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At least 379 records · Page 21

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NASA NTRS1972

The spectra of ten galactic X-ray sources in the southern sky.

Data on 10 galactic X-ray sources located between 320 and 20 deg galactic longitude were obtained during a rocket flight from Brazil in June 1969. Detailed spectra of these sources have been compared with bremsstrahlung, blackbody, and power-law models, each including interstellar absorption. Six of the sources were fitted well by one or more of these models. In only one case were the data sufficient to distinguish the best model. Three of the sources were not fitted by any of the models, which suggests that more complex emission mechanisms are applicable. A comparison of our results with those of previous investigations provides evidence that five of the sources vary in intensity by a factor of two or more, and that three have variable spectra.

Cruddace, R.↗

NASA NTRS1972

The association of X-ray sources with bright stars.

Comparison of selected Uhuru X-ray sources with the SAO star catalog, revealing ten stars within the positional error of the X-ray sources where only about two are expected on the basis of chance. Three of these are already under study as optical candidates for X-ray sources. The new results lend confidence to these candidates as being correct and yield candidate stars for seven other X-ray sources. The association of bright stars with X-ray sources is expected on the basis of the model which invokes mass transfer from a massive star to a compact companion.

Gursky, H.↗

NASA NTRS1972

The resolution of point sources of light as analyzed by quantum detection theory

The resolvability of point sources of incoherent light is analyzed by quantum detection theory in terms of two hypothesis-testing problems. In the first, the observer must decide whether there are two sources of equal radiant power at given locations, or whether there is only one source of twice the power located midway between them. In the second problem, either one, but not both, of two point sources is radiating, and the observer must decide which it is. The decisions are based on optimum processing of the electromagnetic field at the aperture of an optical instrument. In both problems the density operators of the field under the two hypotheses do not commute. The error probabilities, determined as functions of the separation of the points and the mean number of received photons, characterize the ultimate resolvability of the sources.

Helstrom, C. W.↗

NASA NTRS1972

Analytical evaluation of jet noise source location technique utilizing an acoustically hard baffle with aperture

An analytical investigation was conducted on the experimental technique that uses shielding by an acoustically hard baffle to determine the location of noise sources in a jet. The jet exhausts through a circular hole in the baffle and the radiated acoustic power is considered to be generated by a distributed source along the jet axis. It is found that the effect of the baffle on the radiated power can be neglected only when the size of the source is on the order of a wavelength of the emitted sound or larger. Since noise sources in a jet are compact, the experimental technique is insufficient to identify these sources.

Norum, T. D.↗

NASA NTRS1972

The Uhuru catalog of X-ray sources.

A catalog of X-ray sources observed with the Uhuru satellite is presented. About 70 days of data have been analyzed for this catalog resulting in 125 sources. Approximately two-thirds of the sources are located within plus or minus 20 deg of the galactic plane. Some of the sources at higher galactic latitudes are identified with known extragalactic objects. Most of the strong sources near the galactic plane are found to be variable.

Giacconi, R.↗

NASA NTRS1973

Role of pressure transients in the detection and identification of lunar surface gas sources.

The dynamic behavior of neutral gases emitted from lunar surface sources is investigated. Pressure transients to be expected from several types of possible lunar surface sources are constructed. An approximate method is developed to estimate the source-sensor separation distance and the amount of effluent release. The case for which the source location and effluent molecular weight can be determined from ancillary data is discussed. It is shown that, if such data are available, pressure transients detected by either Apollo lunar surface experiment package ion spectrometers or cold cathode gages can be used to determine the release rate and the total amount of effluent release from the source.

Hall, F. G.↗

NASA NTRS1973

Extended extragalactic radio sources.

The most important feature of the extended sources is the double structure characterized in an ideal model by two extended radio-emitting regions situated symmetrically about the nucleus of an optical galaxy or QSO, with a characteristic overall size of about 100 kpc. Not all extended radio sources show this double structure. It has been found that at moderate resolutions about 20% of the well-resolved sources have a core-halo structure. Theoretical proposals for the containment of extended double sources have so far relied either upon self-gravitation of the ejected material or upon assuming the existence of a gaseous intergalactic medium. The advantages and the problems of a model proposed by Burbidge (1967) are discussed, together with the nuclear event serving to release the energy required to make an extended radio source.

De Young, D. S.↗

NASA NTRS1973

High-energy X-ray spectra of five sources.

On October 15-16, 1970, we carried out balloon X-ray observations from Australia at energies above 15 keV. We present the high-energy X-ray spectra of three sources discovered by us, GX 301-2, GX 304-1, and GX 1 + 4. The data suggest that these high-energy sources correspond to the sources 2U 1223-62, 2U 1258-61, and 2U 1728-24 respectively. We also present the spectra for two additional sources, GX 5-1 (2U 1757-25) and GX 3 + 1 (2U 1744-26). The average intensity of the highly variable source GX 301-2 was observed to be as great as Tau X-1 in the energy range 15-50 keV.

Ricker, G. R.↗

NASA NTRS1973

Resolution of point sources of light as analyzed by quantum detection theory.

The resolvability of point sources of incoherent thermal light is analyzed by quantum detection theory in terms of two hypothesis-testing problems. In the first, the observer must decide whether there are two sources of equal radiant power at given locations, or whether there is only one source of twice the power located midway between them. In the second problem, either one, but not both, of two point sources is radiating, and the observer must decide which it is. The decisions are based on optimum processing of the electromagnetic field at the aperture of an optical instrument. In both problems the density operators of the field under the two hypotheses do not commute. The error probabilities, determined as functions of the separation of the points and the mean number of received photons, characterize the ultimate resolvability of the sources.

Helstrom, C. W.↗

NASA NTRS1973

Flux-density-spectral-index relation of extragalactic radio sources

We have analyzed the spectral-index distribution of extragalactic radio sources and its variation with flux density and survey frequency. We find that the effect of K-correction on this variation (which if observed could imply that these sources are at cosmological distances) is small compared with observational uncertainties and other effects. We also derive a form for the intrinsic spectral distribution (assuming it to be independent of redshift and luminosity) which consists of two Gaussians with median spectral indices of about 0.8 and 0.4 and with dispersion of 0.25 and 0.4, respectively. The first class of sources is found in low-frequency surveys. The second class, consisting of only a few percent of all sources, appears in high-frequency surveys and has spectra-index distributions similar to sources identified with quasars.

Petrosian, V.↗

NASA NTRS1974

The generation of gravitational waves. 1. Weak-field sources: A plug-in-and-grind formalism

A plug-in-and-grind formalism is derived for calculating the gravitational waves emitted by any system with weak internal gravitational fields. If the internal fields have negligible influence on the system's motions, then the formalism reduces to standard linearized theory. Whether or not gravity affects the motions, if the motions are slow and internal stresses are weak, then the new formalism reduces to the standard quadrupole-moment formalism. In the general case the new formalism expresses the radiation in terms of a retarded Green's function for slightly curved spacetime, and then breaks the Green's-function integral into five easily understood pieces: direct radiation, produced directly by the motions of the sources; whump radiation, produced by the the gravitational stresses of the source; transition radiation, produced by a time-changing time delay (Shapiro effect) in the propagation of the nonradiative, 1/r field of the source; focussing radiation produced when one portion of the source focusses, in a time-dependent way, the nonradiative field of another portion of the source, and tail radiation, produced by backscatter of the nonradiative field in regions of focussing.

Thorne, K. S.↗

NASA NTRS1975

Vibrationally excited SiO - A new type of maser source in the millimeter wavelength region

The recently discovered strong emission lines at 86.24 GHz from the J = 2-1 rotational transition of SiO in its first excited vibrational state have been found to be a maser source associated with infrared stars. Twelve maser sources have been detected, and almost all are late M-type Mira or semiregular variables. Many of the known OH/H2O/IR sources are also SiO maser sources, and good velocity correlations between SiO and H2O emission profiles were found. Characteristics of these new SiO maser sources are discussed.

Kaifu, N.↗

NASA NTRS1975

Interstellar scintillation of extragalactic radio sources

Observations of 12 compact extragalactic sources were made at 2695 and 8085 MHz in order to detect weak intensity fluctuations caused by interstellar scintillation. Pulsar data are used to estimate the parameters of the interstellar medium needed to interpret the measured upper limits in terms of source angular diameters which are much larger than the scintillation cutoff diameter. It is shown that the observed source rms brightness temperatures are less than 10 to the 15th K and 10 to the 14th K at 2695 and 8085 MHz, respectively, making self-absorbed proton-synchroton radiation and high-brightness coherent mechanisms unlikely. If the sources are composed of 'point' components, each source can contain no fewer than 10,000 such components.

Condon, J. J.↗

NASA NTRS1975

Observations of variable X-ray sources in globular clusters

The discovery is reported of a variable X-ray source at midgalactic latitude in the globular cluster NGC 1851, and observations are reported which confirm the existence of variable X-ray sources in NGC 7078 and NGC 6441. The new source, designated MX 0513-40, was found to vary in intensity by a factor of at least 5 over a sixteen-day period and to have a hard spectrum as compared to that of the Crab Nebula in the energy range from 1 to 10 keV. The source in NGC 7078 (3U 2131+11) was found to vary by more than a factor of 2 over a 500-day period. Strong evidence of variability is presented for the source in NGC 6441 (3U 1746-37). It is concluded that both MX 0513-40 and 3U 2131+11 are probably stellar objects and are no more than about 160 kpc distant.

Clark, G. W.↗

NASA NTRS1975

The generation of gravitational waves. I - Weak-field sources

This paper derives and summarizes a 'plug-in-and-grind' formalism for calculating the gravitational waves emitted by any system with weak internal gravitational fields. If the internal fields have negligible influence on the system's motions, the formalism reduces to standard 'linearized theory'. Independent of the effects of gravity on the motions, the formalism reduces to the standard 'quadrupole-moment formalism' if the motions are slow and internal stresses are weak. In the general case, the formalism expresses the radiation in terms of a retarded Green's function for slightly curved spacetime and breaks the Green's function integral into five easily understood pieces: direct radiation, produced directly by the motions of the source; whump radiation, produced by the 'gravitational stresses' of the source; transition radiation, produced by a time-changing time delay ('Shapiro effect') in the propagation of the nonradiative 1/r field of the source; focusing radiation, produced when one portion of the source focuses, in a time-dependent way, the nonradiative field of another portion of the source; and tail radiation, produced by 'back-scatter' of the nonradiative field in regions of focusing.

Thorne, K. S.↗

NASA NTRS1975

Duct liner optimization for turbomachinery noise sources

An acoustical field theory for axisymmetric, multisectioned lined ducts with uniform flow profiles was combined with a numerical minimization algorithm to predict optimal liner configurations having one, two, and three sections. Source models studied include a point source located on the axis of the duct and rotor/outlet-stator viscous wake interaction effects for a typical research compressor operating at an axial flow Mach number of about 0.4. For this latter source, optimal liners for equipartition-of energy, zero-phase, and least-attenuated-mode source variations were also calculated and compared with exact results. It is found that the potential benefits of liner segmentation for the attenuation of turbomachinery noise is greater than would be predicted from point source results. Furthermore, effective liner design requires precise knowledge of the circumferential and radial modal distributions.

Lester, H. C.↗

NASA NTRS1975

A comparison of X-ray sources in globular clusters and in the galactic bulge

A hypothesis is advanced that the X-ray sources of the galactic bulge and those in globular clusters are of the same type. This is supported by the similarity of the respective stellar populations and the observed incidence of luminous sources. Evolutionary considerations suggest that it is the high stellar density that favors X-ray source formation in those regions. If the correspondence is correct, there must be about 100 additional weak sources in the bulge which have so far eluded detection because of source confusion.

Canizares, C. R.↗

NASA NTRS1976

The problem of spiral galaxies and satellite radio sources

Regions 2-deg x 2-deg in area centered on four spiral galaxies were scanned with the Goldstone 64-m antenna in search of satellite radio sources at 2295 MHz. Four control regions of equal area but free of galaxies brighter than 13th magnitude were also scanned. No significant excess in the number density of sources with flux densities greater than 0.10 Jy relative to the control fields was found. A detailed comparison is made between the results of this program and the results of previous investigators. In particular, attention is directed to the potentially important implications of an investigation by Tovmasyan (1968), who searched a large number of spirals and found evidence that a small percentage of them apparently have radio satellites located up to 20 arcmin from the central galaxy. Fifteen sources selected from Tovmasyan's list of 43 satellite sources were measured. The present results confirm his positions and relative flux densities for each of the sources.

Arp, H.↗