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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Faraday rotation measure variations in the Cygnus region and the spectrum of interstellar plasma turbulence

Linear polarization observations were made of eight double-lobed radio galaxies viewed through the galactic plane in the Cygnus region. These observations have been used to determine intra- and intersource rotation measure differences; in some cases, unambiguous rotation measures have been extracted. The rotation measures are dominated by foreground magnetoionic material. The differences in rotation measure between pairs of sources correlate with angular separation for separations from 10 arcsec to 1.5 deg. These rotation measure fluctuations are consistent with a model in which the electron density varies on roughly 0.1-200 pc scales. The amplitudes of these variations are, in turn, consistent with those electron density variations that cause diffractive interstellar scattering on scales less than 10 to the 11th cm.

Lazio, T. Joseph↗
NASA NTRS1992

Position and morphology of the compact non-thermal radio source at the Galactic Center

We have determined with VLBI the position of the compact nonthermal radio source at the Galactic Center, commonly referred to as SgrA*, in the J2000.0 reference frame of extragalactic radio sources. We have also determined the size of SgrA* at 1.3, 3.6, and 13 cm wavelengths and found that the apparent size of the source increases proportionally to the observing wavelength squared, as expected from source size broadening by interstellar scattering and as reported previously by other authors. We have also established an upper limit of about 8 mJy at 3.6 cm wavelength for any ultracompact component. The actual size of the source is less than 15 AU. Fourier analysis of our very sensitive 3.6 cm observations of this source shows no significant variations of correlated flux density on time scales from 12 to 700 s.

Marcaide, J. M.↗
NASA NTRS2012

Discovery of an Energetic Pulsar Associated with SNR G76.9+1.0

We report the discovery of PSR J2022-<-3842, a 24 ms radio and X-ray pulsar in the supernova remnant G76.9+i.0, in observations with the Chandra X-ray telescope, the Robert C. Byrd Green Bank Radio Telescope, and the Rossi X-ray Timing Explorer (RXTE). The pulsar's spin-down rate implies a rotation-powered luminosity E = 1.2 X 10(exp 38) erg/s, a surface dipole magnetic field strength B(sub S), = 1.0 X 10(exp 12) G, and a characteristic age of 8.9 kyr. PSR J2022+3842 is thus the second-most energetic Galactic pulsar known, after the Crab, as well as the most rapidly-rotating young, radio-bright pulsar known. The radio pulsations are highly dispersed and broadened by interstellar scattering, and we find that a large (delta f/f approximates 1.9 x 10(exp -6)) spin glitch must have occurred between our discovery and confirmation observations. The X-ray pulses are narrow (0.06 cycles FWHM) and visible up to 20 keV, consistent with magnetospheric emission from a rotation-powered pulsar. The Chandra X-ray image identifies the pulsar with a hard, unresolved source at the midpoint of the double-lobed radio morphology of G76.9+ 1.0 and embedded within faint, compact X-ray nebulosity. The spatial relationship of the X-ray and radio emissions is remarkably similar to extended structure seen around the Vela pulsar. The combined Chandra and RXTE pulsar spectrum is well-fitted by an absorbed power-law model with column density N(sub H) = (1.7 +/- 0.3) x 10(exp 22) / sq cm and photon index Gamma = 1.0 +/- 0.2; it implies that the Chandra point-source flux is virtually 100% pulsed. For a distance of 10 kpc, the X-ray luminosity of PSR J2022+3842 is L(sub x){2-1O keV) = 7.0 x 10(exp 33) erg/s. Despite being extraordinarily energetic, PSR J2022+3842 lacks a bright X-ray wind nebula and has an unusually low conversion efficiency of spin-down power to X-ray luminosity, Lx/E = 5.9 X 10(exp-5).

Arzoumanian, Zaven↗
NASA NTRS1982

The scattering phase function of interstellar grains - The case of the reflection nebula NGC 7023

IUE observations of the reflection nebula NGC 7023 and the illuminating star, HD 200775, in the spectral range from 1300 to 3100 A are combined with ground-based measurements of the nebular brightness distribution at 3500, 4100, 4700, and 5500 A to determine the scattering properties of the nebular dust grains. Total nebular fluxes are derived from existing data obtained by satellite and ground-based observations. The relevance of far-IR fluxes from the nebula is considered, and models are derived that are applicable to NGC 7023 both with respect to the ratio of nebular to stellar fluxes and with respect to the nebular surface brightness distribution. It is shown that the average grain albedo in the UV is about 0.54 and that the albedo increases to a level of about 0.6 at 1400 A after reaching a minimum of approximately 0.4 near 2200 A. The results suggest that isotropically scattering particles of high albedo make a significant contribution to interstellar scattering in the far-UV.

Witt, A. N.↗
NASA NTRS1989

New measurements of the far ultraviolet scattering properties of interstellar dust

An analysis is presented for the spectra of the diffuse ultraviolet background taken during shuttle flight STS-61C (January 1986). Eight regions of the sky were observed for approximately 20 minutes each, using a spectrograph designed specifically to perform measurements of the UV background. The field of view was 3.8 deg x 8 min, with imaging along the slit to confine stellar contamination. The instrument featured a shutter mechanism to measure internal background during flight, a low-scatter holographically ruled diffraction grating, photon counting microchannel plate detectors, through baffling, and a crystal window to further attenuate stray light. The spectra covered the range 1400 to 1850 A and was binned in 50 A bands. The procedure for substracting the contribution of stars too faint to be detected as discrete sources during the observations (in general this represents a small fraction of the total intensity detected except at the longest wavelengths). A radiative transfer model used to interpret the data and set confidence intervals on the relevant parameters is described. It was found that the continuum component of the diffuse ultraviolet background arises primarily from two sources. One source is scattering of starlight by interstellar dust with an albedo of about 12 percent and a relatively isotropic phase function. A second source consists of about 150 photons/cm/sec/ster/A of extragalactic light which is attenuated by the dust in the galaxy. Although emission features possibly associated with molecular H2 are detected in one look direction, fluorescence of H2 is not a major contributor to the diffuse UV background, at least at galactic latitudes greater than -10 degrees.

Hurwitz, Mark↗
NASA NTRS1973

Some scattering problems of interstellar grains

Consideration of the general effects of size, shape, chemical composition, index of refraction, temperature, and other physical parameters of small particles upon their scattering properties. A determination of the relative importance of these effects to interstellar dust interpretations is attempted with the ultimate aim of a full definition of interstellar grain optics.

Greenberg, J. M.↗
NASA NTRS1984

Pioneer 10 and Voyager observations of the interstellar medium in scattered emission of the He584 A and H Lya 1216 A lines

The combination of Pioneer photometric and Voyager spectrometric observations of EUV interstellar-interplanetary emissions in the region beyond 5 A was applied to a determination of atomic hydrogen and helium densities. These density estimates obtained from direct measurement of scattered radiation depend on absolute calibration of the instruments in the same way as other earlier determinations based on the same method. However, the spacecraft data were combined with daily full sun averages of the H Lyman 1216 A line obtained by the Solar Mesospheric Explorer satellite to obtain a measure of atomic hydrogen density independent of instrument absolute calibration. The method depends on observations of long and short term temporal variability of the solar line over a one year period, and the fact that the ISM is optically thick. The density estimates from preliminary work on these observations are H = 0.12 cu cm and H = .016 cu cm, giving a density ratio close to the cosmic abundance value in contrast to some earlier results indicating a depletion of atomic hydrogen. Estimates were obtained of galactic background emissions in the signals of both spacecraft.

Shemansky, D. E.↗
NASA NTRS1984

Pioneer 10 and Voyager Observations of the Interstellar Medium in Scattered Emission of the He 584 a and H Lya 1216 a Lines

The combination of Pioneer photometric and Voyager spectrometric observations of EUV interstellar-interplanetary emissions in the region beyond 5 A was applied to a determination of atomic hydrogen and helium densities. These density estimates obtained from direct measurement of scattered radiation depend on absolute calibration of the instruments in the same way as other earlier determinations based on the same method. However, the spacecraft data were combined with daily ll sun averages of the H Lyman 1216 A line obtained by the Solar Mesospheric Explorer satellite to obtain a measure of atomic hydrogen density independent of instrument absolute calibration. The method depends on observations of long and short term temporal variability of the solar line over a one year period, and the fact that the ISM is optically thick. The density estimates from preliminary work on these observations are H = 0.12/cu cm and H = .016/cu cm, giving a density ratio close to the cosmic abundance value in contrast to some earlier results indicating a depletion of atomic hydrogen. Estimates were obtained of galactic background emissions in the signals of both spacecraft.

Shemansky, D. E.↗
NASA NTRS1984

Pioneer 10 and Voyager Observations of the Interstellar Medium in Scattered Emission of the H 584 A and H Lya 1216 A Lines

The combination of Pioneer photometric and Voyager spectrometric observations of EUV interstellar-interplanetary emissions in the region beyond 5 AU have been applied to a determination of atomic hydrogen and helium densities. These density estimates obtained from direct measurement of scattered radiation depend on absolute calibration of the instruments, in the same way as other earlier determinations based on the same method. However. we have combined the spacecraft data with daily full sun averages of the H Lya 1216 A line obtained by the Solar Mesospheric Explorer (SME) satellite, to obtain a measure of atomic hydrogen density independent of instrument absolute calibration. The method depends on observations of long and short term temporal variability of the solar line over a 1 year period, and the fact that the ISM is optically thick. The density estimates from preliminary work on these observations are (H) = 0.12 cm(sup 2) and (He) = .016 cm(sup 2), giving a density ratio close to the cosmic abundance value, in contrast to some earlier results indicating a depletion of atomic hydrogen. We have obtained estimates of galactic background emissions in the signals of both spacecraft.

Shemansky, D. E.↗
NASA NTRS1991

The albedo and scattering phase function of interstellar dust and the diffuse background at far-ultraviolet wavelengths

The diffuse FUV background in a variety of targets has been observed with a nebular spectrometer designed specifically to eliminate potential sources of contamination. The intensity of the background near the Galactic plane is found to be much lower than previously published. The data are analyzed using a radiative transfer model with a limited set of free parameters, including those which describe the scattering properties of the interstellar dust. Strong evidence is presented that scattering of starlight by Galactic dust associated with neutral hydrogen produces the major part of the FUV diffuse background. Few assumptions are necessary to conclude that the albedo of the grains is low in the FUV. The grains, at least at high latitude, scatter fairly isotropically in the FUV. The low FUV albedo and the usual phase factor g are not consistent with predictions of standard models for the size distribution and composition of interstellar grains.

Hurwitz, Mark↗
DOE OSTI2023

Collisional excitation of C2H− by H2: New interaction potential and scattering calculations

Interstellar anions play an important role in astrochemistry as being tracers of the physical and chemical conditions in cold molecular clouds and circumstellar gas. The local thermodynamic equilibrium is generally not fulfilled in media where anions are detected and radiative and collisional data are required to model the observed lines. The C2H− anion has not yet been detected in the interstellar medium; however, collisional data could be used for non-LTE models that would help in identifying the most intense lines. For this purpose, we have computed the first 4D potential energy surface (PES) of the C2H−–H2 complex using an explicitly correlated coupled-cluster approach. The PES is characterized by a single deep minimum with a well-depth of 924.96 cm−1. From this interaction potential, we derived excitation cross sections and rate coefficients of C2H− induced by collisions with para- and ortho-H2. The results obtained for collisions with para-H2 are compared to previous calculations performed using a 2D-PES obtained from an average over H2 rotations.

Chemistry↗
DOE OSTI2025

A Unified Model of Cosmic-Ray Propagation and Radio Extreme Scattering Events from Intermittent Interstellar Structures

Intermittent magnetic structures are a plausible candidate for explaining cosmic-ray (CR) diffusion rates derived from observed CR energy spectra. Independently, studies of extreme scattering events (ESEs) of radio quasars and pulsar scintillation have hinted that very straight, large aspect ratio magnetic current sheets may be responsible for the localized large scattering of radio waves. The required shortest axis of the typical structures producing ESEs is of the same scale (∼au) as the gyroradii of ∼GeV CRs. In this Letter, we propose that the same magnetic/density sheets can produce large scattering of both CRs and radio waves. We demonstrate that the geometry and volume-filling factor of the sheets derived from quasar ESEs can explain the observed mean free path of GeV CRs without introducing free parameters. The model places constraints on the sheet geometry, such as straightness and large aspect ratio, and assumes that the statistics of the sheets are similar throughout the Galactic volume. We therefore discuss observational tests of the sheet model, which includes observations of echoes in pulsars and fast radio bursts, gravitationally lensed quasars, the distribution of ESE durations, and spatial correlations between ESE events and rotation measure fluctuations. Such tests will be enabled by upcoming wide-field radio instruments, including the Canadian Hydrogen Observatory and Radio-transient Detector and Deep Synoptic Array 2000 Antennas.

Kempski, Philipp [Princeton Univ., NJ (United Stat↗
NASA NTRS1992

Sites of fine-scale electron density turbulence in the interstellar medium

Observations of radio-wave scattering effects probe interstellar electron-density turbulence. But with limited use of complementary data sets only broad conclusions have been obtained. The locations of this turbulence are unknown (phases, objects, etc.), as is the mechanism for its production, although it could play a role in various galactic processes. The IRAS data set was used to search for correlations between scattering and structures traced by infrared emitting dust. The existent HI (21-cm line emission), and 408-MHz radio-continuum all-sky surveys were used to search for correlations between scattering and structures traced by these emissions. Only lines of sight outside the galactic plane were considered (to better delimit the structures which might be causing scattering). No correlation is apparent between 21-cm line emission, or infrared continuum emission and interstellar scattering. However, a correlation exists between the scattering toward extragalactic radio sources and 408-MHz continuum emission.

Simonetti, John H.↗
NASA NTRS1978

Far-ultraviolet studies. III - A search for light scattered at large angles by dust

The Apollo 17 far-ultraviolet spectrometer was used during trans-earth coast to measure the brightness of two regions at moderate galactic latitudes. The signal obtained can be entirely accounted for by light from stars in the field of view, with no contribution from galactic-plane starlight scattered off interstellar dust. It is concluded that the interstellar grains are extremely poor large-angle scatterers in the far-ultraviolet.

Henry, R. C.↗
NASA NTRS1986

Detection of scattering in the 2175 A interstellar band

Spectrophotometric observations of two reflection nebulae and their illuminating stars obtained with the International Ultraviolet Explorer Satellite have provided the first evidence for the presence of a scattering contribution to the 2175 A interstellar extinction band. Lower than normal far-UV extinction for the stars embedded in the nebulae indicates that the nebulae have a dust particle size distribution that is dominated by larger particles. The strength of the 2175 A band is larger than normal in both cases. The scattering is found to dominate the long-wavelength wing of the band, without shifting the central wavelength of the band by more than 20 A toward longer wavelengths. These observations are taken to indicate that the solid particles responsible for the 2175 A band can be considerably larger than the Rayleigh limit in some interstellar locations. The absence of a notable shift in the central wavelength of the band in such large particles presents a new severe constraint for models of interstellar grains.

Witt, A. N.↗