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Dent, W. A.

Publications and source records attributed to Dent, W. A..

16 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

New far infrared observations of the central 30 arcmin of the Galaxy

Observations of the 55 micron and 125 micron emissions of the central 30 arcmin of the Galaxy were performed with 1 arcmin resolution with instrumentation on-board the Kuiper flying observatory. The flux distributions detected showed both wavelengths with peaks near the galactic center, some extension along the galactic plane, and the 125 micron feature located 30 arcsec south. An asymmetry was found around Sgr A, and the variation in dust temperature across the region was calculated from the ratio of the two wavelengths. The temperature peak was around Sgr A, with subsidiary peaks associated with the 55 micron emission. A common heating source was indicated by correlations between cm wavelength sources and the temperature maps. Additionally, the distributions of NH3 and formaldehyde were also correlated with the dust distribution at the center.

Dent, W. A.↗
NASA NTRS1979

Search for X-ray emission from bursting radio sources

Results are reported for a systematic search with the A-2 experiment aboard HEAO 1 for X-ray emission from 28 radio sources that were actually bursting at the epoch of the X-ray observations. Two of these sources are found to lie within the positional errors of X-ray sources: the moderately redshifted quasars NRAO 140 and NRAO 530. The positions of 30 historically variable radio sources that were not active at the time of the search are compared with those of detectable X-ray sources, but no X-ray emission is detected from any of these positions. The implications of the results are discussed in terms of models for the radio emission from compact extragalactic sources. Several possible explanations are presented for the general absence of Compton X-ray emission from the bursting radio sources surveyed.

Marscher, A. P.↗
NASA NTRS1979

3C 273 - X-ray variability, spectrum, and position

Observations of the X-ray source associated with 3C 273 have been carried out from 1.3 to 13.3 keV with the scanning modulation collimator on HEAO 1, yielding several precise positions (0.42 sq arcmin), one of which includes 3C 273. The data show an increase in flux by a factor of at least 3 in 6 months. A substantial low-energy (1.3-2.6 keV) flux during the higher state yields a hydrogen column density of less than 2 x 10 to the 22nd per sq cm and removes a possible X-ray distinction between quasars and other extragalactic nuclei, viz., Seyfert type 1 and BL Lacertae objects. The 90-GHz radio flux decreased during this period (1978.0-1978.5), and the optical flux exhibited increased variability after several years of relative quiescence.

Bradt, H. V.↗
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 NTRS1977

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

Centaurus A was observed at radio frequencies of 10.7, 31.4, 85.2, and 89 GHz and at X-ray energies greater than 20 keV. 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 consistent with adiabatic expansion of a trapped plasma in conjunction with turbulent accelerations of the relativistic electrons.

Beall, J. H.↗
NASA NTRS1977

90-GHz flux-density measurements of variable radio sources

Results are presented for measurements of the flux densities of 10 variable extragalactic sources at 85.2 or 90 GHz, which were made over a period of almost seven years with the NRAO 36-ft millimeter-wave antenna. The primary flux-density calibration standards used include Jupiter, Saturn, Mars, and the small-diameter Galactic source DR 21. Measured flux densities are given as a function of time (in years) for the sources 3C 84, NRAO 150, 3C 120, OJ 287, 4C 39.25, 3C 273, 3C 279, 3C 345, BL Lac, and 3C 454.3. No statistically meaningful flux-density changes during an observing interval (1 to 3 days) are detected for any source, and a high degree of correlation between flux-density variations at 85.2 or 90 GHz and those observed at lower frequencies is found in all 10 sources. Some variations observed at different frequencies in several individual sources are briefly discussed.

Hobbs, R. W.↗
NASA NTRS1976

Search for correlated radio and optical events in long-term studies of extragalactic sources

For the first time, long-term records of radio and optical fluxes of a large sample of variable extragalactic sources have been assembled and compared, with linear cross-correlation analysis being used to reinforce the visual comparisons. Only in the case of the BL Lac object OJ 287 is the correlation between radio and optical records strong. In the majority of cases there is no evidence of significant correlation, although nine sources show limited or weak evidence of correlation. The results do not support naive extrapolation of the expanding source model. The general absence of strong correlation between the radio and optical regions has important implications for the energetics of events occurring in such sources.

Pomphrey, R. B.↗
NASA NTRS1974

A high-resolution map of the galactic-center region

A 2-cm wavelength map of the region in the vicinity of the galactic center is presented that represents an improvement in resolution by a factor of 2.7 over Hollinger's (1965) map and an improvement in sensitivity and coverage over the Downes, Maxwell, and Meeks (1965) map. The improved map reveals the great complexity of this region and the existence of a large number of densely clustered discrete sources and many new hitherto unresolved features.

Kapitzky, J. E.↗
NASA NTRS1972

A flux-density scale for microwave frequencies.

Accurate flux-density measurements of the thermal radio source DR 21 have been made at centimeter wavelengths relative to the KPW absolute flux-density scale based on Cas A and at millimeter wavelengths relative to absolute brightness-temperature measurements of Jupiter and Saturn. The form of the absolute spectrum of DR 21 thus defined is given and used to relate two formerly independent flux-density scales. With an accuracy of about 3 percent, this spectrum of DR 21 defines a flux-density scale that can be used to calibrate antennas having beamwidths between 1 and 6 minutes of arc at microwave frequencies above 7 GHz where other methods of absolute calibration are much less accurate.

Dent, W. A.↗
NASA NTRS1972

Evidence for spatially independent outbursts in compact radio sources.

Observations of the 7.8-GHz flux-density variations of 3C 120 suggest that the apparent superrelativistic expansion of this source and others can be explained as being due to independent spatially separated radio outbursts. If confirmed, this result would establish that the energy generated in quasi-stellar sources is released in many separate independent outbursts rather than by repeated eruptions in a single supermassive object.

Dent, W. A.↗
NASA NTRS1972

3C279 - Evidence for a non-superrelativistic model.

Measurements of the observed variation of the total flux density of the quasi-stellar radio source 3C279 at 7.8 GHz are discussed. It is shown that these measurements are consistent with a model for 3C279 that does not involve an apparent source expansion rate of ten times the speed of light.

Dent, W. A.↗