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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Emissivities of strong ultraviolet lines

Using atomic data published during the last few years for Li to Mg sequence ions, we have carried out multilevel calculations over a range of electron pressures to obtain absolute intensities for spectral lines in the UV and EUV regions of the electromagnetic spectrum. Both the pressure dependence of the ionization balance and the dependence of line emissivities on the populations of metastable levels are taken into account. These line emissivities may be used to obtain plane-parallel model atmospheres for different solar transition region structures, and in general for other late-type stars which have observable lines over a wide range of excitation potentials.

Raymond, J. C.↗
NASA NTRS1989

Possible infrared signature of decaying particles

A scenario for massive decaying particles is proposed which accounts for the detection of the submillimeter isotropic radiation component of the IR background (Matsumoto et al., 1988). The peak of the decay photon spectrum in the scenario is at several microns. The observed spectrum of the IR background is compared to that of decay photons with degraded energy. Within the limits to the radiative decays of neutrinos from gamma-ray observations os SN 1987A, it is found that the observed spectrum of decay photons is consistent with the IR background spectrum for decaying particle masses in the range between 5 and 18 keV, lifetimes in the range 2-7 X 10 to the 10th s, and a photon branching range of 0.0004 and 0.0009.

Wang, Boqi↗
NASA NTRS1993

Simultaneous ROSAT, Ginga, VLA, IUE, and optical observations of the bright quasar H1821+643

Simultaneous optical, UV, and X-ray spectroscopy of the low-redshift high-luminosity QSO H1821+643 is presented in order to examine the connection between the optical UV bump and the soft X-ray excess. The correlation of the short-term variability in the optical/UV and soft X-ray band is examined, and the applicability of the accretion disk models to the simultaneous continuum energy distribution is tested. By fitting the continuum energy distribution with the thin accretion disk model, it was found that bare disk models cannot easily account for the emission in both the UV and the steep excess in the soft X-ray band. The intrinsic X-ray spectrum of H1821+643 exhibited a typical power-law slope above about 0.3 keV, as detected by ROSAT and Ginga. The first detection of H1821+643 in the radio band is reported. This radio-quiet QSO was found to have a substantial radio flux at 20 cm and a typical steep spectral slope.

Kolman, Michiel↗
NASA NTRS1995

Balloon-borne measurements of the SN 1987A hard X-ray continuum

SN 1987A hard X-ray continuum spectra obtained on 1987 October 29, 1988 April 9-10, and 1988 November 11 from balloon-flight measurments are presented. The spectra, spanning the energy range from 25 keV to 300 keV, have been analyzed using a detector response matrix inversion technique that converts the spectra form counts/s/sq cm keV to photons/s/sq cm keV allowing direct comparison with theoretical models. The results indicate that the bulk of the (56)Co is mixed moderately through the inner regions of the supernova envelope but they do not preclude the mixing of a small amount of the (56)Co farther out into the envelope necessary to account for the observed (56)Co line fluxes. The effect of the ratio (57)Co to (56)Co on the 1988 November 11 continuum spectrum is discussed.

Pendleton, Geoffrey N.↗
DOE OSTI2018

NuStar View of the Central Region of the Perseus Cluster

Located at the center of the Perseus cluster, 3C 84 is an extremely bright and nearby radio galaxy. Because of the strong diffuse thermal emission from the cluster in X-rays, the detailed properties and the origin of a power-law component from the central active galactic nucleus (AGN) remains unclear in the source. We report here the first NuSTAR observations of 3C 84. The source was observed for 24.2 and 32 ks on February 01 and 04, 2018, respectively. NuSTAR observations spectrally decompose the power-law AGN component above 10 keV. The power-law component dominates the spectrum above 20 keV with a photon index $\sim$1.9 and an energy flux F$_{20-30 keV}$ = 1.0 $\times$10$^{-11}$ erg cm$^{-2}$ s$^{-1}$, corresponding to an isotropic luminosity, $L_{20-30 keV}$ = 7.4$\times$10$^{42}$ erg s$^{-1}$. We discuss possible emitting sites for the power-law component. The expected thermal emission from the accretion disk is not hot enough to account for the hard X-rays detected from the source. Furthermore, wimilar X-ray and $γ$-ray photon indices and long-term flux variations, the absence of cutoff energy in the hard X-ray spectrum of the source, correlated hard X-ray flux and hardness ratio variations, and the similarity of optical-X-ray slope to blazar rather than Seyfert galaxies supports the hard X-ray power-law component originating from the jet.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Signature of collapsars as sources for high-energy neutrinos and r -process nuclei

If collapsars are sources for both high-energy (HE) neutrinos and r-process nuclei, then the profuse low-energy antineutrinos from the β decay of the newly synthesized nuclei can annihilate the HE neutrinos. Considering HE neutrinos produced at internal shocks induced by intermittent mildly magnetized jets, we show that such annihilation suppresses the overall HE neutrino spectrum at ≳300 TeV and produces a corresponding flavor composition of (F v e + $\overline{v}$e : F v μ + $\overline{v}$μ : F v r + $\overline{v}$r )* ≈ (1 : 10 : 1) at source. We find that the emergent HE neutrino flux can well fit the diffuse flux observed at IceCube if contributions from all similar sources are taken into account. Our results highlight the unique role of HE neutrinos in supporting collapsars as sources for r-process nuclei, and can be tested by detection of HE neutrinos from individual sources and accurate measurement of the diffuse HE neutrino flux spectrum and flavor composition.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Wall-resolved spectral cascade-transport turbulence model

A spectral cascade-transport model has been developed and applied to turbulent channel flows (Reτ= 550, 950, and 2000 based on friction velocity, uτ ; or ReδΜ= 8,500; 14,800 and 31,000, based on the mean velocity and channel half-width). This model is an extension of a spectral model previously developed for homogeneous single and two-phase decay of isotropic turbulence and uniform shear flows; and a spectral turbulence model for wall-bounded flows without resolving the boundary layer. Data from direct numerical simulation (DNS) of turbulent channel flow was used to help develop this model and to assess its performance in the 1D direction across the channel width. The resultant spectral model is capable of predicting the mean velocity, turbulent kinetic energy and energy spectrum distributions for single-phase wall-bounded flows all the way to the wall, where the model source terms have been developed to account for the wall influence. We implemented the model into the 3D multiphase CFD code NPHASE-CMFD and the latest results are within reasonable error of the 1D predictions.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2013

Optimisation of optical properties of a long-wavelength GaInNAs quantum-well laser diode

We report optimisation of optical properties of a strained GaInNAs/GaAs quantum-well laser, by taking into account the many-body effect theory and the bowing parameter. The theoretical transition energies and the GaInNAs bowing parameter are fitted into the photoluminescence spectrum of the GaInNAs quantum well, obtained in the experiment. The theoretical results for the photoluminescence spectrum and laser characteristics (light, current and voltage) exhibits a high degree of agreement with the experimental results. (lasers)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

MULTI-WAVELENGTH EMISSION FROM THE FERMI BUBBLE. III. STOCHASTIC (FERMI) RE-ACCELERATION OF RELATIVISTIC ELECTRONS EMITTED BY SNRs

We analyze the model of stochastic re-acceleration of electrons that are emitted by supernova remnants (SNRs) in the Galactic Disk and then propagate into the Galactic Halo in order to explain the origin of nonthermal (radio and gamma-ray) emission from Fermi bubbles (FB). We assume that the energy for re-acceleration in the Halo is supplied by shocks generated by processes of star accretion onto the central black hole. Numerical simulations show that regions with strong turbulence (places for electron re-acceleration) are located high up in the Galactic Halo several kpc above the disk. The energy of the SNR electrons that reach these regions does not exceed several GeV due to synchrotron and inverse Compton energy losses. At appropriate parameters of re-acceleration these electrons can be re-accelerated up to an energy of 10{sup 12} eV, which explains in this model the origin of the observed radio and gamma-ray emission from the FB. However, although the model gamma-ray spectrum is consistent with the Fermi results, the model radio spectrum is steeper than that observed by WMAP and Planck. If adiabatic losses due to plasma outflows from the Galactic central regions are taken into account, then the re-acceleration model nicely reproduces the Planck data points.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THE TURBULENCE VELOCITY POWER SPECTRUM OF NEUTRAL HYDROGEN IN THE SMALL MAGELLANIC CLOUD

We present the results of the Velocity Coordinate Spectrum (VCS) technique to calculate the velocity power spectrum of turbulence in the Small Magellanic Cloud (SMC) in 21 cm emission. We present an updated version of the VCS technique that takes into account regular motions, which is an important factor in our SMC VCS analysis. We have obtained a velocity spectral index of −3.85, a cold phase sonic Mach number of 5.6, and an injection scale of 2.3 kpc. The spectral index is steeper than the Kolmogorov index, which is expected for shock-dominated turbulence. The injection scale of 2.3 kpc suggests that HI supershells or tidal interactions with the Large Magellanic Cloud are the dominant drivers of turbulence in this dwarf galaxy. This implies that turbulence may be driven by multiple mechanisms in galaxies and that galaxy–galaxy interactions may play an important role in addition to supernova feedback.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Beyond the plane-parallel approximation for redshift surveys

Redshift -space distortions privilege the location of the observer in cosmological redshift surveys, breaking the translational symmetry of the underlying theory. This violation of statistical homogeneity has consequences for the modelling of clustering observables, leading to what are frequently called 'wide-angle effects'. We study these effects analytically, computing their signature in the clustering of the multipoles in configuration and Fourier space. We take into account both physical wide-angle contributions as well as the terms generated by the galaxy selection function. Similar considerations also affect the way power spectrum estimators are constructed. We quantify in an analytical way the biases that enter and clarify the relation between what we measure and the underlying theoretical modelling. The presence of an angular window function is also discussed. Motivated by this analysis, we present new estimators for the three dimensional Cartesian power spectrum and bispectrum multipoles written in terms of spherical Fourier-Bessel coefficients. In conclusion, we show how the latter have several interesting properties, allowing in particular a clear separation between angular and radial modes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

An acoustic investigation of the near-surface turbulence on Mars

The Perseverance rover is carrying out an original acoustic experiment on Mars: the SuperCam microphone records the spherical acoustic waves generated by laser sparks at distances from 2 m to more than 8 m. These N-shaped acoustic waves scatter from the multiple local heterogeneities of the turbulent atmosphere. Therefore, large and random fluctuations of sound travel time and intensity develop as the waves cross the medium. The variances of the travel times and the scintillation index (normalized variance of the sound intensity) are studied within the mathematical formalism of the propagation of spherical acoustic waves through thermal turbulence to infer statistical properties of the Mars atmospheric temperature fluctuation field. The comparison with the theory is made by simplifying assumptions that do not include wind fluctuations and diffraction effects. Two Earth years (about one Martian year) of observations acquired during the maximum convective period (10:00–14:00 Mars local time) show a good agreement between the dataset and the formalism: the travel time variance diverges from the linear Chernov solution exactly where the density of occurrence of the first caustic reaches its maximum. Moreover, on average, waves travel faster than the mean speed of sound due to a fast path effect, which is also observed on Earth. To account for the distribution of turbulent eddies, several power spectra are tested and the best match to observation is obtained with a generalized von Karman spectrum with a shallower slope than the Kolmogorov cascade, φ(k)∝(1 + k 2 L 2 ) -4/3 . It is associated with an outer scale of turbulence, L, of 11 cm at 2 m above the surface and a standard deviation of 6 K over 9 s for the temperature. These near-surface atmospheric properties are consistent with a weak to moderate wave scattering regime around noon with little saturation. Overall, this study presents an innovative and promising methodology to probe the near-surface atmospheric turbulence on Mars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Development of New Directions in Axion Dark Matter Searches

The bright objects we observe in the night sky make up less than 20% of the matter in the Universe. More than 80% of matter is invisible dark matter [1], which was postulated because the amount of visible matter in galaxy clusters could not account for the galaxies’ velocities [2]. Observations from cosmology and astrophysics that converge on the existence of dark matter include the cosmic microwave background power spectrum [3], cluster and galactic rotation curves [2], gravitational lensing [4,5] and large-scale structure formation [6]. The existence of dark matter is one of the biggest unsolved mysteries in physics, providing concrete evident for physics beyond the Standard Model (SM) of particle physics. Many particle candidates have been introduced, including weakly interacting massive particles (WIMPs) and ultralight bosons, such as the axion [7]. No candidates have been discovered, despite many searches, and the nature of dark matter is still unknown. The axion is a new fundamental spin-0 particle, which was originally motivated by explaining the strong charge-parity ( CP ) problem of quantum chromodynamics (QCD) [7], the theory of the strong nuclear force within nucleons. The CP -violating term in QCD within the SM is expected to generate a sizable neutron electric dipole moment (EDM), while the experimental upper bound is roughly one trillionth that size [8]. The strong CP problem is the non-observation of this neutron EDM. Discovery of axions would aid in understanding both the cosmological dark matter problem and the strong CP problem.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1974

Measurement of the X-ray emission of the solar atmosphere during a period of low activity

A large-area high-sensitivity X-ray spectrometer has been constructed and used to measure the 1.8- to 5.3-A X-ray emission of the sun under quiescent conditions. The instrument utilizes Bragg reflection from mosaic graphite crystals. The data indicate that the X-ray emission can best be accounted for by a multitemperature model of the solar atmosphere in which both the overall corona and active regions contribute to the X-ray spectrum. Theoretical calculations of the X-ray flux of a hot, optically thin plasma have been used to estimate the solar conditions at the time when the measurements were made.

Wolff, R. S.↗
NASA NTRS1975

Gamma ray astrophysics

Gamma-ray production processes are considered, taking into account Compton scattering between cosmic-ray electrons and starlight microwave background photons, synchrotron radiation, bremsstrahlung interactions, nucleon-antinucleon annihilations, the form of the spectrum from pion production at high energies, and gamma rays from photomeson production at ultrahigh energies. Attention is also given to gamma-ray absorption mechanisms, gamma rays observed from the Galaxy, and extragalactic gamma rays.

Stecker, F. W.↗
NASA NTRS1984

Analysis of the effect on combustor noise measurements of acoustic waves reflected by the turbine and combustor inlet

Spectral analyses of static pressure fluctuations measured in turbine engine combustors at low engine speed show good agreement with theory. At idle speed the high pressure turbine is unchoked. Above idle speed the turbine chokes and a significant change in the shape of the measured combustor pressure spectrum is observed. A simplified theoretical model of the acoustic pressure generated in the combustor due to the turbulence-flame front interaction did not account for acoustic waves reflected from the turbine. By retaining this simplified combustion noise source model and adding a partial reflecting plane at the turbine and combustor inlet, a simple theoretical model was developed that reproduces the undulations in the combustor fluctuating pressure spectra. Plots of the theoretical combustor fluctuating pressure spectra are compared to the measured pressure spectra obtained from the CF6-50 turbofan engine over a range of engine operating speeds. The simplified combustion noise theory when modified by a simple turbine reflecting plane adequately accounts for the changes in measured combustor pressure spectra. It is further concluded that the shape of the pressure spectra downstream of the turbine, neglecting noise generated by the turbine itself, will be the combustion noise spectra unchanged except for the level reduction due to the energy blocked by the turbine.

Huff, R. G.↗
NASA NTRS1984

Analysis of the effect on combustor noise measurements of acoustic waves reflected by the turbine and combustor inlet

Spectral analyses of static pressure fluctuations measured in turbine engine combustors at low engine speed show good agreement with theory. At idle speed the high pressure turbine is unchoked. Above idle speed the turbine chokes and a significant change in the shape of the measured combustor pressure spectrum is observed. A simplified theoretical model of the acoustic pressure generated in the combustor due to the turbulence-flame front interaction did not account for acoustic waves reflected from the turbine. By retaining this simplified combustion noise source model and adding a partial reflecting plane at the turbine and combustor inlet, a simple theoretical model was developed that reproduces the undulations in the combustor fluctuating pressure spectra. Plots of the theoretical combustor fluctuating pressure spectra are compared to the measured pressure spectra obtained from the CF6-50 turbofan engine over a range of engine operating speeds. The simplified combustion noise theory when modified by a simple turbine reflecting plane adequately accounts for the changes in measured combustor pressure spectra. It is further concluded that the shape of the pressure spectra downstream of the turbine, neglecting noise generated by the turbine itself, will be the combustion noise spectra unchanged except for the level reduction due to the energy blocked by the turbine.

Huff, R. G.↗