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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 271 records · Page 15
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NASA NTRS1995

Variability of accretion disks surrounding black holes: The role of inertial-acoustic mode instabilities

The global nonlinear time-dependent evolution of the inertial-acoustic mode instability in accretion disks surrounding black holes has been investigated. The viscous stress is assumed to be proportional to the gas pressure only, i.e., tau = alphap(sub g). It is found that an oscillatory nonsteady behavior exists in the inner regions of disks (r is less than 10r(sub g) where r(sub g) is the Schwarzschild radius) for sufficiently large alpha(greater than or approximately equal to 0.2) and for mass accretion rates less than about 0.3 times the Eddington value. The variations of the integrated bolometric luminosity from the disk, Delta L/L, are less than 3%. A power spectrum analysis of these variations reveals a power spectrum which can be fitted to a power-law function of the frequency Pis proportional to f(exp -gamma), with index gamma = 1.4-2.3 and a low-frequency feature at about 4 Hz in one case. In addition, a narrow peak centered at a frequency corresponding to the maximum epicyclic frequency of the disk at approximately 100-130 Hz and its first harmonic is also seen. The low-frequency modulations are remarkably similar to those observed in black hole candidate systems. The possible existence of a scattering corona in the inner region of the disk and/or other processes contributing to the power at high frequencies in the inner region of the accretion disk may make the detection of the high-frequency component difficult.

Chen, Xingming↗
NASA NTRS1993

Postlaunch calibration of spacecraft attitude instruments

The accuracy of both onboard and ground attitude determination can be significantly enhanced by calibrating spacecraft attitude instruments (sensors) after launch. Although attitude sensors are accurately calibrated before launch, the stresses of launch and the space environment inevitably cause changes in sensor parameters. During the mission, these parameters may continue to drift requiring repeated on-orbit calibrations. The goal of attitude sensor calibration is to reduce the systematic errors in the measurement models. There are two stages at which systematic errors may enter. The first occurs in the conversion of sensor output into an observation vector in the sensor frame. The second occurs in the transformation of the vector from the sensor frame to the spacecraft attitude reference frame. This paper presents postlaunch alignment and transfer function calibration of the attitude sensors for the Compton Gamma Ray Observatory (GRO), the Upper Atmosphere Research Satellite (UARS), and the Extreme Ultraviolet Explorer (EUVE).

Davis, W.↗
NASA NTRS2002

Chandra Observations of the Anomalous X-ray Pulsar 4U 0142+61

We present X-ray imaging, timing, and phase resolved spectroscopy of the anomalous X-ray pulsar 4U 0142+61 using the Chandra X-ray Observatory. The spectrum is well described by a power law plus blackbody model with Gamma = 3.35(2), kT=0.458(3) keV, and N-H = 0.91(2) x 10(exp 22)/sq cm); we find no significant evidence for spectral features (0.5 - 7.0 keV). Time resolved X-ray spectroscopy shows evidence for evolution in phase in either Gamma, or kT or some combination thereof as a function of pulse phase. We derive a precise X-ray position for the source and determine its spin period, P=8.68866(30) s. We have detected emission beyond 4 arcsec from the central source and extending beyond 100 arcsec, likely due to dust scattering in the interstellar medium.

Patel, Sandeep K.↗
NASA NTRS2008

Implications for High Energy Blazar Spectra from Intergalactic Absorption Calculations

Given a knowledge of the density spectra intergalactic low energy photons as a function of redshift, one can derive the intrinsic gamma-ray spectra and luminosities of blazars over a range of redshifts and look for possible trends in blazar evolution. Stecker, Baring & Summerlin have found some evidence hinting that TeV blazars with harder spectra have higher intrinsic TeV gamma-ray luminosities and indicating that there may be a correlation of spectral hardness and luminosity with redshift. Further work along these lines, treating recent observations of the blazers lES02291+200 and 3C279 in the TeV and sub-TeV energy ranges, has recently been explored by Stecker & Scully. GLAST will observe and investigate many blazars in the GeV energy range and will be sensitive to blazers at higher redshifts. I examine the implications high redshift gamma-ray absorption for both theoretical and observational blazer studies.

Stecker, F↗
NASA NTRS2011

Aircraft Lightning Electromagnetic Environment Measurement

This paper outlines a NASA project plan for demonstrating a prototype lightning strike measurement system that is suitable for installation onto research aircraft that already operate in thunderstorms. This work builds upon past data from the NASA F106, FAA CV-580, and Transall C-180 flight projects, SAE ARP5412, and the European ILDAS Program. The primary focus is to capture airframe current waveforms during attachment, but may also consider pre and post-attachment current, electric field, and radiated field phenomena. New sensor technologies are being developed for this system, including a fiber-optic Faraday polarization sensor that measures lightning current waveforms from DC to over several Megahertz, and has dynamic range covering hundreds-of-volts to tens-of-thousands-of-volts. A study of the electromagnetic emission spectrum of lightning (including radio wave, microwave, optical, X-Rays and Gamma-Rays), and a compilation of aircraft transfer-function data (including composite aircraft) are included, to aid in the development of other new lightning environment sensors, their placement on-board research aircraft, and triggering of the onboard instrumentation system. The instrumentation system will leverage recent advances in high-speed, high dynamic range, deep memory data acquisition equipment, and fiber-optic interconnect.

Ely, Jay J.↗
NASA NTRS1995

Time-dependent clustering analysis of the second BATSE gamma-ray burst catalog

A time-dependent two-point correlation-function analysis of the Burst and Transient Source Experiment (BATSE) 2B catalog finds no evidence of burst repetition. As part of this analysis, we discuss the effects of sky exposure on the observability of burst repetition and present the equation describing the signature of burst repetition in the data. For a model of all burst repetition from a source occurring in less than five days we derive upper limits on the number of bursts in the catalog from repeaters and model-dependent upper limits on the fraction of burst sources that produce multiple outbursts.

Brainerd, J. J.↗
NASA NTRS1994

Redshift distortions of galaxy correlation functions

To examine how peculiar velocities can affect the two-, three-, and four-point redshift correlation functions, we evaluate volume-average correlations for configurations that emphasize and minimize redshift distortions for four different volume-limited samples from each of the CfA, SSRS, and IRAS redshift catalogs. We present the results as the correlation length r(sub 0) and power index gamma of the two-point correlations, bar-xi(sub 0) = (r(sub 0)/r)(exp gamma), and as the hierarchical amplitudes of the three- and four-point functions, S(sub 3) = bar-xi(sub 3)/bar-xi(exp 2)(sub 2) and S(sub 4) = bar-xi(sub 4)/bar-xi(exp 3)(sub 2). We find a characteristic distortion for bar-xi(sub 2), the slope gamma is flatter and the correlation length is larger in redshift space than in real space; that is, redshift distortions 'move' correlations from small to large scales. At the largest scales (up to 12 Mpc), the extra power in the redshift distribution is compatible with Omega(exp 4/7)/b approximately equal to 1. We estimate Omega(exp 4/7)/b to be 0.53 +/- 0.15, 1.10 +/- 0.16, and 0.84 +/- 0.45 for the CfA, SSRS, and IRAS catalogs. Higher order correlations bar-xi(sub 3) and bar-xi(sub 4) suffer similar redshift distortions but in such a way that, within the accuracy of our ananlysis, the normalized amplitudes S(sub 3) and S(sub 4) are insensitive to this effect. The hierarchical amplitudes S(sub 3) and S(sub 4) are constant as a function of scale between 1 and 12 Mpc and have similar values in all samples and catalogs, S(sub 3) approximately equal to 2 and S(sub 4) approximately equal to 6, despite the fact that bar-xi(sub 2), bar-xi(sub 3), and bar-xi(sub 4) differ from one sample to another by large factors (up to a factor of 4 in bar-xi(sub 2), 8 for bar-xi(sub 3), and 12 for bar-xi(sub 4)). The agreement between the independent estimations of S(sub 3) and S(sub 4) is remarkable given the different criteria in the selection of galaxies and also the difference in the resulting range of densities, luminosities, and locations between samples.

Fry, J. N.↗
NASA NTRS2020

Comprehensive Radiation Testing of Uncooled, Free Space Coupled, InGaAs Quad Photoreceivers

We have comprehensively tested uncooled, free space coupled, InGaAs Quad Photoreceivers having 0.5mm, 1mm, and 2mm diameter integrated with a low noise transimpedance amplifier (TIA)using 30MeV Protons, 100MeV Protons, 662keV Gamma Rays, 1GeV/n Helium, and 1GeV/n Iron at room temperature of ~20°C.These devices find multiple applications in space for differential wave front sensing as part of a Gravitational Wave Observatory, as well as instrumentation and control for next generation space telescopes. The bandwidth of all receivers was 20MHz which was TIA limited. All 0.5mm and 1mm devices were found to be fully functional at normal operating conditions and at room temperature for Protons, Gamma Rays, 1GeV/n Helium, and 1GeV/n Iron. Only one quadrant of a 2mm In GaAs Quad had hard failure due to 1GeV/n Helium Ions; otherwise it too survived all other radiation tests. Detailed test results follow in the manuscript including recommendations for future space flights. These radiation test results, combined with the earlier successful mechanical shock and vibration testing mean these devices have passed preliminary testing for space qualification.

Abhay M Joshi↗
NASA NTRS2000

The BATSE Gamma-Ray Burst E-Peak Distribution

Gamma-ray burst observed by BATSE are found to have approximately the same characteristic energy, denoted as E_p. We examine whether instrumental effects can give rise to this observation. We simulate the derivation of E-p and determine that the values in the BATSE sample are accurate and complete above a minimum fluence. We simulate the triggering of BATSE on gamma-ray bursts, deriving the efficiency of detecting bursts as a function of characteristic energy. From this simulation, we model the observed E_p distribution function expected when the intrinsic distribution function is a power-law. We find that this distribution produces poor fits to the observations. We find that a log-normal intrinsic distribution with a power-law tail gives a good fit to the data. From these fits, we conclude that instrumental effects cannot produce the observed E_p distribution, and that the observed distribution is a consequence of a narrow intrinsic distribution of E_p in gamma-ray bursts.

Brainerd, Jerome J.↗
NASA NTRS2003

Gamma-Ray Bursts and Cosmology

The unrivalled, extreme luminosities of gamma-ray bursts (GRBs) make them the favored beacons for sampling the high redshift Universe. To employ GRBs to study the cosmic terrain -- e.g., star and galaxy formation history -- GRB luminosities must be calibrated, and the luminosity function versus redshift must be measured or inferred. Several nascent relationships between gamma-ray temporal or spectral indicators and luminosity or total energy have been reported. These measures promise to further our understanding of GRBs once the connections between the luminosity indicators and GRB jets and emission mechanisms are better elucidated. The current distribution of 33 redshifts determined from host galaxies and afterglows peaks near z $\sim$ 1, whereas for the full BATSE sample of long bursts, the lag-luminosity relation predicts a broad peak z $\sim$ 1--4 with a tail to z $\sim$ 20, in rough agreement with theoretical models based on star formation considerations. For some GRB subclasses and apparently related phenomena -- short bursts, long-lag bursts, and X-ray flashes -- the present information on their redshift distributions is sparse or entirely lacking, and progress is expected in Swift era when prompt alerts become numerous.

Norris, Jay P.↗
NASA NTRS1975

An upper limit on interstellar C IV in the spectrum of gamma-2 Velorum

An upper limit on the column density of C IV along the line of sight to gamma-2 Vel is derived from upper limits placed on the equivalent widths of the interstellar C IV doublet with rest wavelengths at 1548.20 A and 1550.77 A. A lower limit of 250,000 K is calculated for the electron temperature of O VI emitting regions by combining the C IV results with a measurement of the column density of interstellar O VI for the same star and using calculations for the relative ionization of some abundant elements as a function of electron temperature in a low-density plasma. Since gamma-2 Vel is in the central part of the Gum Nebula, the high temperature suggested by these results is shown to support the idea that a high-temperature phase of the interstellar medium, possibly maintained by supernova explosions, may exist.-

Lengyel-Frey, D.↗
NASA NTRS1992

ASpect: A new spectrum and line analysis package for IRAF

We examined several publicly available spectral analysis software packages looking for one with enough functionality and versatility to meet the analysis needs of astronomers during the next decade. None of those examined can satisfactorily support the wide variety of panchromatic science programs that are now becoming possible. Furthermore, we concluded that none of these packages can be simply modified to include critical functions because of their original (limited) designs. During the next two years we will write a new spectral analysis package, ASpect, that will incorporate the latest analysis techniques for astronomical spectra in all wavelength domains. The ASpect package has several functional requirements. It must operate on spectra from a wide variety of ground-based and space-based instruments spanning wavelengths from radio to gamma rays. It must accommodate non-linear dispersion relations. It must provide a variety of functions, individually or in combination, with which to fit spectral features and the continuum. It is vitally important that known bad data be masked and that, uncertainties be propagated throughout the calculations in order for astronomers to evaluate the reliability of results. Finally, this new package must provide a powerful, intuitive graphical user interface to handle the burden of data input/output (I/O), on-line 'help,' selection of relevant features for analysis, plotting and graphical interaction, and data base management--all in a comprehensible environment. We anticipate that ASpect will take the form of an external package in IRAF (such as the NOAO and STSDAS packages) and will be layered upon the IRAF virtual Operating System to make it available on as many platforms as possible, while making it resistant to changes in operating systems and compilers. Our choice of IRAF is motivated by its portability, its wide use within the astronomical community, and its rich set of existing user applications.

Hulbert, S. J.↗
NASA NTRS1988

A parameterization of the evaporation of rainfall

A general theoretical expression for the rainfall rate and the total evaporation rate as a function of the distance below cloud base is developed, and is then specialized to the gamma raindrop size distribution. The theoretical framework is used to analyze the data of Rosenfeld and Mintz (1988) on the radar observations of the rainfall rate as a function of the distance below cloud base, for rain falling from continental convective cells in central South Africa, obtaining a parameterization for the evaporation of rainfall.

Schlesinger, Michael E.↗
NASA NTRS1992

Gravitational lensing, time delay, and gamma-ray bursts

The probability distributions of time delay in gravitational lensing by point masses and isolated galaxies (modeled as singular isothermal spheres) are studied. For point lenses (all with the same mass) the probability distribution is broad, and with a peak at delta(t) of about 50 S; for singular isothermal spheres, the probability distribution is a rapidly decreasing function with increasing time delay, with a median delta(t) equals about 1/h month, and its behavior depends sensitively on the luminosity function of galaxies. The present simplified calculation is particularly relevant to the gamma-ray bursts if they are of cosmological origin. The frequency of 'recurrent' bursts due to gravitational lensing by galaxies is probably between 0.05 and 0.4 percent. Gravitational lensing can be used as a test of the cosmological origin of gamma-ray bursts.

Mao, Shude↗
NASA NTRS1974

On the robustness of a Bayes estimate

This paper examines the robustness of a Bayes estimator with respect to the assigned prior distribution. A Bayesian analysis for a stochastic scale parameter of a Weibull failure model is summarized in which the natural conjugate is assigned as the prior distribution of the random parameter. The sensitivity analysis is carried out by the Monte Carlo method in which, although an inverted gamma is the assigned prior, realizations are generated using distribution functions of varying shape. For several distributional forms and even for some fixed values of the parameter, simulated mean squared errors of Bayes and minimum variance unbiased estimators are determined and compared. Results indicate that the Bayes estimator remains squared-error superior and appears to be largely robust to the form of the assigned prior distribution.

Canavos, G. C.↗
NASA NTRS1978

Gravitational experiments on solar probe

A covariance analysis was performed for a solar probe trajectory which encounters the sun at four solar radii. The unknown parameters in the analysis are the six initial cartesian coordinates for the probe, six initial cartesian coordinates for the earth, the astronomical unit, the solar gravitational quadrupole coefficient and two post Newtonian meters (beta, gamma). Errors in the unknown parameters were computed as a function of standard errors on the radio tracking data and on the nongravitational forces which act on the probe. Results were obtained for several tracking geometries and for several orbital inclinations to the ecliptic. The analysis shows that the principal scientific result from the radio tracking of a solar probe would be the determination of the quadrupole moment, which would place a constraint on models of the solar interior.

Anderson, J. D.↗
NASA NTRS1986

Compton scattering in strong magnetic fields

The relativistic cross section for Compton scattering by electrons in strong magnetic fields is derived. The results confirm and extend earlier work which has treated only transitions to the lowest or first excited Landau levels. For the teragauss field strengths expected in neutron star magnetospheres, the relative rates for excited state transitions are found to be significant, especially for incident photon energies several times the cyclotron frequency. Since these transitions must result in the rapid emission of one or more cyclotron photons as well as the Compton-scattered photon, the scattering process actually becomes a photon 'splitting' mechanism which acts to soften hard photon spectra, and also provides a specific mechanism for populating higher Landau levels in the electron distribution function. The results should be significant for models of gamma-ray bursters and pulsating X-ray sources.

Daugherty, Joseph K.↗