and the continuum variability is too fast for the absorber to deviate significantly from that, or the absorber could be in the form of an X-ray-heated wind.

Ptak, A.↗
NASA NTRS1994

Distributed parameter modeling of repeated truss structures

A new approach to find homogeneous models for beam-like repeated flexible structures is proposed which conceptually involves two steps. The first step involves the approximation of 3-D non-homogeneous model by a 1-D periodic beam model. The structure is modeled as a 3-D non-homogeneous continuum. The displacement field is approximated by Taylor series expansion. Then, the cross sectional mass and stiffness matrices are obtained by energy equivalence using their additive properties. Due to the repeated nature of the flexible bodies, the mass, and stiffness matrices are also periodic. This procedure is systematic and requires less dynamics detail. The first step involves the homogenization from a 1-D periodic beam model to a 1-D homogeneous beam model. The periodic beam model is homogenized into an equivalent homogeneous beam model using the additive property of compliance along the generic axis. The major departure from previous approaches in literature is using compliance instead of stiffness in homogenization. An obvious justification is that the stiffness is additive at each cross section but not along the generic axis. The homogenized model preserves many properties of the original periodic model.

Wang, Han-Ching↗
NASA NTRS2005

Multi-scale Rule-of-Mixtures Model of Carbon Nanotube/Carbon Fiber/Epoxy Lamina

A unidirectional carbon fiber/epoxy lamina in which the carbon fibers are coated with single-walled carbon nanotubes is modeled with a multi-scale method, the atomistically informed rule-of-mixtures. This multi-scale model is designed to include the effect of the carbon nanotubes on the constitutive properties of the lamina. It included concepts from the molecular dynamics/equivalent continuum methods, micromechanics, and the strength of materials. Within the model both the nanotube volume fraction and nanotube distribution were varied. It was found that for a lamina with 60% carbon fiber volume fraction, the Young's modulus in the fiber direction varied with changes in the nanotube distribution, from 138.8 to 140 GPa with nanotube volume fractions ranging from 0.0001 to 0.0125. The presence of nanotube near the surface of the carbon fiber is therefore expected to have a small, but positive, effect on the constitutive properties of the lamina.

Frankland, Sarah-Jane V.↗