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At least 1,765 records · Page 98
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NASA NTRS2001

Laser Absorption Measurements of Equivalence Ratios Studied Along With Their Coupling to Pressure Fluctuations in Lean Premixed Prevaporized (LPP) Combustion

Concerns about damaging the Earth's ozone layer as a result of high levels of nitrogen oxides (known collectively as NOx) from high-altitude, high-speed aircraft have prompted the study of lean premixed prevaporized (LPP) combustion in aircraft engines. LPP combustion reduces NOx emissions principally by reducing the peak flame temperatures inside an engine. Recent advances in LPP technologies have realized exceptional reductions in pollutant emissions (single-digit ppm NOx for example). However, LPP combustion also presents major challenges: combustion instability and dynamic coupling effects between fluctuations in heat-release rate, dynamic pressure, and fuel pressure. These challenges are formidable and can literally shake an engine apart if uncontrolled. To better understand this phenomenon so that it can be controlled, we obtained real-time laser absorption measurements of the fuel vapor concentration (and equivalence ratio) simultaneously with the dynamic pressure, flame luminosity, and time-averaged gaseous emissions measurements in a research-type jet-A-fueled LPP combustor. The measurements were obtained in NASA Glenn Research Center's CE-5B optically accessible flame tube facility. The CE-5B facility provides inlet air temperatures and pressures similar to the actual operating conditions of real aircraft engines. The laser absorption measurements were performed using an infrared 3.39 micron HeNe laser in conjunction with a visible HeNe laser for liquid droplet scattering compensation.

Nguyen, Quang-Viet↗
NASA NTRS2006

Discovery of an X-ray Violently Variable Broad Absorption Line Quasar

In this letter, we report on a quasar that is violently variable in the X-rays, XVV. It is also a broad absorption line quasar (BALQSO) that exhibits both high ionization and low ionization UV absorption lines (LoBALQSO). It is very luminous in the X-rays (approximately 10(exp 46) ergs s(sup -l) over the entire X-ray band). Surprisingly, this does not over ionize the LoBAL outflow. The X-rays vary by a factor of two within minutes in the quasar rest frame, which is shorter than 1/30 of the light travel time across a scale length equal to the black hole radius. We concluded that the X-rays are produced in a relativistic jet beamed toward earth in which variations in the Doppler enhancement produce the XVV behavior.

Ghosh, Kajal K.↗
NASA NTRS1994

Enhanced Reverse Saturable Absorption and Optical Limiting in Heavy-Atom Substituted Phthalocyanines

The reverse saturable absorption and optical limiting response of metal phthalocyaninies can be enhanced by using the heavy-atom effect. Phthalocyanines containing heavy metal atoms, such as In, Sn, and Pb show nearly a factor of two enhancement in the ratio of effective excited-state to ground-state absorption cross sections compared to those containing lighter atoms, such as Al and Si. In an f/8 optical geometry, homogeneous solutions of heavy metal phthalocyanines, at 30% linear transmission, limit 8-ns, 532-nm laser pulses to less than or equal to 3 (micro)J (the energy for 50% probability of eye damage) for incident pulses up to 800 (micro)J.

metal phthalocyanines↗
NASA NTRS2006

Highly Ionized Iron Absorption Lines from Outflowing Gases in the X-ray Spectrum of NGC 1365

We present the discovery of four absorption lines in the X-ray spectrum of the Seyfert galaxy NGC 1365, at energies between 6.7 and 8.3 keV. The lines are detected with high statistical confidence (from >20 sigma for the strongest to -4 sigma for the weakest) in two XMM-Newton observations 60 ks long. We also detect the same lines, with a lower signal-to-noise ratio (but still >2 sigma for each line), in two previous shorter (-10 ks) XMM-Newton observations. The spectral analysis identifies these features as Fe XXV and Fe XXVI Kalpha and Kbeta lines, outflowing with velocities varying between -1000 and -5000 km/s among the observations. These are the highest quality detections of such lines so far. The high equivalent widths [EW (Kalpha) approximately 100 eV] and the Kalpha/Kbeta ratios imply that the lines are due to absorption of the AGN continuum by a highly ionized gas with column density NH-5?1023 cm(exp -2) at a distance of -(50-100)RS from the continuum source.

Risaliti, G.↗
NASA NTRS2007

Relativistic Iron K Emission and Absorption in the Seyfert 1.9 Galaxy MCG-05-23-16

We present the results of the simultaneous deep XMM-Newton and Chandra observations of the bright Seyfert 1.9 galaxy MCG-5-23-16, which is thought to have one of the best known examples of a relativistically broadened iron Kalpha line. We detected a narrow sporadic absorption line at 7.7 keV which appears to be variable on a time-scale of 20 ksec. If associated with FeXXVI this absorption is indicative of a possible variable high ionization, high velocity outflow. The time averaged spectral analysis shows that the iron K-shell complex is best modeled with an unresolved narrow emission component (FWHM less than 5000 kilometers per second, EW approx. 60 eV) plus a broad component. This latter component has FWHM approx. 44000 kilometers per second, an EW approx. 50 eV and its profile is well described with an emission line originating from the accretion disk viewed with an inclination angle approx. 40 deg. and with the emission arising from within a few tens of gravitational radii of the central black hole. The time-resolved spectral analysis of the XMM-Newton EPIC-pn spectrum shows that both the narrow and broad components of the Fe K emission line appear to be constant within the errors. The analysis of the XMM-Newton/RGS spectrum reveals that the soft X-ray emission of MCG-5-23-16 is likely dominated by several emission lines superimposed on an unabsorbed scattered power-law continuum. The lack of strong Fe L shell emission together with the detection of a strong forbidden line in the O VII triplet supports a scenario where the soft X ray emission lines are produced in a plasma photoionized by the nuclear emission.

Braito, V.↗
NASA NTRS2007

Temperature Evolution in Excitonic Absorptions of Cd(0.96)Zn(0.04) Materials

We report cryogenic optical properties of Cd(0.96)Zn(0.04)Te wafers that are used as substrate seed layers in HgCdTe focal-plane array detectors. These studies are motivated by the fact that the substrate optical properties influence the overall detector performance. The studies consist of measuring the sample frequency dependent transmittance (T) and reflectance (R) above and below the optical band-gap in the UV/Visible and infrared frequency ranges, and with temperature variation of the sample from 5 to 300 K. These measurements show that the optical gap near 1.49 eV at 300 K increases to 1.62 eV at 5 K. Finally, we observe sharp absorption peaks near this gap energy at low temperatures. The close proximity of these peaks to the optical transition threshold suggests that they originate from the creation of bound electron-hole pairs or excitons. The decay of these excitonic absorptions may contribute to a photoluminescence and transient background response of these back-illuminated HgCdTe CCD detectors.

Quijada, Manuel A.↗
NASA NTRS2007

The Suzaku Observation of NGC 3516: Complex Absorption and the Broad and Narrow Fe K Lines

We present results from a 150 ksec Suzaku observation of the Seyfert 1 NGC 3516 in October 2005. The source was in a relatively highly absorbed state. Our best-fit model is consistent with partial covering by a lowly-ionized absorber with a column density near 5x10(exp 22) cm(exp -2) and with a covering fraction 96-100 percent. Narrow K-shell absorption features due to He- and H-like Fe confirm the presence of a high-ionization absorbing component as well. A broad Fe K(alpha) diskline is required in all fits, even after the complex absorption is taken into account; an additional partial-covering component is an inadequate substitute for the continuum curvature associated with the broad line. The narrow Fe Ka line at 6.4 keV is resolved, yielding a velocity width commensurate with the optical Broad Line Region. The strength of the Compton reflection hump suggests a contribution mainly from the broad Fe line origin. We include in our model soft band emission lines from He- and H-like ions and radiative recombination lines, consistent with photo-ionization, though a small contribution from collisional ionization is possible.

Markowitz, Alex↗
NASA NTRS2007

Temperature Evolution of Excitonic Absorptions in Cd(1-x)Zn(x)Te Materials

The studies consist of measuring the frequency dependent transmittance (T) and reflectance (R) above and below the optical band-gap in the UV/Visible and infrared frequency ranges for Cd(l-x),Zn(x),Te materials for x=0 and x=0.04. Measurements were also done in the temperature range from 5 to 300 K. The results show that the optical gap near 1.49 eV at 300 K increases to 1.62 eV at 5 K. Finally, we observe sharp absorption peaks near this gap energy at low temperatures. The close proximity of these peaks to the optical transition threshold suggests that they originate from the creation of bound electron-hole pairs or excitons. The decay of these excitonic absorptions may contribute to a photoluminescence and transient background response of these back-illuminated HgCdTe CCD detectors.

Quijada, Manuel A.↗
NASA NTRS2008

Absorption in Extended Inhomogeneous Clouds

The launch of several different sensors, including CloudSat, into the A-train constellation of satellites allows us for the first time to compute absorption that can occur in realistic vertically inhomogeneous clouds including multiple cloud decks. CloudSat data show that these situations are common. Therefore, understanding vertically inhomogeneous clouds is important from both climate and satellite atmospheric composition remote sensing perspectives. Satellite passive sensors that operate from the near IR to the UV often rely on radiative cloud pressures derived from absorption in oxygen bands (A, B, gamma, or O2-O2 bands) or from rotational-Raman scattering in order to retrieve information about atmospheric trace gases. The radiative cloud pressure is distinct from the physical cloud top derived from thermal infrared measurements. Therefore, the combination of information from different passive sensors yields some information about the cloud vertical profile. When either or both the clouds or atmospheric absorbers (trace gases and aerosols) are vertically inhomogeneous, the use of an effective cloud pressure derived from these approaches may lead to errors. Here, we focus on several scenarios (deep convective clouds and distinct two layer clouds) based on realistic cloud optical depth vertical profiles derived from the CloudSatfMODIS combination. We focus on implications for trace-gas column amount retrievals (specifically ozone and NO2) and derived surface UV irradiance from the Ozone Monitoring Instrument (OMI) on the Atrain Aura platform.

Joiner, Joanna↗
NASA NTRS2008

Mars Ozone Absorption Line Shapes from Infrared Heterodyne Spectra Applied to GCM-Predicted Ozone Profiles and to MEX/SPICAM Column Retrievals

We present the application of infrared heterodyne line shapes of ozone on Mars to those produced by radiative transfer modeling of ozone profiles predicted by general circulation models (GCM), and to contemporaneous column abundances measured by Mars Express SPICAM. Ozone is an important tracer of photochemistry Mars' atmosphere, serving as an observable with which to test predictions of photochemistry-coupled GCMs. Infrared heterodyne spectroscopy at 9.5 microns with spectral resolving power >1,000,000 is the only technique that can directly measure fully-resolved line shapes of Martian ozone features from the surface of the Earth. Measurements were made with Goddard Space Flight Center's Heterodyne instrument for Planetary Wind And Composition (HIPWAC) at the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii on February 21-24 2008 UT at Ls=35deg on or near the MEX orbital path. The HIPWAC observations were used to test GCM predictions. For example, a GCM-generated ozone profile for 60degN 112degW was scaled so that a radiative transfer calculation of its absorption line shape matched an observed HIPWAC absorption feature at the same areographic position, local time, and season. The RMS deviation of the model from the data was slightly smaller for the GCM-generated profile than for a line shape produced by a constant-with-height profile, even though the total column abundances were the same, showing potential for testing and constraining GCM ozone-profiles. The resulting ozone column abundance from matching the model to the HIPWAC line shape was 60% higher than that observed by SPICAM at the same areographic position one day earlier and 2.5 hours earlier in local time. This could be due to day-to-day, diurnal, or north polar region variability, or to measurement sensitivity to the ozone column and its distribution, and these possibilities will be explored. This work was supported by NASA's Planetary Astronomy Program.

Fast, Kelly E.↗
NASA NTRS2008

Multi-Satellite Observations of Cygnus X-1 to Study the Focused Wind and Absorption Dips

High-mass X-ray binary systems are powered by the stellar wind of their donor stars. The X-ray state of Cygnus X-1 is correlated with the properties of the wind which defines the environment of mass accretion. Chandra-HETGS observations close to orbital phase 0 allow for an analysis of the photoionzed stellar wind at high resolution, but because of the strong variability due to soft X-ray absorption dips, simultaneous multi-satellite observations are required to track and understand the continuum, too. Besides an earlier joint Chandra and RXTE observation, we present first results from a recent campaign which represents the best broad-band spectrum of Cyg X-1 ever achieved: On 2008 April 18/19 we observed this source with XMM-Newton, Chandra, Suzaku, RXTE, INTEGRAL, Swift, and AGILE in X- and gamma-rays, as well as with VLA in the radio. After superior conjunction of the black hole, we detect soft X-ray absorption dips likely due to clumps in the focused wind covering greater than or equal to 95% of the X-ray source, with column densities likely to be of several 10(exp 23) cm(exp -2), which also affect photon energies above 20 keV via Compton scattering.

Hanke, Manfred↗
NASA NTRS2009

An Fe XXVI Absorption Line in the Persistent Spectrum of the Dipping Low Mass X-ray Binary 1A 1744-361

We report on Chandra X-ray Observatory (CXO) High-Energy Transmission Grating (HETG) spectra of the dipping Low Mass X-ray Binary (LMXB) 1A 1744-361 during its July 2008 outburst. We find that its persistent emission is well modeled by a blackbody (kT approx. 1.0 keV) plus power-law (Gamma approx. 1.7) with an absorption edge at 7.6 keV. In the residuals of the combined spectrum we find a significant absorption line at 6.961+/-0.002 keV, consistent with the Fe XXVI (hydrogen-like Fe) 2 - 1 transition. We place an upper limit on the velocity of a redshifted flow of v < 221 km/s. We find an equivalent width for the line of 27+2/-3 eV, from which we determine a column density of 7+/-1 x 10(exp 17)/sq cm via a curve-of-growth analysis. Using XSTAR simulations, we place a lower limit on the ionization parameter of > 10(exp 3.6) erg cm/s. The properties of this line are consistent with those observed in other dipping LMXBs. Using Rossi X-ray Timing Explorer (RXTE) data accumulated during this latest outburst we present an updated color-color diagram which clearly shows that IA 1744-361 is an "atoll" source. Finally, using additional dips found in the RXTE and CXO data we provide an updated orbital period estimate of 52+/-5 minutes.

Gavriil, Fotis P.↗
NASA NTRS2010

Multi-decade Measurements of the Long-Term Trends of Atmospheric Species by High-Spectral-Resolution Infrared Solar Absorption Spectroscopy

Solar absorption spectra were recorded for the first time in 5 years with the McMath Fourier transform spectrometer at the US National solar Observatory on Kitt Peak in southern Arizona, USA (31.91 N latitude, 111.61 W longitude, 2.09 km altitude). The solar absorption spectra cover 750-1300 and 1850-5000 cm(sup -1) and were recorded on 20 days during March-June 2009. The measurements mark the continuation of a long-term record of atmospheric chemical composition measurements that have been used to quantify seasonal cycles and long-term trends of both tropospheric and stratospheric species from observations that began i 1977. Fits to the measured spectra have been performed, and they indicate the spectra obtained since return to operational status are nearly free of channeling and the instrument line shape function is well reproduced taking into account the measurement parameters. We report updated time series measurements of total columns for six atmospheric species and their analysis for seasonal cycles and long-term trends. An sn example, the time series fit shows a decrease in the annual increase rate i Montreal-Protocol-regulated chlorofluorocarbon CCL2F2 from 1.51 plus or minus 0.38% yr(sup -1) at the beginning of the time span to -1.54 plus or minus 1.28 yr(sup -1) at the end of the time span, 1 sigma, and hence provides evidence for the impact of those regulations on the trend.

Rinsland, Curtis P.↗
NASA NTRS2010

UV Absorption Cross Sections of Nitrous Oxide (N2O) and Carbon Tetrachloride (CCl4) Between 210 and 350 K and the Atmospheric Implications

Absorption cross sections of nitrous oxide (N2O) and carbon tetrachloride (CCl4) are reported at five atomic UV lines (184.95, 202.548, 206.200, 213.857, and 228.8 nm) at 27 temperatures in the range 210-350 K. In addition, UV absorption spectra of CCl4 are reported between 200-235 nm as a function of temperature (225-350 K). The results from this work are critically compared with results from earlier studies. For N2O, the present results are in good agreement with the current JPL recommendation enabling a reduction in the estimated uncertainty in the N2O atmospheric photolysis rate. For CCl4, the present cross section results are systematically greater than the current recommendation at the reduced temperatures most relevant to stratospheric photolysis. The new cross sections result in a 5-7% increase in the modeled CCl4 photolysis loss, and a slight decrease in the stratospheric lifetime, from 51 to 50 years, for present day conditions. The corresponding changes in modeled inorganic chlorine and ozone in the stratosphere are quite small. A CCl4 cross section parameterization for use in 37 atmospheric model calculations is presented.

Carlon, Nabilah Rontu↗
NASA NTRS2010

Development of a Coherent Differential Absorption Lidar for Range Resolved Atmospheric CO2 Measurements

A pulsed, 2-m coherent Differential Absorption Lidar (DIAL) / Integrated Path Differential Absorption (IPDA) transceiver, developed under the Laser Risk Reduction Program (LRRP) at NASA, is integrated into a fully functional lidar instrument. This instrument will measure atmospheric CO2 profiles (by DIAL) initially from a ground platform, and then be prepared for aircraft installation to measure the atmospheric CO2 column densities in the atmospheric boundary layer (ABL) and lower troposphere. The airborne prototype CO2 lidar can measure atmospheric CO2 column density in a range bin of 1km with better than 1.5% precision at horizontal resolution of less than 50km. It can provide the image of the pooling of CO2 in lowlying areas and performs nighttime mass balance measurements at landscape scale. This sensor is unique in its capability to study the vertical ABL-free troposphere exchange of CO2 directly. It will allow the investigators to pursue subsequent in science-driven deployments, and provides a unique tool for Active Sensing of CO2 Emissions over Night, Days, and Seasons (ASCENDS) validation that was strongly advocated in the recent ASCENDS Workshop.

Yu, Jirong↗
NASA NTRS2010

Optimization of A 2-Micron Laser Frequency Stabilization System for a Double-Pulse CO2 Differential Absorption Lidar

A carbon dioxide (CO2) Differential Absorption Lidar (DIAL) for accurate CO2 concentration measurement requires a frequency locking system to achieve high frequency locking precision and stability. We describe the frequency locking system utilizing Frequency Modulation (FM), Phase Sensitive Detection (PSD), and Proportional Integration Derivative (PID) feedback servo loop, and report the optimization of the sensitivity of the system for the feed back loop based on the characteristics of a variable path-length CO2 gas cell. The CO2 gas cell is characterized with HITRAN database (2004). The method can be applied for any other frequency locking systems referring to gas absorption line.

Chen, Songsheng↗
NASA NTRS2010

Modeling Broadband X-Ray Absorption of Massive Star Winds

We present a method for computing the net transition of X-rays emitted by shock-heated plasma distributed throughout a partially optically thick stellar wind from a massive star. We find the transmission by an exact integration of the formal solution, assuming the emitting plasma and absorbing plasma are mixed at a constant mass ratio above some minimum radius, below which there is assumed to be no emission. This model is more realistic than either the slab absorption associated with a corona at the base of the wind or the exospheric approximation that assumes all observed X-rays are emitted without attenuation from above the radius of optical depth unity. Our model is implemented in XSPEC as a pre-calculated table that can be coupled to a user-defined table of the wavelength dependent wind opacity. We provide a default wind opacity model that is more representative of real wind opacities than the commonly used neutral ISM tabulation. Preliminary modeling of Chandra grating data indicates that the X-ray hardness trend of OB stars with spectral subtype cars largely be understood as a wind absorption effect.

Leutenegger, Maurice A.↗