Search NASA⌕ Search

Engineering topics

O'Dell, S. L.

Publications and source records attributed to O'Dell, S. L..

34 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1997

AXAF Detector Backgrounds Produced By Cosmic Ray Protons

One of the science instruments on the Advanced X-ray Astrophysics Facility (AXAF), planned for launch in 1998 into a highly elliptical (10,000 km x 140,000 km) orbit, is a microchannel plate High Resolution Camera (HRC). This detector is designed to provide imaging and spectroscopic observations of x-rays emitted by stellar sources in the 0.1 to 10 keV energy range. Described here are analyses made to determine the expected time-dependent detector background from prompt and delayed (activation) radiation initiated by galactic cosmic-ray (GCR) proton interactions in the spacecraft and payload. Numerical simulations were made using the coupled set of Monte Carlo radiation transport codes, analysis software, and data bases shown. The major codes are HETC for nucleon-meson transport, EGS for simulating electromagnetic cascades, and MORSE for low-energy (less than 15 MeV) neutron transport. The simulation follows the transport history of photons in the energy range from - 100 GeV down to approx. 0.1 keV due to gamma-ray sources from neutral pion decay, high-energy (spallation) collisions, and low-energy neutron inelastic scattering and capture reactions. Also included is radioisotope production and the tracking of gamma-rays, electrons, and positrons from induced radioactivity.

Armstrong, T. W.↗
NASA NTRS1997

Uses of continuum radiation in the AXAF calibration

X-ray calibration of the Advanced X-ray Astrophysics Facility (AXAF) observatory at the MSFC X-Ray Calibration Facility (XRCF) made novel use of the x-ray continuum from a conventional electron-impact source. Taking advantage of the good spectral resolution of solid-state detectors, continuum measurements proved advantageous in calibrating the effective area of AXAF's High-Resolution Mirror Assembly (HRMA) and in verifying its alignment to the XRCF's optical axis.

Kolodziejczak, J. J.↗
NASA NTRS1994

Statistical uncertainties in temperature diagnostics for hot coronal plasma using the ASCA SIS

Statistical uncertainties in determining the temperatures of hot (0.5-10 keV) coronal plasmas are investigated. The statistical presicion of various spectral temperature diagnostics is established by analyzing synthetic ASCA solid-state imaging spectrometer (SIS) CCD spectra. The diagnostics considered are the ratio of hydrogen-like to helium-like line complexes of Z greater than or = 14 elements, line-free portions of the continuum, and the entire spectrum. While fits to the entire spectrum yield the highest statistical precision, it is argued that fits to the line-free continuum are less susceptible to atomic data uncertainties but lead to a modest increase in statistical uncertainty over full spectral fits. Temperatures deduced from line ratios can have similar accuracy, but only over a narrow range of temperatures. Convenient estimates of statistical accuracies for the various temperature diagnostics are provided which may be used in planning ASCA SIS observations.

Swartz, Douglas A.↗
NASA NTRS1994

Results of a five-year program of multifrequency monitoring of low-frequency variable radio sources

We present the results of a detailed multifrequency monitoring program of low-frequency variable radio sources. This consists of light curves at 318, 430, 606, 880, and 1400 MHz over a 5 year period. The observations were carried out with the Arecibo 305 m radiotelescope and the Green Bank 91 m radio telescope. The spectral characteristics of the variations confirm the general picture that at least two mechanisms are responsible. The first is clearly intrinsic evolution of synchrotron-emitting components as it is manifested by variations that appear first and most strongly at high frequencies, subsequently drifting to lower frequencies with diminished amplitude. The more common type of low-frequency variability, however, dominates at frequencies below approximately 800 MHz, while the variations near 1 GHz are often quite weak. This spectral property is strong evidence that these variations are interstellar refractive scintillation.

Mitchell, K. J.↗
NASA NTRS1993

Molecular contamination and the calibration of AXAF

By the end of this decade, the two missions comprising the Advanced X-ray Astrophysics Facility (AXAF) will become the X-ray component of NASA's Great Observatories. In order to meet some of the ambitious scientific objectives, the AXAF team plans to calibrate the telescopes and instruments to an accuracy of a few percent. Particulate and molecular contamination potentially limit this precision, because they would likely change between ground calibration and orbital operation. To avoid this requires careful control over procedures affecting the coated optical surfaces, to ensure that neither particulate nor molecular contamination compromise the calibration. In particular, the fractional areal coverage by particulates or condensed droplets must be less than 0.005 projected onto the aperture plane; the thickness of any molecular film must be less than a few tens of angstrom.

Elsner, R. F.↗
NASA NTRS1993

Evidence for dust contamination on the VETA-1 mirror surface

Relative surface brightness data from the VETA-1 test displayed an energy dependence which was inconsistent with models which assume surface roughness is the only cause of scattering. A means of separating an in-plane component of scattering caused by surface roughness from an azimuthally symmetric component, which is expected from dust, was afforded by images taken while only single quarters of the mirror surface were exposed. In this case, in-plane scattered X-rays should populate only opposing 90 deg azimuthal quadrants of an image centered on the focal point. As such, this approach is a novel, high sensitivity test for detecting extremely small (about 10 exp -5) fractional dust coverage on X-ray optics. Comparison of in-plane and out-of-plane quadrants in these tests indicated that most of the scattered data were azimuthally symmetric, and that the symmetric component was enhanced at lower energies. Both results support a model which invokes the wing scan results with a combination of dust and surface roughness induced scattering. The extent to which parameters such as the mirror surface roughness, dust size distribution, spatial distribution, and density may be determined using this approach is also discussed.

Kolodziejeczak, J. J.↗
NASA NTRS1993

X-ray evidence for particulate contamination on the AXAF VETA-1 mirrors

X-ray testing of the AXAF outer mirror pairs, in the VETA-1 configuration, reveals a point spread function (PSF) with unexpectedly large wings at low energies. Although the angular dependence in the wings of the PSF is close to that expected for diffractive scattering from surface roughness, the energy dependence differs substantially. Analyses of the observed X-ray PSF, images near ring focus, and single-quadrant images at conjugate focus suggest that the excess scattering observed at low X-ray energies results from diffractive scattering by relatively small grains (as small as a few tenths micrometer in radius). We develop a simple model for the contribution of scattering by particulates to the PSF. Merging this model with that for scattering by surface roughness, we fit the combined model to the observed energy-dependent PSF, in order to estimate parameters and associated uncertainties characterizing the grain-size distribution and the surface-roughness power spectral density. In particular, we find that the fractional coverage of the mirrors by particulates is approximately 1 x 10 exp -4 (for grain radii between 0.1 and 10 microns), and that the rms surface-roughness is approximately 0.7 nm (for spatial frequencies between 1/mm and 1000/mm).

O'Dell, S. L.↗
NASA NTRS1993

The X-ray reflectivity of the AXAF VETA-I optics

The study measures the X-ray reflectivity of the AXAF VETA-I optic and compares it with theoretical predictions. Measurements made at energies of 0.28, 0.9, 1.5, 2.1, and 2.3 keV are compared with predictions based on ray trace calculations. Results on the variation of the reflectivity with energy as well as the absolute value of the reflectivity are presented. A synchrotron reflectivity measurement with a high-energy resolution over the range 0.26 to 1.8 keV on a flat Zerodur sample is also reported. Evidence is found for contamination of the flat by a thin layer of carbon on the surface, and the possibility of alteration of the surface composition of the VETA-I mirror, perhaps by the polishing technique. The overall agreement between the measured and calculated effective area of VETA-I is between 2.6 and 10 percent. Measurements at individual energies deviate from the best-fitting calculation to 0.3 to 0.8 percent, averaging 0.6 percent at energies below the high energy cutoff of the mirror reflectivity, and are as high as 20.7 percent at the cutoff.

Kellogg, E.↗
NASA NTRS1991

Effective area of the AXAF X-ray telescope - Dependence upon dielectric constants of coating materials

This study examines the dependence of the effective area of the AXAF X-ray telescope upon the complex dielectric constants of possible mirror coatings, over the energy range 0.1-10 keV. At energies near and above the astrophysically important iron-line complex near 6.7 keV, the effective area is very sensitive to the coating density on the three innermost of the telescope's six mirror pairs. Thus, it is desirable to achieve as high a density as feasible. The telescope's spectral response exhibits sharp features at absorption edges of the coating materials. In view of the exceptional energy resolution of the AXAF spectrometers and uncertainties in reflectivities (especially near absorption edges), the AXAF goal of 1-percent accuracy requires both the precise X-ray calibration of the telescope and improved modeling of the telescope and calibration sources. Presented here in the form of plots, the results are also available (on floppy disks) in tabular form.

Elsner, R. F.↗
NASA NTRS1988

Flat-spectrum, variable radio sources

The general behavior of flat-spectrum (compact) radio sources is examined in terms of adiabatic-jet models. Two puzzling properties - namely, (1) the broad, rather flat spectrum (over a large range of radio frequencies) and (2) the relatively slow decay of burst amplitude (with decreasing radio frequency) - are explained. Acceptable models are characterized by the following: (1) a nearly conical, adiabatic jet, with conserved magnetic flux transverse to the axis of the jet; (2) prolonged injection (for which the duration of an event exceeds the apparent transit time scale); and (3) a transparent spectral index which is not too steep. It is suggested that the acceleration mechanism in the core of compact jets may differ substantially from that far from the core, producing a flatter electron number index s = 1.4-2.6.

O'Dell, S. L.↗
NASA NTRS1988

Radio variability of the blazar AO 0235 + 164

The high-redshift blazar A0 0235 + 164 exhibits flux-density variations which are primarily of the less common variety in which low-frequency flux-density variations track the high-frequency variations but are delayed and of smaller amplitude. Observational results based on five years of monitoring are presented which are correlated over at least a factor of 50 frequency range in the sense expected for an expanding synchrotron component: outbursts propagating toward lower frequencies with diminishing amplitudes. A simple, semiempirical jet model is developed which accounts reasonably well for the radio properties of the object. The predictions of the model are compared with observations, examining the radio flux-density histories, the radio spectral evolution, the radio structure, and evidence for relativistic bulk motion.

O'Dell, S. L.↗
NASA NTRS1973

The distribution of redshifts of radio galaxies with different optical spectra and forms

The distribution of redshifts of 222 radio galaxies has been subjected to a power-spectrum analysis. No statistically significant spectral peaks are present for galaxies showing normal absorption-line characteristics; on the other hand, significant spectral peaks are present for those exhibiting strong emission features. Of these spectral peaks, the most significant, at a wavelength of 0.031 in redshift, occurs for those strong-emission radio galaxies which are also compact. The result is consistent with the proposal suggested earlier by Burbidge that the nonrandom features, which may suggest intrinsic redshifts, are confined to those objects - QSOs, N systems, etc. - in which radiation from stars does not dominate. Possible explanations of the apparently nonrandom features are briefly discussed. Finally, a table of the most recently published small-redshift QSOs and compact emission-line galaxies is presented. The redshifts appear to conform to the 0.061 (or 0.031) periodicity found earlier.

Burbidge, G. R.↗
NASA NTRS1973

The redshift-magnitude relation for quasi-stellar objects.

It is shown that the best-fitting slope of the redshift-magnitude relation for the most luminous quasi-stellar object in each redshift range is 4.3 plus or minus 0.4. However, the steepness of this slope is due primarily to the lowest redshift range including 3C 273, without which it is 2.1 plus or minus 0.6. The slopes of the second and third brightest QSOs are 3.1 plus or minus 0.6 and 3.2 plus or minus 0.6, respectively, markedly less than the slope of 5 which is expected for a Friedmann universe with q(0) = +1. Although the departure of the slope from 5 mag per decade is statistically significant at about 95% confidence, due to the heterogeneity and incompleteness of the sample there may be a systematic selectional bias not accounted for either in this analysis or in that of Bahcall and Hills. Consequently, no definite pronouncement can be made until a complete sample of adequate size is examined.

Burbidge, G. R.↗
NASA NTRS1972

Physical associations between quasi-stellar objects and galaxies.

Evaluation of evidence pertaining to apparent associations between galaxies and quasi-stellar objects. It is shown that for the five 3C QSOs which lie very close to bright galaxies the QSO-galaxy angular separations are inversely proportional to the redshifts of the galaxies. This lends additional support to the association hypothesis. However, an analysis of an essentially disjoint sample of QSOs identified from the Parkes radio catalog, and fainter galaxies in the catalog of Zwicky and his associates, reveals no statistically significant tendency toward pairing of QSOs and galaxies. The results can be explained in one of two ways. One is that, despite the evidence to the contrary, the 3C results are due to chance. The other is that real associations in the second sample are masked by selection effects. It is shown that several selectional biases do indeed discriminate against the detection of QSO-galaxy pairs, especially for fainter objects. Finally, on the assumption that the 3C QSO-galaxy associations are real, the relationship of QSOs to galaxies is discussed.

Burbidge, G. R.↗