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All sourcesNASA NTRSDOE OSTI
NASA NTRS2009

QPOs from Random X-ray Bursts around Rotating Black Holes

We continue our earlier studies of quasi-periodic oscillations (QPOs) in the power spectra of accreting, rapidly-rotating black holes that originate from the geometric 'light echoes' of X-ray flares occurring within the black hole ergosphere. Our present work extends our previous treatment to three-dimensional photon emission and orbits to allow for arbitrary latitudes in the positions of the distant observers and the X-ray sources in place of the mainly equatorial positions and photon orbits of the earlier consideration. Following the trajectories of a large number of photons we calculate the response functions of a given geometry and use them to produce model light curves which we subsequently analyze to compute their power spectra and autocorrelation functions. In the case of an optically-thin environment, relevant to advection-dominated accretion flows, we consistently find QPOs at frequencies of order of approximately kHz for stellar-mass black hole candidates while order of approximately mHz for typical active galactic nuclei (approximately equal to 10(exp 7) solar mass) for a wide range of viewing angles (30 degrees to 80 degrees) from X-ray sources predominantly concentrated toward the equator within the ergosphere. As in out previous treatment, here too, the QPO signal is produced by the frame-dragging of the photons by the rapidly-rotating black hole, which results in photon 'bunches' separated by constant time-lags, the result of multiple photon orbits around the hole. Our model predicts for various source/observer configurations the robust presence of a new class of QPOs, which is inevitably generic to curved spacetime structure in rotating black hole systems.

Kukumura, Keigo↗
NASA NTRS2004

The X-Ray Variable Sky

A novel approach to search for rapid time variability of every source, whether persistent or transient, within the entire ROSAT detector field has been developed. Semi-automated software statistically tests the photon arrival time event list for source variability and reports significant (greater than 5 sigma) variable enhancement in the photon count rate on time scales ranging from 5-500 seconds. The computational algorithms have been thoroughly tested, and are able to discriminate between false detections induced by instrumental effects, and real X-ray variable sources (XRVs); when carefully scrutinized by the PI. The analysis has proven to be successful, with both periodic and transient XRVs having been found. Some of these are entirely new discoveries, while others have been detected in previous X-ray missions, yet their nature remained elusive since source identification and analysis had yet to be performed. On average, one new (unidentified) XRV is found per ROSAT PSPC field analyzed. In some cases the XRVs show only a transient signature (a burst type event) with no persistent X-ray or other known (optical) counterpart. The ambitious goal of this project was to process as much of the ROSAT PSPC archive data as feasibly possible to discover XRVs and analyze this data.

Stringfellow, Guy↗
NASA NTRS1991

Young stellar objects detected by IRAS

Fluxes of 453 IRAS point sources selected for proximity to dense gas have been accurately determined by co-adding IRAS data. From this set, 258 sources are detected in all four bands: 12, 25, 60, and 100 microns. As distance from dense molecular material increases, these sources exhibit systematic trends in color and a markedly decreasing luminosity. These trends are broadly consistent with expected evolutionary changes in these stars, with younger objects found closer to dense gas. These data suggest a subtle variation on the criterion for identifying the youngest objects detectable by IRAS, in addition to that of steeply rising spectra: a color temperature which is uniform across all four IRAS bands in the range 40-50 K. These characteristics are used as a criterion to select point sources as candidate very young pre-main-sequence stars.

Clark, Frank O.↗
NASA NTRS2011

Estimated Accuracy of Three Common Trajectory Statistical Methods

Three well-known trajectory statistical methods (TSMs), namely concentration field (CF), concentration weighted trajectory (CWT), and potential source contribution function (PSCF) methods were tested using known sources and artificially generated data sets to determine the ability of TSMs to reproduce spatial distribution of the sources. In the works by other authors, the accuracy of the trajectory statistical methods was estimated for particular species and at specified receptor locations. We have obtained a more general statistical estimation of the accuracy of source reconstruction and have found optimum conditions to reconstruct source distributions of atmospheric trace substances. Only virtual pollutants of the primary type were considered. In real world experiments, TSMs are intended for application to a priori unknown sources. Therefore, the accuracy of TSMs has to be tested with all possible spatial distributions of sources. An ensemble of geographical distributions of virtual sources was generated. Spearman s rank order correlation coefficient between spatial distributions of the known virtual and the reconstructed sources was taken to be a quantitative measure of the accuracy. Statistical estimates of the mean correlation coefficient and a range of the most probable values of correlation coefficients were obtained. All the TSMs that were considered here showed similar close results. The maximum of the ratio of the mean correlation to the width of the correlation interval containing the most probable correlation values determines the optimum conditions for reconstruction. An optimal geographical domain roughly coincides with the area supplying most of the substance to the receptor. The optimal domain s size is dependent on the substance decay time. Under optimum reconstruction conditions, the mean correlation coefficients can reach 0.70 0.75. The boundaries of the interval with the most probable correlation values are 0.6 0.9 for the decay time of 240 h and 0.5 0.95 for the decay time of 12 h. The best results of source reconstruction can be expected for the trace substances with a decay time on the order of several days. Although the methods considered in this paper do not guarantee high accuracy they are computationally simple and fast. Using the TSMs in optimum conditions and taking into account the range of uncertainties, one can obtain a first hint on potential source areas.

Kabashnikov, Vitaliy P.↗
NASA NTRS2011

Power-Law Template for IR Point Source Clustering

We perform a combined fit to angular power spectra of unresolved infrared (IR) point sources from the Planck satellite (at 217,353,545 and 857 GHz, over angular scales 100 < I < 2200), the Balloonborne Large-Aperture Submillimeter Telescope (BLAST; 250, 350 and 500 microns; 1000 < I < 9000), and from correlating BLAST and Atacama Cosmology Telescope (ACT; 148 and 218 GHz) maps. We find that the clustered power over the range of angular scales and frequencies considered is well fit by a simple power law of the form C_l\propto I(sup -n) with n = 1.25 +/- 0.06. While the IR sources are understood to lie at a range of redshifts, with a variety of dust properties, we find that the frequency dependence of the clustering power can be described by the square of a modified blackbody, nu(sup beta) B(nu,T_eff), with a single emissivity index beta = 2.20 +/- 0.07 and effective temperature T_eff= 9.7 K. Our predictions for the clustering amplitude are consistent with existing ACT and South Pole Telescope results at around 150 and 220 GHz, as is our prediction for the effective dust spectral index, which we find to be alpha_150-220 = 3.68 +/- 0.07 between 150 and 220 GHz. Our constraints on the clustering shape and frequency dependence can be used to model the IR clustering as a contaminant in Cosmic Microwave Background anisotropy measurements. The combined Planck and BLAST data also rule out a linear bias clustering model.

Addison, Graeme E.↗
NASA NTRS1978

SS Cygni as a hard X-ray source identified with the scanning modulation collimator on HEAO 1

Hard X-ray emission from a 15 min by 1.5 deg area containing SS Cygni had previously been reported and identified with SS Cygni. This paper reports a 0.4 sq arc min measurement of the position of the X-ray source with the HEAO 1 modulation collimator experiment which leads to an unequivocal identification of SS Cygni as an X-ray emitter in the 2-10 keV energy range. The historical light curve in hard X-rays was also investigated by analyzing relevant Uhuru data, and two instances of possible source detection in the 2-6 keV band were noted. From Ariel 5 observations it is calculated that the X-ray luminosity of SS Cygni in the 2-10 keV energy range is of the order of 10 to the 33rd erg/sec in the optical low state and probably at least an order of magnitude less in the high state. By contrast, the soft X-ray luminosity increases by an order of magnitude between the optical low and high states. A coronal model or accretion onto the white dwarf can account for the inverse correlation of the soft and hard X-rays.

Fabbiano, G.↗
NASA NTRS1975

A balloon-borne Ge/Li/ spectrometer for gamma-ray astronomy

On June 10, 1974, a new high-resolution gamma ray spectrometer operating in the energy range of 0.05 to 10 MeV was successfully balloon-flight tested. This system uses four 40 cu cm Ge(Li) crystals in a single cryostat as prime sensors. A 6.35 cm thick CsI (Na) shield provides background rejection and collimation, defining a solid angle of 0.095 sr. Cooling is provided by a liquid-nitrogen dewar. The system was highly stable with spectral resolution of better than 2.5 keV at 1 MeV for the flight duration. The background total count rate was about 20 counts/sec. Numerous lines were observed, most of which originated from secondary neutron interactions in the spectrometer. The sensitivity of the system to line emissions originating in cosmic point sources is 0.0001 to 0.001 photons per sq cm s over the energy range.

Jacobson, A. S.↗
NASA NTRS1979

Pioneer Venus Star Sensor

The design predictions and orbital performance verification of the solid state star scanner used in the Onboard Attitude Control of the Pioneer Venus Orbiter and Multiprobe are presented. The star sensor extended the scanner use to active despin control, and it differs from previous sensors in solid state detection, redundant electronics for reliability, larger field of view, and large dynamic spin range. The star scanner hardware and design performance based on the ability to predict all noise sources, signal magnitudes, and expected detection probability are discussed. In-flight data collected to verify sensor ground calibration are tabulated and plotted in predicted accuracy curves. It is concluded that the Pioneer Venus Star Sensor has demonstrated predictable star calibration in the range of .1 magnitude uncertainties and usable star catalogs of 100 stars with very high probabilities of detection, which were much better than expected and well within the mission requirements.

Gutshall, R. L.↗
NASA NTRS1971

Voltage-programed control of turboalternator load

A power source and load-control concept was designed and tested for use with a Brayton cycle turboalternator. The concept provides power that adjusts to a range of loads while controlling alternator speed and isolating load-switching transients. The concept is also applicable to power-generating sources other than the Brayton cycle.

Young, R. N.↗
NASA NTRS1972

Remote sensing as a source of data for outdoor recreation planning

Specific data needs for outdoor recreation planning and the ability of tested remote sensors to provide sources for these data are examined. Data needs, remote sensor capabilities, availability of imagery, and advantages and problems of incorporating remote sensing data sources into ongoing planning data collection programs are discussed in detail. Examples of the use of imagery to derive data for a range of common planning analyses are provided. A selected bibliography indicates specific uses of data in planning, basic background materials on remote sensing technology, and sources of information on environmental information systems expected to use remote sensing to provide new environmental data of use in outdoor recreation planning.

Reed, W. E.↗
NASA NTRS1990

A radiological assessment of space nuclear power operations near Space Station Freedom

In order to accomplish NASA's more ambitious exploration goals, nuclear reactors may be used in the vicinity of Space Station Freedom (SSF) either as power sources for coorbiting platforms or as part of the propulsion system for departing and returning personnel or cargo vehicles. This study identifies ranges of operational parameters, such as parking distances and reactor cooldown times, which would reasonably guarantee that doses to the SSF crew from all radiation sources would be below guidelines recently recommended by the National Council of Radiation Protection and Measurements. The specific scenarios considered include: (1) the launch and return of a nuclear electric propulsion vehicle, (2) the launch and return of a nuclear thermal rocket vehicle, (3) the operation of an SP-100 class reactor on a coorbiting platform, (4) the activation of materials near operating reactors, (5) the storage and handling of radioisotope thermal generator units, and (6) the storage and handling of fresh and previously operated reactors. Portable reactor shield concepts were examined for relaxing the operational constraints imposed by unshielded (for human proximity operations) reactors and that might also be used to provide additional SSF crew protection from natural background radiation.

Stevenson, Steve↗
NASA NTRS1993

A large eddy simulation scheme for turbulent reacting flows

The recent development of the dynamic subgrid-scale (SGS) model has provided a consistent method for generating localized turbulent mixing models and has opened up great possibilities for applying the large eddy simulation (LES) technique to real world problems. Given the fact that the direct numerical simulation (DNS) can not solve for engineering flow problems in the foreseeable future (Reynolds 1989), the LES is certainly an attractive alternative. It seems only natural to bring this new development in SGS modeling to bear on the reacting flows. The major stumbling block for introducing LES to reacting flow problems has been the proper modeling of the reaction source terms. Various models have been proposed, but none of them has a wide range of applicability. For example, some of the models in combustion have been based on the flamelet assumption which is only valid for relatively fast reactions. Some other models have neglected the effects of chemical reactions on the turbulent mixing time scale, which is certainly not valid for fast and non-isothermal reactions. The probability density function (PDF) method can be usefully employed to deal with the modeling of the reaction source terms. In order to fit into the framework of LES, a new PDF, the large eddy PDF (LEPDF), is introduced. This PDF provides an accurate representation for the filtered chemical source terms and can be readily calculated in the simulations. The details of this scheme are described.

Gao, Feng↗
NASA NTRS2011

The LBA Calibrator Survey of Southern Compact Extragalactic Radio Sources - LCS1

We present a catalogue of accurate positions and correlated flux densities for 410 flat-spectrum, compact extragalactic radio sources previously detected in the Australia Telescope 20 GHz (AT20G) survey. The catalogue spans the declination range [-90deg, -40deg] and was constructed from four 24-h very long baseline interferometry (VLBI) observing sessions with the Australian Long Baseline Array at 8.3 GHz. The VLBI detection rate in these experiments is 97 per cent, the median uncertainty of the source positions is 2.6 mas and the median correlated flux density on projected baselines longer than 1000 km is 0.14 Jy. The goals of this work are (1) to provide a pool of southern sources with positions accurate to a few milliarcsec, which can be used for phase-referencing observations, geodetic VLBI and space navigation; (2) to extend the complete flux-limited sample of compact extragalactic sources to the Southern hemisphere; and (3) to investigate the parsec-scale properties of high-frequency selected sources from the AT20G survey. As a result of this VLBI campaign, the number of compact radio sources south of declination -40deg which have measured VLBI correlated flux densities and positions known to milliarcsec accuracy has increased by a factor of 3.5.

Petrov, Leonid↗
NASA NTRS2017

Chorus Whistler Wave Source Scales As Determined From Multipoint Van Allen Probe Measurements

Whistler mode chorus waves are particularly important in outer radiation belt dynamics due to their key role in controlling the acceleration and scattering of electrons over a very wide energy range. The key parameters for both nonlinear and quasi-linear treatment of wave-particle interactions are the temporal and spatial scales of the wave source region and coherence of the wave field perturbations. Neither the source scale nor the coherence scale is well established experimentally, mostly because of a lack of multipoint VLF waveform measurements. We present an unprecedentedly long interval of coordinated VLF waveform measurements (sampled at 16384 s(exp -1)) aboard the two Van Allen Probes spacecraft-9 h (0800-1200 UT and 1700-2200 UT) during two consecutive apogees on 15 July 2014. The spacecraft separations varied from about 100 to 5000 km (mostly radially); measurements covered an L shell range from 3 to 6; magnetic local time 0430-0900, and magnetic latitudes were approximately 15 and approximately 5 deg during the two orbits. Using time-domain correlation techniques, the single chorus source spatial extent transverse to the background magnetic field has been determined to be about 550-650 km for upper band chorus waves with amplitudes less than 100 pT and up to 800 km for larger amplitude, lower band chorus waves. The ratio between wave amplitudes measured on the two spacecraft is also examined to reveal that the wave amplitude distribution within a single chorus element generation area can be well approximated by a Gaussian exp(-0.5 x r (exp 2)/r(sub 0)(exp 2)), with the characteristic scale r(sub 0) around 300 km. Waves detected by the two spacecraft were found to be coherent in phase at distances up to 400 km.

Agapitov, O.↗
NASA NTRS2002

Description and Sensitivity Analysis of the SOLSE/LORE-2 and SAGE III Limb Scattering Ozone Retrieval Algorithms

The SOLSE (Shuttle Ozone Limb Sounding Experiment) and LORE (Limb Ozone Retrieval Experiment) instruments are scheduled for reflight on Space Shuttle flight STS-107 in July 2002. In addition, the SAGE III (Stratospheric Aerosol and Gas Experiment) instrument will begin to make limb scattering measurements during Spring 2002. The optimal estimation technique is used to analyze visible and ultraviolet limb scattered radiances and produce a retrieved ozone profile. The algorithm used to analyze data from the initial flight of the SOLSE/LORE instruments (on Space Shuttle flight STS-87 in November 1997) forms the basis of the current algorithms, with expansion to take advantage of the increased multispectral information provided by SOLSE/LORE-2 and SAGE III. We also present detailed sensitivity analysis for these ozone retrieval algorithms. The primary source of ozone retrieval error is tangent height misregistration (i.e., instrument pointing error), which is relevant throughout the altitude range of interest, and can produce retrieval errors on the order of 10-20 percent due to a tangent height registration error of 0.5 km at the tangent point. Other significant sources of error are sensitivity to stratospheric aerosol and sensitivity to error in the a priori ozone estimate (given assumed instrument signal-to-noise = 200). These can produce errors up to 10 percent for the ozone retrieval at altitudes less than 20 km, but produce little error above that level.

Loughman, R.↗
NASA NTRS1989

Characterization of a Ho:Tm:Cr:YAG laser with a Cr:GSAG laser as pumping source

Rare earth lasers were in existance since the first laser was developed. The primary lasing elements for the class of lasers in the infrared was neodymium and chromium. However, the need for eye safe lasers in the mid-infrared range has prompted an enormous amount of research to the use of other elements. Holmium was investigated extensively as the source of infrared radiation for atmospheric research as well as medical research. The results and procedure are briefly discussed.

Henderson, George W.↗
NASA NTRS1995

Simulation of proton radiation belt formation during the March 24, 1991 SSC

The rapid formation of a new proton radiation belt at L approximately = 2.5 following the March 24, 1991 Storm Sudden Commencement (SSC) observed at the Combined Release and Radiation Effects Satellite (CRRES) satellite is modeled using a relativistic guiding center test particle code. The SSC is modeled by a bipolar electric field and associated compression and relaxation in the magnetic field, superimposed on a dipole magnetic field. The source population consists of both solar and trapped inner zone protons. The simulations show that while both populations contribute to drift echoes in the 20-80 MeV range, primary conditions is from the solar protons. Proton acceleration by the SSC differs from relativistic electron acceleration in that different source populations contribute and nonrelativistic conservation of the first adiabatic invariation leads to greater energization of protons for a given decrease in L. Model drift echoes and flux distribution in L at the time of injection compare well with CRRES observations.

Hudson, M. K.↗
NASA NTRS1978

An anemometer for measuring velocities on the .25 to 4.25 M/sec range

A vane type anemometer was developed as a secondary standard for the on site calibration of wind velocity sensors. The instrument outputs an electrical pulse for each interruption of a light source by an eight vane fan. The pulse rate is proportional to the fan's angular speed and linearly related to wind velocity over the range 0.25 to 4.27 m/sec.

Carpini, T. D.↗