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NASA NTRS1975

Spectroradiometric calibration techniques in the far ultraviolet - A stable emission source for the Lyman bands of molecular hydrogen

The problems associated with making accurate spectroradiometric measurements in the far UV region are sketched briefly. The equipment and methods that were developed for providing absolute sensitivity calibration of an Apollo 17 far UV spectrometer are described. The absolute reference standards were photoelectric diodes calibrated at the National Bureau of Standards. A complete vacuum optical facility, which included a premonochromator and stable UV light sources, was developed to calibrate the Apollo 17 instrument, and it has been used for a number of other tasks. Absolute radiometric calibrations between about 1200 and 1700 A were performed with an absolute accuracy of + or - 10%. The light source, which was designed to provide a very stable light output, is a low-pressure molecular hydrogen lamp in which the pressure is stabilized by thermal control of uranium hydride powder.-

Fastie, W. G.↗

NASA NTRS1999

The X-Ray Spectrum of WZ Sagittae

We carried out an Advanced Satellite for Cosmology and Astrophysics (ASCA) observation of WZ Sagittae, to obtain the first X-ray spec- trum and to search for an X-ray counterpart near the well-known optical signal of 27.87 s. At around the same epoch, we carried out a similar search in ultraviolet radiation with the Faint Object Spectrograph (FOS) aboard the Hubble Space Telescope (HST). These experiments were done to test the hypothesis that the signal arises from rapid white dwarf rotation. The result generally confirmed that hypothesis, because signals were apparently detected at 27.87 s in both UV and hard X-ray light. But the detailed results were somewhat confusing, not exactly as simple as we dreamed in the proposal. The X-ray signal only emerges with fairly subtle methods of data analysis: either a "period-biased" search, or a sorting of X-ray photons into hard/soft - thus the detections are slightly marred by the taint of a posterz'ori reasoning. The UV data suffer a different but also worrisome problem: although 27.87 s is definitely detected, it's weaker than two other signals, at 28.22 and 29.69 s. That's quite surprising - we expected that the UV power spectrum would be simple, on the grounds that the UV flux distribution is nearly a pure white dwarf. Considered in isolation, such behavior from a white dwarf should be considered evidence for the pulsation hypothesis. The X-ray spectrum was interesting, mostly by virtue of what it did not show. It was essentially a pure thermal bremsstrahlung with kT = 5 keV, with only a very weak Fe K complex. This probably signifies origin near the central white dwarf and heating from accretion (propellor heating farther out would give lower densities and more cooling by emission lines). We published a paper reporting most of these details (Patterson et al. (1998), PASP, 110, 403), Much of the world accepts now that WZ Sge contains a rapidly spinning white dwarf - the fastest spin of any accreting white dwarf.

Patterson, Joseph↗

NASA NTRS2022

Inferring iron-oxide species content in atmospheric mineral dust from DSCOVR EPIC observations

The iron-oxide content of dust in the atmosphere and most notably its apportionment between hematite (α-Fe2O3) and goethite (α-FeOOH) are key determinants in quantifying dust’s light absorption, its top of atmosphere UV radiances used for dust monitoring, and ultimately shortwave dust direct radiative effects (DRE). Hematite and goethite column mass concentrations and iron-oxide mass fractions of total dust mass concentration were retrieved from the DeepSpace Climate Observatory (DSCOVR) Earth Polychromatic Imaging Camera (EPIC) measurements in the ultraviolet–visible (UV–Vis) channels. The retrievals were performed for dust-identified aerosol plumes over land using aerosol optical depth (AOD) and spectral imaginary refractive index provided by the Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm over six continental regions (North America, North Africa, West Asia, Central Asia, EastAsia, and Australia). The dust particles are represented as an internal mixture of non-absorbing host and absorbing hematite and goethite. We use the Maxwell–Garnett effective medium approximation with carefully selected complex refractive indices of hematite and goethite that produce mass fractions of iron oxides species consistent with in situ values found in the literature to derive the hematite and goethite volumetric/mass concentrations from MAIAC EPIC products. We compared the retrieved hematite and goethite concentrations with in situ dust aerosol mineralogical content measurements, as well as with published data. Our data display variations within the published range of hematite, goethite, and iron-oxide mass fractions for pure mineral dust cases. A specific analysis is presented for 15 sites over the main dust source regions. Sites in the central Sahara, Sahel, and Middle East exhibit a greater temporal variability of iron oxides relative to other sites. Niger site(13.52°N, 2.63°E) is dominated by goethite over Harmattan season with median of ~2 weight percentage (wt.%) of iron-oxide. Saudi Arabia site (27.49°N, 41.98°E) over Middle East also exhibited surge of goethite content with the beginning of Shamal season. The Sahel dust is richer in iron-oxide than Saharan and northern China dust except in Summer. The Bodélé Depression area shows a distinctively lower iron-oxide concentration (~1 wt.%) throughout the year. Finally, we show that EPIC data allow to constrain the hematite refractive index. Specifically, we select 5 out of 13 different number of hematite refractive indices widely variable in published laboratory studies by constraining the iron-oxide mass ratio to the known measured values. Provided climatology of hematite and goethite mass fractions across main dust regions of the Earth will be useful for dust shortwave DRE studies and climate modeling.

Sujung Go↗

NASA NTRS1981

The International Ultraviolet Explorer

The instrumentation and functioning of the IUE are discussed. The design of the telescope, fine error sensors, echelle spectrographs, and television cameras, as well as their integration, is described. The optical characteristics of the IUE are listed and the schematic arrangement of the detector is depicted. Sources of background noise are assessed, including the thermionic effects in the UV converters and television tubes, radiation from the earth's trapped electron belts, wide angle white light scattering from the earth and sun, scattering of dispersed light caused by low dispersion gratings, photometric calibration, and photometric stability. IUE sensitivity changes during the first three years of operation are shown together with sensitivity curves for short and long wavelengths. Modes of wavelength determination are considered.

Boggess, A.↗

NASA NTRS1991

All-optical photochromic spatial light modulators based on photoinduced electron transfer in rigid matrices

A single material (not a multi-element structure) spatial light modulator may be written to, as well as read out from, using light. The device has tailorable rise and hold times dependent on the composition and concentration of the molecular species used as the active components. The spatial resolution of this device is limited only by light diffraction as in volume holograms. The device may function as a two-dimensional mask (transmission or reflection) or as a three-dimensional volume holographic medium. This device, based on optically-induced electron transfer, is able to perform incoherent to coherent image conversion or wavelength conversion over a wide spectral range (ultraviolet, visible, or near-infrared regions).

Beratan, David N.↗

NASA NTRS1992

The Ultraviolet Imaging Telescope - Design and performance

The instrumental configuration, calibration, and operations during the first flight of the Ultraviolet Imaging Telescope on the Astro-1 mission, December 2-10, 1990, are described. The UV images of a wide variety of astronomical objects were recorded with a 40-arcmin diameter field of view. Images of targets as faint as magnitude 21 (UV) were secured with a resolution of about 3 arcsec. The optics, light baffling, and image motion compensation system are summarized, and detectors and electronic subsystems are described.

Stecher, Theodore P.↗

NASA NTRS2001

Multiwavelength Modeling of Nove Atmospheres

LMC 1988 #1 was a slow, CO type, dust forming classical nova. It was the first extragalactic nova to be observed with the IUE satellite. We have successfully fitted observed ultraviolet and optical spectra of LMC 1988 #1 taken within the first two months of its outburst (when the atmosphere was still optically thick) with synthetic spectra computed using PHOENIX nova model atmospheres. The synthetic spectra reproduce most of the features seen in the spectra and provide V band magnitudes consistent with the observed light curve. The fits are improved by increasing the CNO abundances to 10 times the solar values. The bolometric luminosity of LMC 1988 #1 was approximately constant at 2 x 10(exp 38) ergs per second at a distance of 47.3 kpc for the first 2 months of the outburst until the formation of the dust shell.

Huschildt, P. H.↗

NASA NTRS2004

Optical Detection of Organic Chemical Biosignatures at Hydrothermal Vents

We have developed a non-contact, optical life detection instrument that can detect organic chemical biosignatures in a number of different environments, including dry land, shallow aqueous, deep marine or in ice. Hence, the instrument is appropriate as a biosignature survey tool both for Mars exploration or in situ experiments in an ice-covered ocean such as one might wish to explore on Europa. Here, we report the results we obtained on an expedition aboard the Russian oceanographic vessel Akademik Mstislav Keldysh to hydrothermal vent sites in the Pacific Ocean using our life detection instrument MCDUVE, a multichannel, deep ultraviolet excitation fluorescence detector. MCDUVE detected organic material distribution on rocks near the vent, as well as direct detection of organisms, both microbial and microscopic. We also were able to detect organic material issuing directly from vent chimneys, measure the organic signature of the water column as we ascended, and passively observe the emission of light directly from some vents.

Conrad, P. G.↗

NASA NTRS2000

Comparing galaxy morphology at ultraviolet and opical wavelengths

We have undertaken an imaging survey of 34 nearby galaxies in far-ultraviolet(FUV, [sim]1500 u and optical (UBVRI) passbands to characterize galaxy morphology as a function of wavelength. This sample, which includes a range of classical Hubble types from elliptical to irregular, with emphasis on spirals at low inclination angle, provides a valuable database for comparison with images of high-z galaxies whose FUV light is redshifted into the optical and near-infrared bands.

galaxies UBVRI imaging morphology star formation s↗

NASA NTRS1991

Spectroscopic binary orbits from ultraviolet radial velocities. VIII - LZ Cephei (HD 209481)

We present a study of the double-lined spectroscopic binary LZ Cep based on high-resolution IUE spectra. We determine the radial velocities by using a cross-correlation technique incorporating profile fitting; this allows measurements of velocity separations down to unusually small values. The final velocity curve has a small but significant eccentricity. By considering other constraints, we construct a model for the system, which correctly predicts the light curve; in the model, the secondary fills its critical equipotential lobe. The secondary is conspicuously overluminous for its mass, and must have reached its present configuration following a phase of case A mass transfer.

Howarth, Ian D.↗

NASA NTRS1987

Measurements of OH(X2pi) in the stratosphere by high resolution UV spectroscopy

This paper reports the first results obtained using high spectral resolution imaging ultraviolet spectroscopy to observe multiple rotational lines of OH A2 Sigma-X2pi (0-0) band. A 9.2 A spectral segment from 3075.8 A to 3085.0 A is imaged at 0.08 A FWHM spectral resolution, allowing the simultaneous acquisition of six of the brightest OH resonance fluorescence emission lines. The high spectral resolution and low scattered light design of the instrument allows these lines to be detected above the Rayleigh scattered sunlight background. The technique permits remote sensing of stratospheric OH from a high altitude instrument. The instrument was flown to an altitude of 40 km on Aug. 25, 1983, and again on June 12, 1986, on scientific balloons from Palestine, TX. The OH profiles inverted from the limb scans made during these flights are reported here. These profiles represent the first measurements of the temporal variation of OH over an extended height range. The results demonstrate that the technique can be used to monitor OH from orbit.

Torr, D. G.↗

NASA NTRS1988

A search for the binary companion of Polaris

Polaris has a spectroscopic orbit determined from an extensive series of observations as well as a more uncertain astrometric orbit. The determination of its mass and evolutionary state is of considerable interest because it is a low-amplitude classical Cepheid with unusual period and amplitude variations. In this study, IUE spectra are investigated to search for light from the companion. The spectra of Polaris from 1600 A to 3200 A are a good match for nonvariable supergiants of similar spectral type. The lack of any excess flux at the shortest wavelengths implies that a main-sequence companion must be later than A8 V. Although this is the most likely companion, the ultraviolet observations cannot rule out a white dwarf 15,000 K or cooler. Both these companions are consistent with either an evolutionary mass or a smaller pulsation mass for the Cepheid.

Evans, Nancy Remage↗

NASA NTRS1995

Photoelectrochemical etching of silicon carbide (SiC) and its characterization

Silicon carbide (SiC) is an attractive semiconductor material for high speed, high density, and high temperature device applications due to its wide bandgap (2.2-3.2 eV), high thermal conductivity, and high breakdown electric field (4 x 10(exp 6) V/cm). An instrumental process in the fabrication of semiconductor devices is the ability to etch in a highly controlled and selective manner for direct patterning techniques. A novel technique in etching using electrochemistry is described. This procedure involves the ultraviolet (UV) lamp-assisted photoelectrochemical etching of n-type 3C- and 6H-SiC to enhance the processing capability of device structures in SiC. While under UV illumination, the samples are anodically biased in an HF based aqueous solution since SiC has photoconductive properties. In order for this method to be effective, the UV light must be able to enhance the production of holes in the SiC during the etching process thus providing larger currents with light from the photocurrents generated than those currents with no light. Otherwise dark methods would be used as in the case of p-type 3C-SiC. Experiments have shown that the I/V characteristics of the SiC-electrolyte interface reveal a minimum etch voltage of 3 V and 4 V for n- and p-type 3C-SiC, respectively. Hence it is possible for etch-stops to occur. Etch rates calculated have been as high as 0.67 micrometer/min for p-type, 1.4 micrometer/min for n-type, and 1.1 micrometer/min for pn layer. On n-type 3C- SiC, an oxide formation is present where after etching a yellowish layer corresponds to a low Si/C ratio and a white layer corresponds to a high Si/C ratio. P-type 3C-SiC shows a grayish layer. Additionally, n-type 6H-SiC shows a brown layer with a minimum etch voltage of 3 V.

Collins, D. M.↗

NASA NTRS1998

The Variability and Spectrum of NGC 4051 from Deep, Simultaneous EUVE and XTE Observations

We present timing and spectral analysis of the data collected by the Extreme Ultraviolet Explorer Satellite (EUVE) for the Seyfert 1 galaxy NGC 4051 during 1996. NGC 4051 was observed twice in May 1996 and again in December 1996 for a total of more than 200 ksec. The observations were always simultaneous with hard X-ray observations conducted with the X-Ray Timing Explorer (XTE). The EUVE light curves are extremely variable during each observation, with the maximum variability during May 1996 when we registered changes by a factor of 21 over 8 hours and more than a factor of 24 variations from peak to minimum. We detected signal in the EUVE spectrograph in the 75-100 Arange which is well fitted by absorbed power law models. We will illustrate the results of our spectral and detailed power spectrum analysis for the simultaneous EUVE and XTE spectra and light curves and discuss the consequences on possible emission mechanisms.

Fruscione, Antonella↗

NASA NTRS2019

Constraining the Metallicities, Ages, Star Formation Histories, and Ionizing Continua of Extragalactic Massive Star Populations

We infer the properties of massive star populations using the far-ultraviolet stellar continua of 61 star-forming galaxies: 42 at low redshift observed with the Hubble Space Telescope and 19 at z ~ 2 from the MegaSaura sample. We fit each stellar continuum with a linear combination of up to 50 single-age and single-metallicity starburst99 models. From these fits, we derive light-weighted ages and metallicities, which agree with stellar wind and photospheric spectral features, and infer the spectral shapes and strengths of the ionizing continua. Inferred light-weighted stellar metallicities span 0.05–1.5 Z(sub ⊙) and are similar to the measured nebular metallicities. We quantify the ionizing continua using the ratio of the ionizing flux at 900 Å to the non-ionizing flux at 1500 Å and demonstrate the evolution of this ratio with stellar age and metallicity using theoretical single-burst models. These single-burst models only match the inferred ionizing continua of half of the sample, while the other half are described by a mixture of stellar ages. Mixed-age populations produce stronger and harder ionizing spectra than continuous star formation histories, but, contrary to previous studies that assume constant star formation, have similar stellar and nebular metallicities. Stellar population age and metallicity affect the far-UV continua in different and distinguishable ways; assuming a constant star formation history diminishes the diagnostic power. Finally, we provide simple prescriptions to determine the ionizing photon production efficiency (ξ(sub ion)) from the stellar population properties. The ξ(sub ion) inferred from the observed star-forming galaxies has a range of log(ξ(sub ion)) = 24.4–25.7 Hz erg(exp −1) that depends on the stellar population age, metallicity, star formation history, and contributions from binary star evolution. These stellar population properties must be observationally determined to accurately determine the number of ionizing photons generated by massive stars.

Chisholm, J.↗

NASA NTRS1993

Variations of the ultraviolet Fe II and Balmer continuum emission in the Seyfert galaxy NGC 5548

We present measurements of the Balmer continuum/Fe II emission blend between 2160 and 4130 A in the Seyfert galaxy NGC 5548. The measurements are from spectra obtained as part of the combined space-based and ground-based monitoring program of this object in 1988-1989. An iterative scheme is used to determine and subtract the continuum emission underlying the emission blend so as to obtain a light curve sampled once every four days. The small blue bump is an important component of the emission-line cooling, constituting about one third of the line flux in this object. Its flux varies with an amplitude of approximately +/- 20 percent about the mean, similar to the amplitude of the Balmer line variations during the same period. Its light curve resembles that of Ly-alpha, with a lag of about 10 days behind the continuum variations. The bump variation amplitude is independent of the wavelength interval where it is measured, which indicates that both the Balmer continuum and Fe II emission have comparable variation amplitudes. These results suggest that the Fe II UV multiplets and the Balmer continuum are emitted in the same parts of the broad-line region as most other broad emission lines in this object.

Maoz, D.↗

NASA NTRS1990

UV observations of x ray binaries

IUE (International Ultraviolet Explorer) has observed both high and low mass x ray binaries throughout its life. The UV spectra of high mass systems reveal the nature of the massive companion star and the effects of the x ray illumination of the stellar wind. In loss mass systems, the x ray illuminated disk or companion star dominates the UV light. System parameters and the characteristics of the accretion disk can be inferred.

Raymond, John C.↗

NASA NTRS1996

The Nearest Neutron Stars

Extreme Ultraviolet Explorer (EUVE) satellite observations of the Pulsar PSR J0437-4715, the Seyfert Galaxy RX J0437.4-4711, and the Geminga Pulsar are reported on. The main purpose of the PSR J0437-4715 investigation was to examine its soft X-ray flux. The 20 day EUVE observation of RX J0437.4-4711 constitutes a uniformly sampled soft X-ray light curve of a highly variable Seyfert galaxy whose power spectrum can be examined on timescales from 3 hrs. to several days. A unique aspect of the EUVE observation of RX J0437.4-4711 is its long light curve which we have used to measure the power spectrum of soft X-ray variability at low frequencies. Approximately 2100 counts were detected for the Geminga pulsar in a period of 251,000 s by the EUVE Deep Survey instrument. Geminga presents an unusually difficult problem because its multicomponent X-ray spectrum and pulse profile are indicative of a complex distribution of surface emission, and possibly a contribution from nonthermal emission as well.

Halpern, Jules P.↗