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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 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS2014

Dust and Gas in the Magellanic Clouds from the Heritage Herschel Key Project. I. Dust Properties and Insights into the Origin of the Submm (Submillimeter) Excess Emission

The dust properties in the Large and Small Magellanic Clouds are studied using the HERITAGE Herschel Key Project photometric data in five bands from 100 to 500 micromillimeters. Three simple models of dust emission were fit to the observations: a single temperature blackbody modified by a powerlaw emissivity (SMBB), a single temperature blackbody modified by a broken power-law emissivity (BEMBB), and two blackbodies with different temperatures, both modified by the same power-law emissivity (TTMBB). Using these models we investigate the origin of the submillimeter excess; defined as the submillimeter (submm) emission above that expected from SMBB models fit to observations < 200 micromillimeters. We find that the BEMBB model produces the lowest fit residuals with pixel-averaged 500 micromillimeters submillimeter excesses of 27% and 43% for the Large and Small Magellanic Clouds, respectively. Adopting gas masses from previous works, the gas-to-dust ratios calculated from our fitting results show that the TTMBB fits require significantly more dust than are available even if all the metals present in the interstellar medium (ISM) were condensed into dust. This indicates that the submillimeter excess is more likely to be due to emissivity variations than a second population of colder dust. We derive integrated dust masses of (7.3 plus or minus 1.7) x 10 (sup 5) and (8.3 plus or minus 2.1) x 10 (sup 4) solar masses for the Large and Small Magellanic Clouds, respectively. We find significant correlations between the submillimeter excess and other dust properties; further work is needed to determine the relative contributions of fitting noise and ISM physics to the correlations.

Magellanic↗
DOE OSTI2021

Learning curves for drug response prediction in cancer cell lines

Motivated by the size and availability of cell line drug sensitivity data, researchers have been developing machine learning (ML) models for predicting drug response to advance cancer treatment. As drug sensitivity studies continue generating drug response data, a common question is whether the generalization performance of existing prediction models can be further improved with more training data. We utilize empirical learning curves for evaluating and comparing the data scaling properties of two neural networks (NNs) and two gradient boosting decision tree (GBDT) models trained on four cell line drug screening datasets. The learning curves are accurately fitted to a power law model, providing a framework for assessing the data scaling behavior of these models. The curves demonstrate that no single model dominates in terms of prediction performance across all datasets and training sizes, thus suggesting that the actual shape of these curves depends on the unique pair of an ML model and a dataset. The multi-input NN (mNN), in which gene expressions of cancer cells and molecular drug descriptors are input into separate subnetworks, outperforms a single-input NN (sNN), where the cell and drug features are concatenated for the input layer. In contrast, a GBDT with hyperparameter tuning exhibits superior performance as compared with both NNs at the lower range of training set sizes for two of the tested datasets, whereas the mNN consistently performs better at the higher range of training sizes. Moreover, the trajectory of the curves suggests that increasing the sample size is expected to further improve prediction scores of both NNs. These observations demonstrate the benefit of using learning curves to evaluate prediction models, providing a broader perspective on the overall data scaling characteristics. A fitted power law learning curve provides a forward-looking metric for analyzing prediction performance and can serve as a co-design tool to guide experimental biologists and computational scientists in the design of future experiments in prospective research studies.

60 APPLIED LIFE SCIENCES↗
NASA NTRS1980

Cosmic ray studies with a gas Cherenkov counter in association with an ionization spectrometer

The results from a balloon-borne gas Cherenkov counter (threshold 16.5 GeV/nucleon) and an ionization spectrometer are presented. The gas Cherenkov counter provides an absolute energy distribution for the response of the calorimeter for 5 or = Z 26 nuclei of cosmic rays. The contribution of scintillation to the gas Cherenkov pulse height was obtained by independently selecting particles below the gas Cherenkov threshold using the ionization spectrometer. Energy spectra were derived by minimizing the chi squared between Monte Carlo simulted data and flight data. Best fit power laws, dN/dE = AE-gamma, were determined for C, N, O, Ne, Mg, and Si. The power laws, all consistent with E (-2.7) are not good fits to the data. A better fit is obtained using the spectrum derived from the spectrometer. The data from the ionization calorimeter and the gas Cherenkov are thus completely self-consistent.

Balasubrahmanyan, V. K.↗
NASA NTRS1980

Numerical study of baroclinic instability

The effect of a power law gravity field on baroclinic instability is examined with emphasis on the case of inverse fifth power gravity, since this is the power law produced when terrestrial gravity is simulated in spherical geometry by electrostatic means. Growth rates of unstable normal modes were obtained as a function of parameters of the problem by solving a second order differential equation numerically. Results are compared with those from an earlier study where gravity was a constant. The conclusion is that, over the range of parameter space explored here, there is no significant change in the character of theoretical regime diagrams if the vertically averaged gravity is used as a parameter.

Geisler, J. E.↗
NASA NTRS1986

X-ray spectra of BL Lacertae objects

An analysis of previous X-ray spectral measurements of BL Lac objects indicates that the ultraviolet-through-X-ray spectra have a two-component, concave-up power law structure and that the soft X-ray spectrum is an extrapolation of the ultraviolet power law. The X-ray intensities of these objects were found to be highly variable. Though no significant amount of cold, low ionization gas is found to be present in these objects, evidence is presented of intrinsic excess absorption that may be due to hot highly ionized gas.

Urry, C. Megan↗
NASA NTRS1990

Coarsening in binary solid-liquid mixtures

A theory of Ostwald ripening has been developed for a solid-liquid mixture cosisting of a low volume fraction array of spherical solid particles in a liquid wherein the coarsening process proceeds via the transport of both heat and mass. It is found that the simultaneous transport of heat and mass during ripening does not alter the exponents of the temporal power laws governing the ripening process from their classical values but does alter the amplitudes of these power laws. The growth rate of the cube of the average particle radius, the rate constant, is found to depend both on the alloy solute concentration and the ratio of the thermal to solutal diffusivities. In most metallic systems, a large decrease in the rate constant can be expected with small additions of solute to a pure metal. Possible extensions of this theory to the analogous problem of ripening in isothermal ternary alloys are also discussed.

Voorhees, P. W.↗
NASA NTRS1991

Monte-Carlo calculations of Ge detector escape-peak efficiencies

The authors present calculated first and second escape peak efficiencies for a variety of n-type coaxial Ge detectors using the Monte Carlo method. The results are given for right-circular cylindrical geometries of size ranging from 3-cm diameter x 3-cm thick to 8-cm diameter x 8-cm thick. For completeness, the intrinsic interaction and photopeak efficiencies for energies ranging from 20 keV to 20 MeV are listed. The calculations have been tested against experimental measurement and found to be in agreement to within the quoted errors (typically 10 percent). It is found that the ratio of the second escape peak efficiency to the first escape peak efficiency, R, is approximately independent of energy for small detector volumes but has a weak power law energy dependence for large volumes. It is shown that R also varies as an inverse power law of the active volume.

Owens, Alan↗
NASA NTRS1991

A Compton reflection model for the cosmic X-ray and gamma-ray backgrounds

The gamma-ray spectrum of an AGN in the Compton reflection model is calculated and the contribution of such sources to the cosmic gamma-ray background is considered. The spectrum is composed of three parts: an X-ray power law, a broad hump from roughly 10 to 120 keV due to reprocessing of the power-law X-ray spectrum by cold gas, and gamma rays which are due to the inverse-Compton scattering of X-rays. The contribution of such sources to the cosmic gamma-ray background is computed and it is found that they provide an excellent fit to the entire background spectrum from 3 keV to 10 MeV. Constraints on the physical conditions in these sources are derived based on these results and the possibility of observing such sources with GRO is considered.

Rogers, R. D.↗
NASA NTRS2013

Kepler Observations of Rapid Optical Variability in the BL Lac Object W2r192+42

We present the first Kepler monitoring of a strongly variable BL Lac, W2R1926+42. The light curve covers 181 days with approx. 0.2% errors, 30 minute sampling and >90% duty cycle, showing numerous delta-I/I > 25% flares over timescales as short as a day. The flux distribution is highly skewed and non-Gaussian. The variability shows a strong rms-flux correlation with the clearest evidence to date for non-linearity in this relation. We introduce a method to measure periodograms from the discrete autocorrelation function, an approach that may be well-suited to a wide range of Kepler data. The periodogram is not consistent with a simple power-law, but shows a flattening at frequencies below 7x10(exp -5) Hz. Simple models of the power spectrum, such as a broken power law, do not produce acceptable fits, indicating that the Kepler blazar light curve requires more sophisticated mathematical and physical descriptions than currently in use.

galaxies↗
NASA NTRS2013

The First FERMI-LAT Gamma-Ray Burst Catalog

In three years of observations since the beginning of nominal science operations in 2008 August, the Large Area Telescope (LAT) on board the Fermi Gamma-Ray Space Telescope has observed high-energy great than (20 MeV) gamma-ray emission from 35 gamma-ray bursts (GRBs). Among these, 28 GRBs have been detected above 100 MeV and 7 GRBs above approximately 20 MeV. The first Fermi-LAT catalog of GRBs is a compilation of these detections and provides a systematic study of high-energy emission from GRBs for the first time. To generate the catalog, we examined 733 GRBs detected by the Gamma-Ray Burst Monitor (GBM) on Fermi and processed each of them using the same analysis sequence. Details of the methodology followed by the LAT collaboration for the GRB analysis are provided. We summarize the temporal and spectral properties of the LAT-detected GRBs. We also discuss characteristics of LAT-detected emission such as its delayed onset and longer duration compared with emission detected by the GBM, its power-law temporal decay at late times, and the fact that it is dominated by a power-law spectral component that appears in addition to the usual Band model.

gamma-ray burst: general↗
NASA NTRS2019

Observing the Transient Pulsations of SMC X-1 with NuSTAR

We report on Nuclear Spectroscopic Telescope Array observations of transient pulsations in the neutron star X-ray binary SMCX-1. The transition from nonpulsing to pulsing states was not accompanied by a large change in flux.Instead, both pulsing and nonpulsing states were observed in a single observation during the low-flux super-orbital state. During the high state, we measure a pulse period of P=0.70117(9) s at Tref=56145 MJD. Spectral analysis during nonpulsing and pulsing states reveals that the observations can be consistently modeled by an absorbed power law with a phenomenological cutoff resembling a Fermi–Dirac distribution, or by a partially obscured cutoff power law. The shapes of the underlying continua show little variability between epochs, while the covering fraction and column density vary between super-orbital states. The strength of pulsations also varies, leading usto infer that the absence and reemergence of pulsations are related to changing obscuration, such as by a warped accretion disk. SMCX-1 is accreting near or above its Eddington limit, reaching an unabsorbed X-ray luminosity of LX(2–10 keV)5×1038 erg s1. This suggests that SMCX-1 may be a useful local analog to ultraluminous X-ray pulsars (ULXPs), which likewise exhibit strong variability in their pulsed fractions, as well as flux variability onsimilar timescales. In particular, the gradual pulse turn-on, which has been observed in M82X-2, is similar to the behavior we observe in SMCX-1. Thus we propose that pulse fraction variability of ULXPs may also be due tovariable obscuration.

Pike, Sean N.↗
DOE OSTI2025

Scaling laws in jet classification

We demonstrate the emergence of scaling laws in the benchmark top versus QCD jet classification problem in collider physics. Six distinct physically-motivated classifiers exhibit power-law scaling of the binary cross-entropy test loss as a function of training set size, with distinct power law indices. This result highlights the importance of comparing classifiers as a function of dataset size rather than for a fixed training set, as the optimal classifier may change considerably as the dataset is scaled up. We speculate on the interpretation of our results in terms of previous models of scaling laws observed in natural language and image datasets.

Batson, Joshua↗
NASA NTRS1989

Marine stratocumulus structure

Thirty-three Landsat TM scenes of California stratocumulus cloud fields were acquired as part of the FIRE Marine Stratocumulus Intensive Field Observations in July 1987. They exhibit a wide variety of stratocumulus structures. Analysis has so far focused upon the July 7 scene, in which aircraft from NASA, NCAR, and the British Meteorological Office repeatedly gathered data across a stratocumulus-fair weather cumulus transition. The aircraft soundings validate the cloud base temperature threshold determined by spatial coherence analysis of the TM thermal band. Brightness variations in the stratocumulus region exhibit a -5/3 power-law decrease of the wavenumber spectra for scales larger than the cloud thickness, about 200 m, changing to a -3 power at smaller scales. Observations by an upward-looking three-channel microwave radiometer on San Nicolas Island also show the -5/3 power-law in total integrated liquid water, suggesting that the largest-scale TM brightness variations are primarily due to variations in the liquid water. The Kolmogorov 5/3 power suggests that for some purposes liquid water in turbulent stratocumulus clouds may be treated as a passive scalar, simply reflecting variations in vertical velocity. This may be tested using the velocities measured by the aircraft.

Cahalan, Robert F.↗
DOE OSTI2023

Statistical characterization of a shock interacting with an inclined gas column

We present a statistical characterization of the interaction between a planar shock and a finite-diameter, cylindrical column of dense gas based on three-dimensional, large-eddy simulation results. In the simulation, the column of gas is initially inclined at an angle $\alpha _0$ with respect to the shock plane. Effects of the initial column angle on the mixing characteristics are examined at Mach number 2.0 for column incline angles $1^\circ$ , $5^\circ$ , $10^\circ$ and $30^\circ$ . Mean velocity profiles show that the column angle affects the gas velocity components in the vertical plane, but not in the spanwise direction. The gas undergoes higher initial upward acceleration at larger initial column incline angles. With time, the gas motion tends to become one-dimensional in the streamwise direction. Initially, velocity fluctuations are most intense within the interior of the column, but concentrate near the column leading edge over time. At high wavenumbers $\kappa$ , the turbulent kinetic energy spectra follow a power-law scaling of $\kappa ^{-1}$ . The structure functions of the mass fraction do not clearly demonstrate power-law scaling except at early times for $\alpha _0=30^\circ$ , manifesting overall trends very similar to those observed in earlier experiments. Probability distributions of the mass fraction show independence of the mean and the standard deviation of the mixed gas on $\alpha _0$ . The column angle was also found to have little effect on the mixing efficiency characterized by the molecular mixedness. Velocity components in the streamwise and transverse directions tend towards a bimodal distribution for larger $\alpha _{0}$ .

42 ENGINEERING↗
DOE OSTI2023

VarIabiLity seLection of AstrophysIcal sources iN PTF (VILLAIN): I. Structure function fits to 71 million objects

Light-curve variability is well-suited to characterizing objects in surveys with high cadence and a long baseline. This is especially relevant in view of the large datasets to be produced by the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST). We aim to determine variability parameters for objects in the Palomar Transient Factory (PTF) and explore differences between quasars (QSOs), stars, and galaxies. We relate variability and colour information in preparation for future surveys. We fit joint likelihoods to structure functions (SFs) of 71 million PTF light curves with a Markov chain Monte Carlo method. For each object, we assume a power-law SF and extract two parameters: the amplitude on timescales of one year, A, and a power-law index, γ. With these parameters and colours in the optical (Pan-STARRS1) and mid-infrared (WISE), we identify regions of parameter space dominated by different types of spectroscopically confirmed objects from SDSS. Candidate QSOs, stars, and galaxies are selected to show their parameter distributions. QSOs show high-amplitude variations in the R band, and the highest γ values. Galaxies have a broader range of amplitudes and their variability shows relatively little dependency on timescale. With variability and colours, we achieve a photometric selection purity of 99.3% for QSOs. Even though hard cuts in monochromatic variability alone are not as effective as seven-band magnitude cuts, variability is useful in characterizing object subclasses. Through variability, we also find QSOs that were erroneously classified as stars in the SDSS. We discuss perspectives and computational solutions in view of the upcoming LSST.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1975

Long-term differential energy spectrum for solar-flare iron-group particles

A long-term solar-flare differential energy spectrum for iron-group nuclei from approximately 0.1 to approximately 600 MeV/amu is derived from track density profile measurements in sample 64455 and sample 68815. Measurements from uneroded surfaces were obtained from quench crystals of plagioclase in 64455, and a Kr-81/Kr method indicates that the exposure age of this sample is 2,010,000 yrs. The power laws which best fit the normalized track density data are reported; the energy spectrum consists of two power law curves smoothly joined together which in turn are smoothly connected to a modulated galactic cosmic-ray spectrum. Standard track production versus depth profiles can be used to determine solar-flare track exposure ages and erosion rates for lunar samples.

Blanford, G. E.↗
NASA NTRS1980

Theoretical regime diagrams for thermally driven flows in a beta-plane channel in the presence of variable gravity

The effect of a power law gravity field on baroclinic instability is examined, with a focus on the case of inverse fifth power gravity, since this is the power law produced when terrestrial gravity is simulated in spherical geometry by a dielectric force. Growth rates are obtained of unstable normal modes as a function of parameters of the problem by solving a second order differential equation numerically. It is concluded that over the range of parameter space explored, there is no significant change in the character of theoretical regime diagrams if the vertically averaged gravity is used as parameter.

Geisler, J. E.↗
NASA NTRS1982

On the coronae of rapidly rotating stars. III - An improved coronal rotation-activity relation in late type dwarfs

While it has previously been claimed that X-ray luminosity, or surface flux, varies as a power of the rotational velocity in G dwarfs, it is demonstrated that a single power law does not provide an acceptable fit to all the data. The data are describable by a power law, with a break in slope near a 12-day period which occurs at the same rotational period as the bifurcation in Vaughan and Preston's (1980) chromospheric study of G dwarfs. It is speculated that the two breaks are both related and due to a change in the stellar dynamo mode, which, at a rotation period of 12 days, corresponds to an age of one billion years for single G dwarfs.

Walter, F. M.↗