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NASA NTRS1991

Early results from the Cosmic Background Explorer (COBE)

Data obtained with the FIR Absolute Spectrophotometer, Differential Microwave Radiometers, and Diffuse IR Background Experiment (DIRBE) on the COBE satellite since its launch in November 1989 are briefly characterized. The COBE spacecraft and its 900-km 99-deg orbit are described; the scientific goals and capabilities of the instruments are reviewed; and sample DIRBE data are presented in a map and graph. Upper limits on the Comptonization parameter (y less than 0.001) and the chemical potential (mu less than 0.01 at the 3sigma level) are determined, and the spectrum of the dipole anisotropy is shown to be that of a Doppler-shifted blackbody. The DIRBE 100-micron sky brightness values at the ecliptic poles are found to be significantly lower than those measured by IRAS.

Mather, J. C.↗

NASA NTRS1991

First results of the COBE satellite measurement of the anisotropy of the cosmic microwave background radiation

The concept and operation of the Differential Microwave Radiometers (DMR) instrument aboard NASA's Cosmic Background Explorer satellite are reviewed, with emphasis on the software identification and subtraction of potential systematic effects. Preliminary results obtained from the first six months of DMR data are presented, and implications for cosmology are discussed.

Smoot, G. F.↗

NASA NTRS1991

Evolving starburst galaxies, faint number counts, and the 2 keV background

Blue and 60 micron extragalactic number counts and redshift distributions are modeled using evolutionary models in which galaxies periodically undergo transient starburst events. Support is found for the hypothesis that the same starburst phenomenon is responsible for the excess source counts seen at both wavelengths, though observed at lower mean redshift in the far infrared than in the blue. The soft X-ray emission from these evolving starburst events is not likely to exceed the 2 keV background unless the evolutionary rate is greater than (1 + z)-cubed (luminosity or density evolution) and the redshift of galaxy formation is greater than about five.

Lonsdale, C.↗

NASA NTRS1991

Test facility requirements for the thermal vacuum thermal balance test of the Cosmic Background Explorer Observatory

The Cosmic Background Explorer Observatory (COBE) underwant a thermal vacuum thermal balance test in the Space Environment Simulator (SES). This was the largest and most complex test ever conducted at this facility. The 4 x 4 m (13 x 13 ft) spacecraft weighed approx. 2223 kg (4900 lbs) for the test. The test set up included simulator panels for the inboard solar array panels, simulator panels for the flight cowlings, Sun and Earth Sensor stimuli, Thermal Radio Frequency Shield heater stimuli and a cryopanel for thermal control in the Attitude Control System Shunt Dissipator area. The fixturing also included a unique 4.3 m (14 ft) diameter Gaseous Helium Cryopanel which provided a 20 K environment for the calibration of one of the spacecraft's instruments, the Differential Microwave Radiometer. This cryogenic panel caused extra contamination concerns and a special method was developed and written into the test procedure to prevent the high buildup of condensibles on the panel which could have led to backstreaming of the thermal vacuum chamber. The test was completed with a high quality simulated space environment provided to the spacecraft. The test requirements, test set up, and special fixturing are described.

Milam, Laura J.↗

NASA NTRS1990

The cosmic microwave background - A probe of particle physics

The current status of spectral distortions and angular anisotropies in the cosmic microwave background is reviewed, with emphasis on the role played by weakly interacting particle dark matter. Theoretical predictions and recent observational results are described, and prospects for future progress are summarized.

Silk, Joseph↗

NASA NTRS1991

An accelerated lambda iteration method for multilevel radiative transfer. I - Non-overlapping lines with background continuum

A method is presented for solving multilevel transfer problems when nonoverlapping lines and background continuum are present and active continuum transfer is absent. An approximate lambda operator is employed to derive linear, 'preconditioned', statistical-equilibrium equations. A method is described for finding the diagonal elements of the 'true' numerical lambda operator, and therefore for obtaining the coefficients of the equations. Iterations of the preconditioned equations, in conjunction with the transfer equation's formal solution, are used to solve linear equations. Some multilevel problems are considered, including an eleven-level neutral helium atom. Diagonal and tridiagonal approximate lambda operators are utilized in the problems to examine the convergence properties of the method, and it is found to be effective for the line transfer problems.

Rybicki, G. B.↗

NASA NTRS1991

ROSAT detection of an X-ray shadow in the 1/4-keV diffuse background in the Draco nebula

The detection by the Roentgen satellite (ROSAT) X-ray telescope of a shadow in the 1/4-keV (C-band, 0.1 to 0.284 keV) cosmic diffuse background is reported. The location and morphology of the local minimum in X-rays are in clear agreement with a discrete H I cloud. The shadow is very deep with a minimum level at 50 percent of the surrounding emission; therefore, a minimum of 50 percent of the observed off-cloud flux must originate on the far side of the cloud. The analysis of H I velocity components links the cloud with the Draco nebula (distance of about 600 parsecs); it then follows that there is significant 1/4-keV X-ray emission at large distance (more than 400 parsecs) from the galactic plane along this line of sight. The extent of the distant emission region is uncertain, and if it indicates the existence of a hot galactic corona, it must be patchy in nature.

Snowden, S. L.↗

NASA NTRS1991

Do clusters of galaxies affect the spectrum of the microwave background?

The distortion, averaged over the sky, due to the Compton scattering of background photons with electrons in the hot gas in clusters of galaxies is calculated. Using an existing sample of X-ray clusters, various values of the density parameter Omega, and plausible models for cluster evolution, Monte Carlo realizations of the microwave sky are generated. The spatial structure of these simulations shows a network of discrete sources whose properties can be a strong function of both evolution and Omega. The amount of spectral distortion in the models is greatest for models characterized by self-similar cluster evolution in an open universe and is within an order of magnitude of the current upper limits. Thus, improved observational sensitivity must inevitably detect some deviation from a blackbody spectrum.

Markevitch, M.↗

NASA NTRS1991

Cross-correlation of the X-ray background with nearby galaxies

The detection of a signal in the cross-correlation of the diffuse 2-10 keV HEAO 1 A-2 X-ray surface brightness with the galaxy surface density derived from diameter-limited samples from the Uppsala General Catalogue is reported. An ad hoc relationship between the X-ray flux and the galaxy counts is used to estimate the local X-ray volume emissivity at 2.8 + or - 1.0 x 10 to the 38th ergs/s/cu Mpc. This result implies that unevolved populations of X-ray sources correlated with present-epoch galaxies can contribute only 13 + or - 5 percent of the cosmic X-ray background.

Jahoda, Keith↗

NASA NTRS1991

The origin of the cosmic X-ray background

A high-resolution image from the Rosat X-ray satellite reveals many faint discrete sources in the 0.1-2-keV energy range. Optical spectroscopy of these sources performed at the Anglo-Australian Telescope shows that many of them are quasars, and the inferred density of quasars on the sky contributes at least 30 percent of the cosmic X-ray background at 1 kev.

Shanks, T.↗

NASA NTRS1991

Small-scale fluctuations and angular correlations of the X-ray background in the HEAO 1 A-2 energy band - Constraints on clustering of X-ray sources

HEAO 1 A-2 all-sky survey data have been used to determine the amplitude of intensity fluctuations of the extragalactic 2-10 keV X-ray background (XRB) over an effective solid angle of 1.84 sq deg and their angular correlation function on angular scales of less than 3 deg. A good empirical fit to the data is obtained assuming that the integral counts in the A-2 band have a slope of 1.65 + 0.06 or - 0.07. Alternatively, the data may imply a significant clustering of extragalactic X-ray sources.

Martin-Mirones, J. M.↗

NASA NTRS1991

The intensity and spectrum of the diffuse X-ray background

The paper reports the first measurement of the intensity and spectrum of the diffuse X-ray background in the 0.16-3.5 keV band which is free from contamination by sources with fluxes greater than 4 x 10 to the -14th ergs/sq cm s. These results have been made possible by a number of techniques for reducing cosmic-ray contamination and instrumental artifacts in the data collected by the Einstein Observatory imaging proportional counter.

Wu, X.↗

NASA NTRS1991

Background issues for on-line aircraft documentation

Currently, almost all aircraft documentation in commercial aircraft cockpits is presented via hardcopy manuals. Several recent projects are aimed at eliminating all paper documentation in cockpits using electronic libraries. Electronic libraries encompass diverse information bases including aircraft system documentation, operations and procedures, checklists, maintenance logs, minimum equipment lists, maps and charts, and flight management information. These electronic libraries are envisioned to be embedded in the avionics so as to provide real time monitoring and display of information. Background issues are examined (motivation, information retrieval models, and preliminary designs) for the on-line presentation of aircraft systems documentation including operations, procedures, and checklists.

Russell, C. Ray↗

NASA NTRS1991

Cosmic background explorer (COBE) navigation with TDRSS one-way return-link Doppler in the post-helium-venting phase

A navigation experiment was performed which establishes Ultra-Stable Oscillator (USO) frequency stabilized one way return link Doppler TDRSS tracking data as a feasible option for mission orbit determination support at the Goddard Space Center Flight Dynamics Facility. The study was conducted using both one way and two way Tracking and Data Relay Satellite System (TDRSS) tracking measurements for the Cosmic Background Explorer (COBE) spacecraft. Tracking data for a 4 week period immediately follow the depletion of the helium supply was used. The study showed that, for both definitive orbit solution and short term orbit prediction (up to 4 weeks), orbit determination results based on one way return link Doppler tracking measurements are comparable to orbit determination results based on two way range and two way Doppler tracking measurements.

Nemesure, M.↗

NASA NTRS1992

Spectral-luminosity evolution of active galactic nuclei and the cosmic X- and gamma ray background

Coherent electromagnetic dynamo acceleration processes, which act on charge particles within the context of black hole accretion disk scenarios, are generally regarded as the underlying central power source for active galactic nuclei (AGN). If the precursor active galaxies (PAG) for such AGN are formed at high redshift and contain initial seed black holes with mass approximately equal to 10(exp 4) solar masses, then the Eddington limited X-ray radiation emitted during their lifetime will undergo the phenomenon of 'spectral-luminosity evolution'. When accretion disks are first formed at the onset of galaxy formation the accretion rate occurs at very high values of luminosity/size compactness parameter L/R greater than 10(exp 30) erg/cm-sec. In the absence of extended structure, such high values of L/R generate dynamic constraints which suppress coherent, black hole/accretion disk dynamo particle acceleration processes. This inhibits nonthermal radiation processes and causes the spectrum of X-radiation emitted by PAG to be predominantly thermal. A superposition of PAG sources at z is greater than or equal to 6 can account for the residual cosmic X-ray background (CXB) obtained from the total CXB after subtraction of foreground AGN sources associated with present epoch Seyfert galaxies. The manner in which the PAG undergo spectral-luminosity evolution into Seyfert galaxies is investigated.

Leiter, Darryl↗

NASA NTRS1992

Sensitivity of LDEF foil analyses using ultra-low background germanium versus large NaI(T1) multidimensional spectrometers

Cobalt samples were analyzed for cosmic ray induced Co-60 with both an ultralow background germanium gamma ray spectrometer and with a large NaI(T1) multidimensional spectrometer using electronic anticoincidence shielding. Aluminum samples were analyzed for Na-22. The results are presented along with the relative sensitivities and precision afforded by the two methods.

Reeves, James H.↗

NASA NTRS1992

High background photon counting lidar

Photon counting with lidar returns is usually limited to low light levels, while wide dynamic range is achieved by counting for long times. The broad emission spectrum of inexpensive high-power semiconductor lasers makes receiver filters pass too much background light for traditional photon counting in daylight. Very high speed photon counting is possible, however, at more than 500 MHz which allows the construction of eyesafe lidar operating in the presence of bright clouds. Detector improvements are possible to count to 20 GHz producing a single shot dynamic range of ten decades.

Lentz, W. J.↗

NASA NTRS1991

The lightest supersymmetric particle and the extragalactic gamma-ray background

The possibility that cosmological photino annihilation is caused by the extragalactic gamma-ray background (EGB) is examined with particular attention given to the lightest supersymmetric particle (LSP). The LSP is considered a general type of the best-motivated candidates for cosmic dark matter (CDM). The theoretical analysis employs a corrected assumption for the annihilation cross section, and cosmological integrations are performed through the early phases of the universe. Romberg's method is used for numerical integration, and the total optical depth is developed for the gamma-ray region. The computed LSP-type annihilation fluxes are found to be negligible when compared to the total EGB observed, suggesting that the LSP candidates for CDM are not significant contributors to the EGB.

Gao, Yi-Tian↗