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NASA NTRS1977

Spatial distribution of Compton-produced X-ray flux from rich and regular clusters of galaxies

The spatial distribution of radio and X-ray fluxes from rich and regular clusters of galaxies is discussed. A diffusion model is investigated in which high-energy electrons are ejected from a point source at the center of the cluster. While diffusing out, the electrons lose energy by Compton scattering with the universal blackbody photons, thus producing the X-ray emission. Traversing intracluster magnetic fields, the electrons emit synchrotron radio emission. For the Coma cluster of galaxies good agreement is found between the observed and predicted X-ray counting-rate distribution for the Compton model. From the observed radio data a value of about 3 by 10 to the -8th power gauss is deduced for the average magnetic-field strength in the central region of Coma.

Rephaeli, Y.↗

NASA NTRS1977

Far-infrared spectrometry of H II regions and the Galactic Center

The H II regions W3, M17, and W51, as well as the Galactic center regions Sgr A and Sgr B2, were observed at a spectral resolving power of roughly 15 in the range from 45 to 115 microns. At that resolution the spectra appear featureless and can be approximated by dilute blackbody curves whose temperatures range from approximately 90 K down to about 40 K.

Ward, D. B.↗

NASA NTRS1977

Radiometry of satellites and of the rings of Saturn

Brightness temperatures at wavelengths between 8 microns and 3.7 cm are tabulated for satellites of known size; except for Titan, they are consistent with blackbody emission values. For Titan a more complex thermal radiation spectrum with three separate regimes is identified. Radiometric brightness measurements, when used in conjunction with photometry, are shown to determine satellite sizes and albedos; this technique is calibrated by using satellites and asteroids with radii evaluated from other methods. Eclipse radiometry indicates the thermophysical properties of surface layers; such observations show that the uppermost coverings of Phobos, Callisto, and Ganymede have remarkably low thermal conductivities. Infrared and radar detections of the rings of Saturn are surveyed; the important constraints that they provide on ring particle sizes and bulk composition are outlined.

Morrison, D.↗

NASA NTRS1977

Spectrum and isotropy of the submillimeter background radiation

Infrared evidence referable to the spectrum and isotropy of the background radiation of the universe is considered, along with theoretical implications. Difficulties in the submillimeter region are indicated. Two types of anisotropy are treated: small-scale granularity (attributable to discrete sources such as galaxies or to condensation in the primordial plasma) and large-scale anisotropies; the anisotropies are discussed as possible indicators of the early history of the universe, and in relation to future possible generalized Michelson-Morley experiments (to detect effects on local physics of the existence of a universal rest frame). A complete blackbody spectrum is seen as the clear signature of the primordial fireball, and slight deviations from a perfect thermal spectrum (or perfect isotropy) are to be sought and scrutinized.

Muehlner, D.↗

NASA NTRS1977

Precision measurements of NH3 spectral lines near 11 microns using the infrared heterodyne technique

Absolute line-center frequencies for eleven lines of ammonia in near coincidence with CO2-laser transitions have been determined to accuracies of + or -3 MHz by infrared heterodyne detection. These results were obtained by heterodyning a blackbody with a stable grating-tuned CO2 gas laser. A discussion of the apparatus and method of calculation, including error analysis, is presented. With these accurately determined line-center positions, the ammonia molecule will be a useful secondary-frequency standard for diode-laser spectroscopy in the 11-micron wavelength region.

Hillman, J. J.↗

NASA NTRS1977

Observations of the X-ray burst source MXB 1636-53

X-ray bursts have been observed from MXB 1636-53, almost certainly associated with the strong steady X-ray source 2S 1636-536 (Norma X-1, 3U 1636-53). The steady source was observed in January 1977 at roughly half the intensity reported in the 3U catalog and showed about 15% variability on a time scale of hours. The spectra of the X-ray bursts are well fitted by blackbody radiation whose temperature rises rapidly to a maximum of approximately 28 million K and then cools slowly. If the source is at a distance of 10 kpc, the radius of the projected burst emission region is about 10 km, similar to the size of a neutron star.

Hoffman, J. A.↗

NASA NTRS1977

Calibration analysis for a multi-channel infrared scanning radiometer

A procedure for calibrating an infrared scanning spectroradiometer by a computerized parametric error analysis technique was developed. The uncertainties in the radiometric measurements of scene radiance and (for the case of a blackbody scene) temperature due to possible uncertainties in the calibration target temperature, calibration target emissivity, and instrument temperature were calculated for a range of uncertainty levels in the parameters, as well as for a gamut of scent temperatures corresponding to a given spectral channel. This technique is applicable to the radiometric calibration of any infrared radiometer. It was applied specifically to the Cloud-Top Scanning (C.T.S) Radiometer, a three-channel instrument designed for aircraftborne cloud radiance measurements in the 6.75 and 11.5 micron thermal emission spectral regions.

Walden, H.↗

NASA NTRS1976

Broadband photoexcitation of lasers

Existing techniques are reviewed and novel approaches for broadband photoexcitation of lasers are discussed. This subject is of interest because of the emergence of future intense near equilibrium and blackbody sources, such as the gas core reactor, and electric discharge plasma devices, which are the key to the development of intense broadband radiation sources. The possible use of radiation from solar concentrators in space also fits into this general area. The varied use of photoexcitation of lasers are discussed ranging over direct optical pumping, photodissociation, and photoreionization. In addition, the new uses of laser transitions and chemical reactions is shown. This is accomplished through multiline operation of high pressure tunable lasers which can use the broadband radiation for multiphoton excitation.

Hess, R. V.↗

NASA NTRS1978

Calibration of longwavelength exotech model 20-C spectroradiometer

A brief description of the Exotech model 20-C field spectroradiometer which measures the spectral radiance of a target in the wavelength ranges 0.37 to 2.5 microns (short wavelength unit), 2.8 to 5.6 microns and 7.0 to 14 microns (long wavelength unit) is given. Wavelength calibration of long wavelength unit was done by knowing the strong, sharp and accurately known absorption bands of polystyrene, atmospheric carbon dioxide and methyl cyclohexane (liquid) in the infrared wavelength region. The spectral radiance calibration was done by recording spectral scans of the hot and the cold blackbodies and assuming that spectral radiance varies linearly with the signal.

Kumar, R.↗

NASA NTRS1977

Magnetic fields greater than 10 to the 20th power gauss

Zaumen (1976) found that spontaneous pair production in a uniform magnetic field should be a feasible process for field strengths at least of the order of 10 to the 20th power gauss. This note points out that a magnetic field of this order of magnitude is most unlikely to occur in realistic astrophysical situations because of the large dynamical and quantum-mechanical effects such a field would produce. It is suggested that Zaumen's calculation would probably have little bearing on the suspected evolution of astrophysical systems since other processes (either dynamical or quantum-mechanical) apparently limit the field strength before such high magnetic fields would be reached. An upper limit of about 10 to the 16th power gauss is obtained by considering the isotropy of the 3-K blackbody radiation, the formation of collapsed objects in very high magnetic fields, and magnetic bremsstrahlung processes in quantum electrodynamics.

Lerche, I.↗

NASA NTRS1977

Thunderstorm monitoring from a geosynchronous satellite

It has been shown that SMS digital IR data can be used to calculate parameters which appear to be related to cloud or thunderstorm growth rates and variations in cloud top structure. The data appear to be useful in examining both clearly defined single clouds and cold areas within mature cirrus anvils. The data used for this study were blackbody temperatures derived from digital IR data from SMS-2 for a series of storms over the midwest United States on May 6, 1975.

Adler, R. F.↗

NASA NTRS1978

Spectrophotometry of OH 26.5+0.6 from 2 to 40 microns

Airborne and ground-based observations show that OH 26.5+0.6 has strong 10 micrometers and weak 18 micrometers silicate absorptions superposed on an overall energy distribution much like a blackbody. The flux level, color temperature, and depth of the 10 micrometers absorption have varied during two years of observations. A model of the source as a late-type variable star that has ejected an optically thick dust shell is suggested; the mass-loss rate implied is greater than about 0.00001 solar masses per year. The fact that significant flux from the source is observed between 4 and 7 micrometers is evidence that oxygen-rich dust has significant opacity in that wavelength range.

Forrest, W. J.↗

NASA NTRS1978

The structure and spectrum of a colliding-cloud system and its possible relationship to QSOs

A collision between two gas clouds with initial densities of approximately 10 million per cu cm, velocities of about 1000 km/s, and radii of approximately 1 pc is investigated quantitatively by coupling a calculation of the radiation spectrum resulting from the anticipated shock fronts with a computation for the conversion of this high-energy radiation into optical emission in adjacent photoionized regions. The detailed structure of the colliding clouds is discussed, and the effects of an ambient magnetic field are considered. The combined emission-line spectrum is presented along with continuum emission estimates for thermal, synchrotron, and very-high-energy bremsstrahlung mechanisms. It is shown that significant continua can be produced over the range from 300 microns to 3 keV, including a blackbody contribution from a high-density neutral region between the shock fronts, free-free and free-bound radiation from the cooling zones directly behind the shocks, and free-free, free-bound, and two-photon radiation from the photoionized regions immediately ahead of and behind the cooling zones. The theoretical spectrum of the structure resulting from the collision is found to be similar in general and in some details to those observed for typical quasars.

Daltabuit, E.↗

NASA NTRS1978

Lamellar grating observations of the Orion Nebula from 100 to 500 microns

Results are presented for spectroscopic observations of the Kleinmann-Low nebula in the 100-500-micron region taken from the Kupier Airborne Observatory. These results, in conjunction with previous work at shorter wavelengths, are consistent with a dilute blackbody spectrum modified by grain emissivity and spatial variations in the grain density.

Pipher, J. L.↗

NASA NTRS1978

Radio and X-ray variability of the nucleus of Centaurus A /NGC 5128/

Centaurus A (NGC 5128) has been observed at radio frequencies of 10.7, 31.4, 85.2, and 89 GHz and at X-ray energies greater than 20 keV. These observations, together with results reported by other workers, are interpreted in terms of models of the nucleus of this radio galaxy. The radio observations cover the period from 1973 through early 1977. The X-ray observations cover two 10-day intervals, one in July and August (1975) and the other in July and August 1976. The source exhibits significant variability in all the observed radio frequencies. The observed radio and X-ray intensities show some concurrent variations but do not track one another throughout the observations. A model of the source in which X-rays are produced by inverse Compton scattering of blackbody photons by relativistic electrons is proposed to explain these observations. The observed variations in the electromagnetic spectrum are shown to be consistent with adiabatic expansion of a trapped plasma in conjunction with turbulent accelerations of the relativistic electrons. Upper limits obtained with the model indicate that there may be sufficient energy available in the nucleus to form radio lobes with the same total energy as those already present.

Beall, J. H.↗

NASA NTRS1978

Airborne photometric observations between 1.25 and 3.25 microns of late-type stars

The stars Alpha Aur (G5 III + G0 III), Alpha Boo (K2 IIIp), Alpha Ori (M1-M2 Ia-Ib), Alpha Sco (M1.5 Iab), Mu Gem (M3 III), and Alpha Her (M5 Ib-II) have been observed using interference filters in five photometric bands between 1.25 and 3.25 microns during seven flights with NASA's Lear Jet Infrared Observatory. The filters were designed to measure molecular features, primarily from CN and CO, and continuum fluxes. By calibrating the photometer in the laboratory against a stabilized blackbody source, relative flux curves have been derived. The energy distributions and the strength of molecular features are discussed. The most interesting result obtained is that the fluxes from Mu Gem and Alpha Her in the filter centered at 3.25 microns seem to be depressed by at least some tenths of a magnitude. Tentatively this depression is proposed to be due to the wings of the two vibration-rotation bands (about 2.7 microns) of hot water vapor. Since water vapor is an important opacity source and its abundance is a sensitive C/O indicator, the proposed interpretation makes renewed efforts to detect water bands in early M stars highly desirable.

Nordh, H. L.↗

NASA NTRS1978

Venus - The 17- to 38-micron spectrum

A far-IR emission spectrum of Venus covering the wavelength range from 17 to 38 microns is examined which was obtained on five nights at an altitude of 14 km with the 30-cm telescope of the NASA Lear Jet. The spectrum is found to be characterized by an overall continuum level with noticeable absorption shortward of 20 microns and longward of 30 microns as compared with a 245-K blackbody. The continuum level is taken as implying a continuous source of opacity in the Venusian atmosphere over the entire range from 17 to 38 microns with increased opacity shortward of 20 microns and longward of 30 microns. It is shown that a haze of sulfuric acid droplets can provide the necessary opacity and explain the observed depressions. A pressure level of roughly 200 mb is deduced for this spectrum.

Reed, R. A.↗

NASA NTRS1978

Thermal limit for spherical accretion and X-ray bursts

A mechanism is described whereby the rate of spherical accretion on a collapsed object and the resulting X-ray luminosity can be limited by thermal heating and evaporation of the accreting gas. The luminosity limit can be a factor of the order of 100 below the corresponding Eddington limit. Steady accretion flow is not possible within a range above this limit, and X-ray bursts can be produced by accretion surges resulting from the cooling and collapse of a heated gas shell above a critical optical depth. The luminosity and approximate recurrence rate of the bursts from NGC 6624 and other burst sources may then be understood. A general model for bursters is proposed in which massive (about 10 to 100 solar masses) black holes, from massive stars and the cores of disrupted globular clusters, undergo supercritical spherical accretion in interstellar clouds. Both X-ray and gamma-ray bursts are predicted to have similar hard X-ray (about 200 keV) spectra, though high average accretion rates and central optical depths also yield a comparable luminosity at around 5 keV from a Comptonized blackbody spectrum in the X-ray bursts and associated steady sources.

Grindlay, J. E.↗