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Cohen, M. H.

Publications and source records attributed to Cohen, M. H..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

VLBI monitoring of the superluminal quasar 3C 273, 1977-1982

The compact ratio structure of the quasar 3C 273 has been monitored with a VLBI array at 5.0 and 10.7 GHz at six-month intervals during 1977-1982. THe VLBI monitoring observations, data reduction, calibration, and mapping techniques are described, and hybrid maps from the monitoring program are presented along with a brief discussion of the main features. Results on the source kinematics (superluminal motion of the knots and their time evolution) are given, and synchro-Compton models of the knots are presented and limits to their Doppler factors (delta) derived from X-ray measurements. The constraints on jet geometry provided by the combined measurements of delta and v/c are considered. For the core, the inhomogeneous jet model of Koenigl (1981) is compared with the available data. The relation between 3C 273 and the similar superluminal source 3C 345 is discussed.

Unwin, S. C.↗
NASA NTRS1983

VLBI maps of 3C 273 and 3C 345 at 2.3 GHz

Maps of the quasars 3C 273 and 3C 345 made at 2.3 GHz with an eight station VLBI array are presented. Five compact components spanning 45 milli-arcsec are seen in 3C 273, and four are seen in 3C 345. Comparisons with maps at higher and lower frequencies show that in both sources the core, which dominates the high-frequency 'core-jet' structure, is strongly self-absorbed at 2.3 GHz and below. The jet components have flat spectra, peaking between 2 and 5 GHz. In both sources the VLBI jet curves toward the direction of the arcsecond-scale outer jet.

Cohen, M. H.↗
NASA NTRS1982

Proposed Integrated Radio-Telescope Network

Proposed network of radio telescopes, controlled by a central computer and managed by a single organization, offer potential for research on a scale that could not be matched by present privately and publicly-owned radio telescopes. With 10 antenna sites, network would establish base lines thousands of miles long. Antennas will be linked to computer center by telephone circuits.

Cohen, M. H.↗
NASA NTRS1980

Structure and optical properties of evaporated films of the Cr- and V-group metals

Thin films of Cr, Mo, and W rapidly evaporated in high vacuum (5 x 10 to the -7th torr) onto room-temperature substrates show anomalously low reflectance (compared to bulk samples). From electron and X-ray diffraction and electron microscopy, the normal bcc crystal structure is found, but with very fine grains. Columnar grains about 100 A in diameter were separated by a less dense grain-boundary network about 10-A wide. The measured optical conductivity agrees with an inhomogeneous-medium model that assumes the normal crystalline conductivity for the grain interiors, with model parameters that correlate to the observed columnar grain size. In contrast, V and Nb films rapidly evaporated onto room-temperature substrates have the reflectance of bulk crystalline material. On liquid-nitrogen temperature substrates, however, V and Nb have normal bcc crystal structure but with small flat-plate grains, and the same model, with appropriate parameters, accounts for the optical conductivity. The difference between these two groups apparently depends on residual gases segregated at the grain boundaries in the Cr-group films.

Nestell, J. E., Jr.↗
NASA NTRS1979

Misalignment in the radio jets of NGC 6251

The compact nuclear source in NGC 6251 has been observed with VLBI techniques at 2.8 and 13 cm. The source is asymmetric with a bright optically thick core and an optically thin jet. The jet has a small but significant difference in position angle from the outer jet.

Cohen, M. H.↗
NASA NTRS1977

An angular size for the compact radio source at the galactic center

Simultaneous very-long-baseline (100 million wavelengths) and intermediate-baseline (4 million wavelengths) interferometer observations at a wavelength of 3.7 cm in 1976.2 show that the galactic-center compact radio source has a linear size of approximately 140 AU. The observations also set an upper limit of about 0.1 Jy to the emission from any core of size less than roughly 10 AU at that epoch. The total flux density of the compact source was slightly higher than the value reported previously.

Lo, K. Y.↗
NASA NTRS1977

The compact radio sources in 4C 39.25 and 3C 345

Long-baseline interferometry of the quasars 4C 39.25 and 3C 345 at 10.65 and 14.77 GHz shows that the centimeter radio source in each object is double, with component separations of 0.0020 arcsec (4C 39.25) and 0.0013 arcsec (3C 345 at 1974.5). For each source, the separation is the same at both frequencies, as well as similar to the structure observed at 7.85 GHz (and 5.0 GHz for 4C 39.25). The spectra of the individual components are derived and shown to vary with time approximately as expected for expanding self-absorbed synchrotron sources. The magnetic fields in the components are estimated to be as high as 0.1 gauss, but the structure of the sources appears to be unrelated to the magnetic-field orientation derived from low-resolution polarization measurements. The component separation in 4C 39.25 has not changed for several years, whereas 3C 345 shows rapid expansion.

Shaffer, D. B.↗
NASA NTRS1973

Theoretical and experimental investigations of superconductivity. Amorphous semiconductors, superconductivity and magnetism

The research activities from 1 March 1963 to 28 February 1973 are summarized. Major lectures are listed along with publications on superconductivity, superfluidity, electronic structures and Fermi surfaces of metals, optical spectra of solids, electronic structure of insulators and semiconductors, theory of magnetic metals, physics of surfaces, structures of metals, and molecular physics.

Cohen, M. H.↗
NASA NTRS1971

The small-scale structure of radio galaxies and quasi-stellar sources at 3.8 centimeters.

Observation of fringes from 31 compact radio sources, including eight known or suspected galaxies and 20 known or suspected QSSs, by using the Goldstack interferometer at lambda = 3.8 cm (d/lambda = 10 to the 8th power). Fringe visibility curves were obtained for nine sources showing structure on a scale of .001 sec of arc, and simple models are fitted to the data. Results for 3C 273 and 3C 279 are compared with data taken by Knight et al. (1971) at an earlier epoch. The apparent changes in brightness distribution of 3C 273 and 3C 279 are difficult to explain.

Cohen, M. H.↗