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NASA NTRS1986

Search for brown dwarfs in the IRAS data bases

The results of two searches for brown dwarfs using IRAS data bases are reviewed. From one survey, it is concluded that all of the 5776 high-latitude point sources in the IRAS Point Source Catalog can be positionally associated with stars or galaxies by simple comparisons with optical catalogs and atlases. Only one object near the north Galactic pole possesses an optical/IR color cool enough to be considered as a candidate; it is found to be an unusual carbon star of 1300 K with a probable distance far out in the Galactic halo. The more sensitive Serendipitous Survey, which detected objects about five times fainter than the other survey, also found no brown dwarfs. This survey found fewer ordinary stars than the number extrapolated from the less sensitive survey.

Low, F. J.↗

NASA NTRS1995

ROSAT observations of the unusual supernova remnant CTB 80 containing the pulsar PSR 1951 + 32

The unusal supernova remnant CTB, 80 containing the 39.5 ms pulsar PSR 1951 + 32, has been observed with the Position Sensitive Proportional Counter (PSPC) and the High Resolution Imager (HRI) aboard ROSAT. The HRI image, centered on the pulsar, is composed of a bright compact core of approximately 1 arcminute radius containing the pulsar and a compact nebula, as well as a diffuse nebula extending approximately 5 arcminutes eastward of the pulsar. The PSPC allowed us to model the spectra of the point source, the compact nebula and the 5 arcminute diffuse nebula. For a power-law spectrum with photon index Gamma approximately 2 and an interstellar column density of N(sub H) approximately 3 x 10(exp 21)/cm(exp 21) the derived luminosities are approximately 2.3 x 10(exp 33) d(sub 2.5)(exp 2) ergs/s from the pointlike source, approximately 3.9 x 10(exp 33) d(sup 2.5)(exp 2) ergs/s from the compact nebula, and approximately 1.8 x 10(exp 33) d(sub 2.5)(exp 2) ergs/s from the 5 arcminutes diffuse nebula. In addition, the 2 deg diameter circular field of view of the PSPC reveals a hard emission feature southeast of the pulsar with a conical geometry extending out to the edge of the detector. The spectrum from this region is well described by a two-temperature Raymond-Smith thermal plasma with an average temperature of approximately 10(exp 7) K and a luminosity of approximately 10(exp 34)d(sub 2.5)(exp 2) ergs/s. Pulsations from the 39.5 ms puslar, PSR 1951 + 32, are detected at the 99% confidence level. The implied pulsed fration is approximately 35% with a complicated energy-dependent behavior. The compact core and the extended diffuse nebula can be explained as synchrotron radiation from the relativistic pulsar wind confined by the ram pressure of the surrounding inhomgeneous medium. The conelike feature detected southeast of PSR 1951 + 32 is consistent with emission from an optically thin SNR in the radiative cooling phase of its evolution.

Safi-Harb, Samar↗

NASA NTRS1999

Design of a 2.5 Gbps Optical Transmitter for the International Space Station

A high data rate laser transmitter assembly (LTA) has been designed as the source for an optical free-space communication link between the International Space Station and the 1-meter Optical Communication Telescope Laboratory (OCTL) to be built at the Table Mountain Facility (IMF, Wrightwood, CA). The transmitter design concept uses a fiber-based master oscillator power amplifier (MOPA) configuration with an average output power of 200 made at a 1550 nm transmit wavelength. This transmitter source is also designed to provide a signal at 980 nm to the Silicon-based focal plane array for the point-ahead beam control function. This novel integration of a 980 nm boresight signal allows the use of Silicon based imagers for the acquisition/tracking and point-ahead functions, yet permits the transmit signal to be at any wavelength outside the Silicon sensitivity. The LTA, a sub-system of the Flight Terminal, has been designed to have a selectable data rate from 155 - 2500 Mbps in discrete steps. It also incorporates a 2.5 Gbps pseudo-random bit sequence (PRBS) generator for complete link testing and diagnostics. The design emphasizes using commercial off the shelf components (COTS).

Ortiz, Gerardo G.↗

NASA NTRS2015

Crosscutting Airborne Remote Sensing Technologies for Oil and Gas and Earth Science Applications

Airborne imaging spectroscopy has evolved dramatically since the 1980s as a robust remote sensing technique used to generate 2-dimensional maps of surface properties over large spatial areas. Traditional applications for passive airborne imaging spectroscopy include interrogation of surface composition, such as mapping of vegetation diversity and surface geological composition. Two recent applications are particularly relevant to the needs of both the oil and gas as well as government sectors: quantification of surficial hydrocarbon thickness in aquatic environments and mapping atmospheric greenhouse gas components. These techniques provide valuable capabilities for petroleum seepage in addition to detection and quantification of fugitive emissions. New empirical data that provides insight into the source strength of anthropogenic methane will be reviewed, with particular emphasis on the evolving constraints enabled by new methane remote sensing techniques. Contemporary studies attribute high-strength point sources as significantly contributing to the national methane inventory and underscore the need for high performance remote sensing technologies that provide quantitative leak detection. Imaging sensors that map spatial distributions of methane anomalies provide effective techniques to detect, localize, and quantify fugitive leaks. Airborne remote sensing instruments provide the unique combination of high spatial resolution (<1 m) and large coverage required to directly attribute methane emissions to individual emission sources. This capability cannot currently be achieved using spaceborne sensors. In this study, results from recent NASA remote sensing field experiments focused on point-source leak detection, will be highlighted. This includes existing quantitative capabilities for oil and methane using state-of-the-art airborne remote sensing instruments. While these capabilities are of interest to NASA for assessment of environmental impact and global climate change, industry similarly seeks to detect and localize leaks of both oil and methane across operating fields. In some cases, higher sensitivities desired for upstream and downstream applications can only be provided by new airborne remote sensing instruments tailored specifically for a given application. There exists a unique opportunity for alignment of efforts between commercial and government sectors to advance the next generation of instruments to provide more sensitive leak detection capabilities, including those for quantitative source strength determination.

hyperspectral↗

NASA NTRS1984

High Sensitivity, High Angular Resolution Far-infrared Photometry from the KAO

Most of the luminosity of embedded sources is reemitted in the far-infrared continuum. Measurements in the far-infrared are essential to understand the energetics of the interstellar medium, and of star formation regions in particular. Measurements from the KAO, are made in diffraction limited beams that sample a spatial scale considerably smaller than that given by IRAS. The KAO instrument technology has matured to the point that the single scan limiting flux of IRAS at 100 micro can be reached in a diffraction limited beam in a single typical KAO observing leg. The far-infrared photometer system and selections of recent observations are presented.

Lester, D.↗

NASA NTRS1992

OSSE spectral analysis techniques

Analysis of the spectra from the Oriented Scintillation Spectrometer Experiment (OSSE) is complicated because of the typically low signal to noise (approx. 0.1 percent) and the large background variability. The OSSE instrument was designed to address these difficulties by periodically offset-pointing the detectors from the source to perform background measurements. These background measurements are used to estimate the background during each of the source observations. The resulting background-subtracted spectra can then be accumulated and fitted for spectral lines and/or continua. Data selection based on various environmental parameters can be performed at various stages during the analysis procedure. In order to achieve the instrument's statistical sensitivity, however, it will be necessary for investigators to develop a detailed understanding of the instrument operation, data collection, and the background spectrum and its variability. A brief description of the major steps in the OSSE spectral analysis process is described, including a discussion of the OSSE background spectrum and examples of several observational strategies.

Purcell, W. R.↗

NASA NTRS2021

JWST/MIRI Simulated Imaging: Insights into Obscured Star Formation and AGNs for Distant Galaxies in Deep Surveys

The James Webb Space Telescope MIRI instrument will revolutionize extragalactic astronomy with unprecedentedsensitivity and angular resolution in mid-IR. Here we assess the potential of MIRI photometry to constrain galaxyproperties in the Cosmic Evolution Early Release Science(CEERS)survey. We derive estimated MIRIfluxes fromthe spectral energy distributions(SEDs)of real sources that fall in a planned MIRI pointing. We also obtain MIRIfluxes for hypothetical active galactic nucleus(AGN)–galaxy mixed models varying the AGN fractionalcontribution to the total IR luminosity(fracAGN). Based on these modelfluxes, we simulate CEERS imaging(3.6 hrexposure)in six bands from F770W to F2100W usingMIRISIMand reduce these data usingJWST PIPELINE.Weperform point-spread-function-matched photometry withTPHOTandfit the source SEDs withX-CIGALE,simultaneously modeling photometric redshift and other physical properties. Adding the MIRI data, the accuracyof both redshift and fracAGNis generally improved by factors of2 for all sources atz3. Notably, for pure-galaxy inputs(fracAGN=0), the accuracy of fracAGNis improved by∼100 times thanks to MIRI. The simulatedCEERS MIRI data are slightly more sensitive to AGN detections than the deepest X-ray survey, based on theempiricalLX–L6μmrelation. Like X-ray observations, MIRI can also be used to constrain the AGN accretionpower(accuracy≈0.3 dex). Our work demonstrates that MIRI will be able to place strong constraints on the mid-IR luminosities from star formation and AGNs and thereby facilitate studies of the galaxy/AGN coevolution.

G Yang↗

NASA NTRS1979

X-ray all-sky monitors

Considerations for an all-sky monitor for the Ariel-5 mission are evaluated. The monitors with the best signal-to-noise utilize the pinhole-camera principle, and are also least expensive in hardware and data analysis costs. The sensitivity sufficient to sensibly monitor compact extragalactic sources requires a system with approximately 100 times the sensitivity of the Ariel-5 ASM. A system identical in size to the Ariel-5 ASM can be made with approximately 5 times better sensitivity, and can satisfy the minimum requirements for this mission if it spins. An all-sky monitor appears to be essential to assure that rare occurrences in compact objects can be observed with the pointed experiment, and to ascertain that large amounts of pointing time will not be wasted by observing at the wrong times.

Holt, S. S.↗

NASA NTRS1998

X-Ray Emission from Protostellar Jets

The goals of this program were to identify the termination shocks in several parsec-scale protostellar jets through the thermal soft X-rays emitted by the high-velocity terminal shock waves, and to assess the impact these outflows have on the surrounding interstellar and intercloud medium. The terminal shock waves that plow into the undisturbed interstellar medium may have shock velocities commensurate with the observed space motions of several hundred km s(exp -1). Such shocks will heat and ionize the interstellar medium, perhaps creating large ovoid bubbles of hot gas. Identifying the location of the terminal shocks in these outflows would also allow us to place constraints on the ages of these outflows, and hence the duration of the mass-loss phase that accompanies the formation of a star. We targeted four outflows where the outer optical shock waves are projected against low-extinction backgrounds. From the two dozen or so parsec-scale flows known, we chose those that have the highest velocities, brightest optical counterparts, and/or the richest concentration of shock waves in a small area on the sky. Four giant HH flows have been observed with the ROSAT HRI at Priority A for 30 ksec each. Results: Unfortunately, we failed to detect X-ray emission from the terminal bow shocks of the giant HH flows using the ROSAT HRI. The reasons are likely to be: (1) The shock velocities may be too low to emit in the soft X-rays. (2) The sensitivity of the HRI is too low to detect the diffuse emission. The fields that we observed were chosen to be low extinction sight-lines, with the consequence that the tenuous media into which the outer bow shocks are propagating produce low fluxes. Despite the failure to achieve the primary science goals, we have identified in each image a half a dozen or so point-source young stellar objects. The HRI images have been very useful for discerning potential outflow sources and are being combined with optical, near-IR, and radio imaging data of each outflow field. The first paper on L1551 NE is currently in preparation.

Morse, Jon A.↗

NASA NTRS1973

Processing of thermionic power on an electrically propelled spacecraft

A study to define the power processing equipment required between a thermionic reactor and an array of mercury-ion thrusters for a nuclear electric propulsion system is reported. Observations and recommendations that resulted from this study were: (1) the preferred thermionic-fuel-element source voltages are 23 V or higher; (2) transistor characteristics exert a strong effect on power processor mass; (3) the power processor mass could be considerably reduced should the magnetic materials that exhibit low losses at high frequencies, that have a high Curie point, and that can operate at 15 to 20 kG become avaliable; (4) electrical component packaging on the radiator could reduce the area that is sensitive to meteoroid penetration, thereby reducing the meteoroid shielding mass requirement; (5) an experimental model of the power processor design should be built and tested to verify the efficiencies, masses, and all the automatic operational aspects of the design.

Macie, T. W.↗

NASA NTRS2014

Model Evaluation of Methods for Estimating Surface Emissions and Chemical Lifetimes from Satellite Data

Column densities from satellite retrievals can provide valuable information for estimating emissions and chemical lifetimes objectively across the globe. To better understand the uncertainties associated with these estimates, we test four methods using simulated column densities from a point source: a box model approach, a 2D Gaussian fit, an Inverse Radius fit and an Exponentially-Modified Gaussian fit. The model results were simulated using the WRF and CAMx models for the year 2005, for a single point source outside Atlanta in Georgia, USA with specified emissions and three chemical scenarios: no chemical reactions, 12 h chemical lifetime and 1 h chemical lifetime. No other sources were included in the simulations. We find that the box model provides reliable estimates irrespective of plume speed and plume direction, if the plume speed and the chemical lifetime are known accurately. The 2D Gaussian fit was found to be sensitive to plume speed and direction, and requires omnidirectional dispersion in order to have a decent fit. However, the 2D Gaussian fit is only an approximate fit to the data, and the discrepancies mean that the results are dependent on the geographical domain used for the optimization. An Inverse Radius fit is introduced to correct this issue, which is found to provide improved emissions and lifetime estimates. The Exponentially-Modified Gaussian fit also gave improved estimates. It is however dependent on accurate plume rotation such that reported chemical lifetimes with this method could be significantly underestimated.

Emission inventory↗

NASA NTRS2004

Passive Acoustic Tomography Tested for Measuring Gas Temperatures

The requirements of higher performance, better fuel economy, and lower emissions place an increasing premium on knowing the internal operating parameters of jet engines. One of the most important is the gas temperature in the post combustor section of the engine. Typically the gas temperature is measured with a thermocouple probe or by some optical technique such as Rayleigh scattering. Probes, while providing valuable information, have several limitations. The probe signal must be corrected for radiation and conduction losses, probes provide only a point measurement, and probes must be constructed of materials whose melting points are lower than the temperature of the environment into which they are inserted. Some of the disadvantages of probes are overcome by various optical techniques. Nothing needs to be inserted into the flow, and the temperature can be directly related to the signal by known physical laws. However, optical techniques require optical access (i.e., a window) and a light source (such as a laser), and they are very sensitive to the presence of particles in the flow. To overcome these problems, researchers from the NASA Glenn Research Center and The University of Nevada are developing a technique that uses sound instead of light to measure gas temperature. Like optical techniques, it is nonintrusive--no probe need be exposed to the combustion environment--and the temperature is directly related to a measured quantity--the speed of sound, which is proportional to the square root of the absolute temperature. The temperature profile inside the engine is constructed from the differences in arrival time between correlated signals from an array of microphones placed around the circumference of the engine. In much the same way as a complete picture of the inside of your body can be constructed from an array of x-ray photographs taken at different angles, the temperature profile in the engine is constructed from the angular array of microphones. It is tomography by sound waves. Active acoustic tomography, in which a sound pulse is injected into the flow and the time delays between members of an array of microphones are used to construct the temperature field has been used successfully in the stacks of power plants. However, the flow field inside a jet engine is much too noisy for it to be possible to detect an externally injected sound pulse. Instead we are developing passive acoustic tomography, which uses the sound already present in the flow.

Fralick, Gustave C.↗

NASA NTRS2022

Opportunistic Experiments to Constrain Aerosol Effective Radiative Forcing

Aerosol-cloud interactions (ACI) are considered to be the most uncertain driver of present-day radiative forcing due to human activities. The non-linearity of cloud-state changes to aerosol perturbations make it challenging to attribute causality in observed relationships of aerosol radiative forcing. Using correlations to infer causality can be challenging when meteorological variability also drives both aerosol and cloud changes independently. Natural and anthropogenic aerosol perturbations from well defined sources provide ‘opportunistic experiments’ (also known as natural experiments) to investigate ACI in cases where causality may be more confidently inferred. These perturbations cover a wide range of locations and spatio-temporal scales, including point sources such as volcanic eruptions or industrial sources, plumes from biomass burning or forest fires, and tracks from individual ships or shipping corridors.We review the different experimental conditions and conduct a synthesis of the available satellite data sets and field campaigns to place these opportunistic experiments on a common footing, facilitating new insights and a clearer understanding of key uncertainties in aerosol radiative forcing. Cloud albedo perturbations are strongly sensitive to background meteorological conditions. Strong liquid water path increases due to aerosol perturbations are largely ruled out by averaging across experiments. Opportunistic experiments have significantly improved process-level understanding of ACI, but it remains unclear how reliably the relationships found can be scaled to the global level, thus demonstrating a need for deeper investigation in order to improve assessments of aerosol radiative forcing and climate change.

Matthew W Christensen↗

NASA NTRS1981

Very high resolution observations of SS433 at 10.65 GHz

Observations of SS433 made on June 12, 1979, from West Germany, Massachusetts, and West Virginia are discussed. It is noted that SS433 did not show fringes on any baseline although all the calibration sources were seen at their expected strengths. The measured total flux density of SS433 was found to be approximately 0.5 Jy, consistent with previous observations. The source was observed by on-offs at each telescope, which indicates that they were all pointed properly during the observations. The absence of fringes is not attributed to poor observing conditions or instrumental difficulties. It is concluded that if all the 10.65 GHz radiation emanates from a single component, then that component is at least 0.005 arcsec (approximately 10 to the 14th cm) in size. The measurements made on more sensitive intercontinental baselines indicate that there is no component of SS433 smaller than 0.001 arcsec emitting 10.65 GHz radiation above a level of 50 mJy.

Geldzahler, B. J.↗

NASA NTRS1990

Receiving efficiency of monostatic pulsed coherent lidars. I - Theory. II - Applications

Pulsed coherent radars' receiving efficiency, eta, is presently investigated as a function of range z on the basis of a theory which relates eta(z) to both the transmitted laser intensity and the point-source receiving efficiency; this efficiency is calculated by a backward method employing the back-propagated local oscillator (BPLO) approach. The theory is applied to the ideal case, in order to study system optimization when both the transmitted and the BPLO fields at the antenna are Gaussian. In the second part of this work, eta(z) is calculated for various conditions of the NOAA/ERL Wave Propagation Laboratory CO2 Doppler lidar; the sensitivity of eta(z) to transmitted laser beam quality, telescope focal setting, telescope power, scanner astigmatism, and system misalignment.

Zhao, Yanzeng↗

NASA NTRS2019

The Imaging X-Ray Polarimetry Explorer (IXPE): Technical Overview II

The Imaging X-ray Polarimetry Explorer (IXPE) will add polarization to the properties (time, energy, and position) observed in x-ray astronomy. A NASA Astrophysics Small Explorer (SMEX) in partnership with the Italian Space Agency (ASI), IXPE will measure the 2–8-keV polarization of a few dozen sources during the first 2 years following its 2021 launch. The IXPE Observatory includes three identical x-ray telescopes, each comprising a 4-m-focal-length (grazingincidence) mirror module assembly (MMA) and a polarization-sensitive (imaging) detector unit (DU), separated by a deployable optical bench. The Observatory’s Spacecraft provides typical subsystems (mechanical, structural, thermal, power, electrical, telecommunications, etc.), an attitude determination and control subsystem for 3-axis stabilized pointing, and a command and data handling subsystem communicating with the science instrument and the Spacecraft subsystems.

X-ray astronomy↗

NASA NTRS1984

Images in the rocket ultraviolet - Photometry of M101

It is pointed out that ultraviolet imagery of spiral galaxies is a sensitive probe of the large-scale morphology of the star-forming regions and the associated interstellar dust. A rocket-borne Ritchey-Chretien telescope was employed to obtain three ultraviolet images of the Sc 1 galaxy M101 (NGC 5457). The present investigation is concerned with the procedures for reduction and absolute calibration of the obtained images and an analysis in terms of discrete sources. Attention is also given to the radial variation and the azimuthal variation of disk and arms. It is found that the emission from M101 in the UV bandpass is dominated by extreme Population I, in the form of discrete OB/H II complexes.

Hill, J. K.↗

NASA NTRS1989

Silicon chemistry in interstellar clouds

Interstellar SiO was discovered shortly after CO but it has been detected mainly in high density and high temperature regions associated with outflow sources. A new model of interstellar silicon chemistry that explains the lack of SiO detections in cold clouds is presented which contains an exponential temperature dependence for the SiO abundance. A key aspect of the model is the sensitivity of SiO production by neutral silicon reactions to density and temperature, which arises from the dependence of the rate coefficients on the population of the excited fine structure levels of the silicon atom. This effect was originally pointed out in the context of neutral reactions of carbon and oxygen by Graff, who noted that the leading term in neutral atom-molecule interactions involves the quadrupole moment of the atom. Similar to the case of carbon, the requirement that Si has a quadrupole moment requires population of the J = 1 level, which lies 111K above the J = 0 ground state and has a critical density n(cr) equal to or greater than 10(6)/cu cm. The SiO abundance then has a temperature dependence proportional to exp(-111/T) and a quadratic density dependence for n less than n(cr). As part of the explanation of the lack of SiO detections at low temperatures and densities, this model also emphasizes the small efficiencies of the production routes and the correspondingly long times needed to reach equilibrium. Measurements of the abundance of SiO, in conjunction with theory, can provide information on the physical properties of interstellar clouds such as the abundances of oxygen bearing molecules and the depletion of interstellar silicon.

Langer, William D.↗