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At least 1,207 records · Page 67
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NASA NTRS1985

On the emission region of gamma ray bursts

Within the last few years the rapid accumulation of gamma ray burst spectral data, especially those of KONUS and solar maximum mission, has made the confrontation between theories of the gamma ray emission mechanisms and observations much more urgent and challenging. At present the most viable mode seems to be some combination of inverse Comptonization and synchrotron emission. Here, an attempt is made to limit the acceptable parameter space of the emission region by taking into account the maximum set of observational constraints. These are then applied to two specific scenarios: surface emission versus magnetospheric emission. Some observable predictions based on these scenarios are considered.

Liang, E. P.↗
NASA NTRS1984

A search for 183 GHz emission from water in late-type stars

A search was made for 183 GHz line emission from water vapor in the direction of twelve Mira and two semiregular variables. Upper limits to the emission are in the range of 2000 to 5000 Jy. It is estimated that thermal emission from the inner regions of late type stellar envelopes will be on the order of ten Jy. Maser emission, according to one model, would be an order of magnitude stronger. From the limited set sampled, the possibility of very strong maser emission at 183 GHz cannot yet be ruled out.

Kuiper, T. B. H.↗
NASA NTRS1984

Radio emissions from the planets earth, Jupiter, and Saturn

The present investigation is concerned with a comparison of the general observable properties of the radio emission from the earth, Jupiter, and Saturn. Attention is given to typical dynamo spectra, average power, average sense of circular polarization, external control of radio emission, source locations, and emission efficiencies. A data summary regarding planetary radio observations is presented in a table. The polarization of northern hemisphere emission for the three planets is considered along with possibilities regarding the total radiated power of the Uranus radio emission.

Kaiser, M. L.↗
NASA NTRS1984

The complex X-ray emission of NGC 253

High-resolution X-ray observations of the starburst galaxy NGC 253 with the Einstein Observatory show that there are at least four components to its X-ray emission. Eight bright point-like sources were detected. Diffuse emission is visible in the inner disk of NGC 253. A more intense emission region is associated with the nuclear region and with an extension along the southern portion of the minor axis. This last component, in particular, suggests the presence of shock-heated gas associated with outflow from the starburst nucleus. A siimlar feature was seen in the X-ray emission of M82. A comparison between the nuclear regions of NGC 253, M82, and the Milky Way suggests an excess of X-ray emission in the nucleus of M82.

Fabbiano, G.↗
NASA NTRS1986

Hard X-ray imaging and the relative contribution of thermal and nonthermal emission in flares

The question of whether the impulsive 25 to 100 keV X-ray emission from solar flares is thermal or nonthermal has been a long-standing controversy. Both thermal and nonthermal (beam) models have been developed and applied to the hard X-ray data. It now seems likely that both thermal and nonthermal emission have been observed at hard X-ray energies. The Hinotori classification scheme, for example, is an attempt to associate the thermal-nonthermal characteristics of flare hard X-ray emission with other flare properties. From a theoretical point of view, it is difficult to generate energetic, nonthermal electrons without dumping an equal or greater amount of energy into plasma heating. On the other hand, any impulsive heating process will invariably generate at least some nonthermal particles. Hence, strictly speaking, although thermal or nonthermal emission may dominate the hard X-ray emission in a given energy range for a given flare, there is no such thing as a purely thermal or nonthermal flare mechanism.

Holman, G. D.↗
NASA NTRS1985

On stellar X-ray emission

Stellar X-ray astronomy represents an entirely new astronomical discipline which has emerged during the past five years. It lies at the crossroads of solar physics, stellar physics, and general astrophysics. The present review is concerned with the main physical problems which arise in connection with a study of the stellar X-ray data. A central issue is the extent to which the extrapolation from solar physics is justified and the definition (if possible) of the limits to such extrapolation. The observational properties of X-ray emission from stars are considered along with the solar analogy and the modeling of X-ray emission from late-type stars, the modeling of X-ray emission from early-type stars, the physics of stellar X-ray emission, stellar X-ray emission in the more general astrophysical context, and future prospects.

Rosner, R.↗
NASA NTRS1986

The Cygnus loop - A detailed comparison of X-ray and optical emission

A comparison at a resolution of 17 arcsec is presented between optical emission from forbidden O III and forbidden S II and the thermal X-ray emission for a field on the southeast edge of the Cygnus Loop SNR. The relationship between optical and X-ray emission in the field is described, generalizing to the Loop as a whole when possible. Several possible explanations for the presence of bright X-ray emission in the vicinity of optical emission are presented and evaluated based on the data. These mechanisms involve evaporation, gradual variations and gradients in the density of the preshock intercloud medium, and additional compression of material which has already been heated to X-ray temperatures by the adiabatic blast wave. The additional compression could result from rapid deceleration of the blast wave itself or from reshocking by reflected or bow shocks around dense clouds. Implications of the observations for models of SNR evolution and the interstellar medium are discussed.

Cox, D. P.↗
NASA NTRS1986

Is IRAS cirrus cloud emission largely fine-structure radiation?

The contribution of line emission to the diffuse 'cirrus' radiation components observed by IRAS at 60 and 100 microns, respectively, is evaluated. The cirrus radiation was observed straddling the galactic plane to latitudes of + or - 80 degrees. Comparison of the observed brightness from cirrus clouds with forbidden line emission of O I and O III at 63 and 88 microns showed that the flux levels observed by IRAS are of the same order as the fine-structure line emission predicted by Stacey et al. (1985). It is argued that the fine-structure line emission from neutral gas at 63 microns as well as the line emission from diffuse ionized clouds at 88 microns, are significant components in most clouds, in some clouds they are the prime source of cirrus radiation.

Harwit, M.↗
NASA NTRS1985

Emission in the hydrogen-Brackett lines of Epsilon Aurigae during eclipse

Results of high-resolution infrared spectroscopy of Epsilon Aur performed from 1982 and 1985 in order to investigate the effects of eclipse on spectral lines at near-infrared wavelengths are reported. The profiles of the Brackett-alpha and Brackett-gamma lines of hydrogen are reported. The B-alpha emission has an integrated flux of 9 x 10 to the -15th W/sq m, a FWHM of 60 km/s, and a radial velocity that follows the predicted velocity curve of the primary star more closely than that of the secondary. The mean B-alpha/B-gamma flux ratio was approximately unity on the three dates both lines were observed, indicating optically thick emission. It is concluded that the emission originates in a flow rather than from material in Keplerian revolution; the Brackett line emission probably has the same source as the previously observed H-alpha emission.

Backman, D. E.↗
NASA NTRS1986

IRAS surface brightness maps of visible reflection nebulae: Evidence for non-equilibrium infrared emission

Surface brightness maps at 12, 25, 60, and 100 microns of 16 visible reflection nebulae were extracted from the Infrared Astronomy Satellite (IRAS) database. The maps were produced by coadding IRAS survey scans over areas centered on the illuminating stars, and have spatial resolutions of 0.9' x 4' at 12 and 25 microns, 1.8' x 4.5' at 60 microns, and 3.6' x 5' at 100 microns. Extended emission in the four IRAS bandpasses was detected in fourteen of the reflection nebulae. The IRAS data were used to measure the flux of the infrared emission associated with each source. The energy distributions show that the 12 micron flux is greater than the 25 micron flux in 11 of the nebulae, and the peak flux occurs in the 60 or 100 micron bandpass in all 16 nebular. The 60 and 100 micron flux can be approximated by blackbodies with temperatures between 30 and 50 K, consistent with temperatures expected from extrapolation of greybody fits to the 60 and 100 micron data. The excess 12 and 25 micron emission is attributed to a nonequilibrium process such as emission from thermal fluctuations of very small grains excited by single ultraviolet photons, or emission from polycyclic aromatic hydrocarbons (PAHs) excited by ultraviolet radiation. The common features of the energy distributions of the 16 reflection nebulae, also seen in the reflection nebulae associated with the Pleiades, suggest that PAHs or very small grains may be found in most reflection nebulae.

Castelaz, M. W.↗
NASA NTRS1986

A nonisothermal emissivity and absorptivity formulation for water vapor

An emissivity approach is taken to modeling fluxes and cooling rates in the atmosphere. The nonisothermal water vapor long wave radiation emissivity and absorptivity model that is developed satisfies the requirements of defining a monochromatic transfer equation for predicting water vapor emissions. Predictions made with the model compare favorably with fluxes predicted by a radiation model for narrow-band emissions in 5 kayser intervals. The spectral resolution assumed in narrow-band models is shown to be an arbitrary parameter and, if a far wing continuum-type opacity is included in the emissivity scheme presented, results can be obtained which are as accurate as predictions made with state of the art line-by-line (LBL) calculations.

Ramanathan, V.↗
NASA NTRS1986

Altitude variation of EUV emissions and evidence for proton precipitation at low latitudes in the Saturnian atmosphere

Extreme ultraviolet observations of Saturn from Voyager 1 and 2 are analyzed. The Lyman alpha and H2 band emissions extend throughout the upper atmosphere, from the hydrocarbon homopause to well above the exobase. Analysis of the Lyman alpha emissions with a radiative transfer model indicates that the Lyman alpha source temperature is very high. This suggests that energetic protons or hydrogen atoms are responsible for a fraction of the emissions. Calculation of the solar-scattered component of the emissions based on the neutral atmosphere of Smith et al. (1983) reveals that only 1-2 kR out of a total of 3.5 kR of the observed Lyman alpha intensity is due to solar scatter for the Voyager 2 disk observations; the remainder of the Lyman alpha emissions are collisionally excited. The Lyman alpha and H2 bands are constant in longitude but decrease in local time by a factor of 2 from dawn to dusk. This correlation of the Lyman alpha and H2 band intensities is further evidence that most of the Lyman alpha is collisionally excited.

Yelle, R. V.↗
NASA NTRS1986

Continued observations of the H Ly alpha emission from Uranus

Observations of Uranus obtained over four years with the IUE Observatory supports the initial identification of a bright H Ly alpha flux which varies independently of the solar H Ly alpha flux, implying a largely self-excited emission. An average brightness of 1400 Rayleighs is derived, and limits for the possible contribution by reflected solar H Ly alpha emission, estimated to be about 200 Rayleighs, suggest that the remaining self-excited emission is produced by an aurora. Based on comparison with solar wind measurements obtained in the vicinity of Uranus by Voyager 2 and Pioneer 11, no evidence for correlation between the solar wind density and the H Ly alpha brightness is found. The upper limit to H2 emission gives a lower limit to the ratio of H Ly alpha/H2 emissions of about 2.4, suggesting that the precipitating particles may be significantly less energetic on Uranus than those responsible for the aurora on Jupiter. The average power in precipitating particles is estimated to be of the order of 10 to the 12th W.

Clarke, J.↗
NASA NTRS1987

Long-period dynamic spectrograms of low-frequency interplanetary radio emissions

Dynamic spectrograms of the low-frequency interplanetary radio emissions as observed by Voyagers 1 and 2 from 1983 through mid-1986 are reported. The radio emissions were observed to be most intense in the latter portion of 1983 at 3 kHz but have also been detected at 2 kHz. The emission has been present almost continuously at either 2 or 3 kHz since late 1983. The spectrograms presented herein show that the phenomenon appears almost identically as observed by the two spacecraft separated by more than 10 AU, at least at the higher frequency. One feature revealed by the dynamic spectrograms which had not been noticed previously is a gradual rise in frequency of the 3-kHz component following the onset of the late 1983 event. These new observations reinforce the conclusion that the low-frequency emissions are freely propagating radio waves, but the two-component spectral structure implies that the previous model of emission at twice the plasma frequency at the inner heliosphere shock is inadequate to fully account for the observations. Either an additional source region or an additional source mechanism is suggested.

Kurth, W. S.↗
NASA NTRS1986

Excitation of O(1D) atoms in aurorae and emission of the forbidden OI 6300-A line

The electron aurora leads to six processes capable of exciting the O(1D2) metastable state of the atomic-oxygen ground-state configuration, the parent state of the 6300-A red line. Altitude profiles of the volume emission rate resulting from each process are computed for Maxwellian electron spectra with characteristic energies between 0.1 and 2.0 keV. Since each process peaks at a different altitude, the sum or total volume emission rate extends over a wide altitude range. Measurements of 6300-A emission obtained by rocket and satellite-borne instruments are summarized, and it is shown that the chemical reaction of N(2D) with O2 is the major source of O(1D) atoms in the electron aurora. New calculations of the 6300-A:4728-A column emission-rate ratio are presented for a range of characteristic energies in an assumed Maxwellian electron spectrum. An approximate equation for the red-line emission per unit energy input is given as a function of electron-spectrum characteristic energy.

Rees, M. H.↗
NASA NTRS1987

Anharmonicity and the interstellar polycyclic aromatic hydrocarbon infrared emission spectrum

The hypothesis that interstellar infrared emission originates from vibrationally excited polycyclic aromatic hydrocarbons (PAHs) requires that emission can arise from all vibrational levels that are energetically accessible. Due to anharmonicity, the emission from the upper vibrational levels is shifted to longer wavelengths from that of the v = 1-0 transition. It is shown that structure in the 3-micron region is readily and quantitatively explained by emission from upper vibrational levels of excited PAHs that contain a maximum of 20-30 carbon atoms. The asymmetrical broadening of the 11.3-micron emission band may also be due to anharmonicity.

Barker, John R.↗
NASA NTRS1986

Improved acoustic emission locations

A technique for locating acoustic emission in brittle rock specimens is described. An array of eight piezoelectric transducers was used to detect acoustic emission and to receive velocity measurement pulses. Axial and transverse velocities were calculated from the transmission data; the use of the velocity data to locate the acoustic emissions is discussed. Waveforms were recorded for about 140 acoustic emissions during the deformation of the Westerly granite specimen and about 100 acoustic emissions were located. The effect of transducer size and incident angle on apparent arrival times is evaluated using a lead zirconium titinate transducer bonded to a small brass rider with a curved surface matched to the brass specimen. The relation between location accuracy in various regions of the specimen and the geometry of the receiver array is studied. It is observed that for incidence angles away from the normal, the velocity appears to be higher than for normal incidences.

Getting, I. C.↗
NASA NTRS1987

Physical processes and infrared emission from the Cassiopeia A supernova remnant

IRAS 12-100 micron data on the Cas A remnant are presented, and various physical mechanisms and astrophysical sites that may contribute to the observed infrared emission are analyzed. The contributions of various sources of infrared emission to the IRAS fluxes are found to be small. The residual infrared emission is attributed to thermal emission from dust which is swept up by the expanding supernova blast wave and collisionally heated by the postshock X-ray emitting gas. The calculations are consistent with a shock velocity of 1800 km/s and a preshock gas density of about 2/cu cm. The mass of the swept-up gas is about 0.6 solar mass. An excess of 12 micron thermal emission in the spectrum of Cas A suggests the presence of very small particles in the preshocked gas.

Dwek, E.↗