Engineering topics

Dreher, J. W.

Publications and source records attributed to Dreher, J. W..

4 records

NASA NTRS1991

Multifrequency radio observations of Cygnus A - Spectral aging in powerful radio galaxies

A detailed analysis of the radio spectrum across the lobes of Cygnus A is presented in order to critically test the synchroton spectral aging theory. The results are in good agreement with the jet model for powerful radio galaxies, involving particle acceleration at the hot spots and outflow into the radio lobes, with subsequent energy loss due to synchrotron radiation. The hot spot spectra are well represented by a spectral aging model involving continuous injection of relativistic particles. Both hot spots have spectral break frequencies around 10 GHz. An injection index of 0.5 is found for both hot spots, consistent with diffusive shock acceleration at a strong nonrelativistic shock in a Newtonian fluid. The LF hot spot emission spectrum falls below the injected power law. This effect is isolated to the hot spots, and is best explained by a low-energy cutoff in the particle distribution.

Carilli, C. L.↗

NASA NTRS1989

Rejection of RFI by means of interferometry

The feasibility of using radio interferometry to reduce the levels of RF interference (RFI) encountered in the NASA Targeted Search SETI program is discussed. It is shown that a radio interferometer will be much less sensitive to RFI than a single antenna system. However, the use of an interferometer would entail a reduction in the noise-limited sensitivity of the system as well as some additional costs due to added complexity.

Dreher, J. W.↗

NASA NTRS1985

The binary system MWC 349

The ionized zone surrounding MWC 349 has been resolved using the VLA. On the basis of a radio continuum map, it is concluded that there is a physical interaction between the two stellar components of MWC 349, and that the system is therefore a binary. The B component can be classified as a B0 III star, yielding a distance of about 1.2 kpc for the binary. The low ratio of infrared to short-wavelength luminosity in the A component suggests a circumstellar dusty region of special geometry. It is proposed that an appreciable dust and gas disk surrounds the star, modifying the symmetry of its H II region, and yielding the low infrared luminosity and perhaps its extra circumstellar extinction.

Cohen, M.↗

NASA NTRS1984

Rings and wiggles in Hercules A

The first results from extensive observations of Her A, the fourth brightest extragalactic radio source in the sky at low frequencies, made with the Very Large Array of the National Radio Astronomy Observatory are reported. It is found that Her A possesses an unusual jet-dominated morphology instead of the Cygnus-A-like morphology expected for such a powerful radio source. Its two jets are quite different in appearance; one appears to be a continuous twisting jet, while the other suggests repeated ejection of individual plasmons by the central object. The questions raised by the presence of such different morphologies in the same source concerning the generation and evolution of extended extragalactic radio sources are discussed. Variations in the central engine provide the simplest interpretation for some of the features of this object; the role of environmental effects is less clear.

Dreher, J. W.↗