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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Saturn as a radio source

Magnetospheric radio emissions, Saturn electrostatic discharges, inferred source locations, and emission theories are addressed.

Kaiser, M. L.↗
NASA NTRS1984

Saturn as a radio source

Magnetospheric radio emissions, Saturn electrostatic discharges, inferred source locations, and emission theories are addressed.

Kaiser, M.L.↗
NASA NTRS2010

Long-Term Stability of Radio Sources in VLBI Analysis

Positional stability of radio sources is an important requirement for modeling of only one source position for the complete length of VLBI data of presently more than 20 years. The stability of radio sources can be verified by analyzing time series of radio source coordinates. One approach is a statistical test for normal distribution of residuals to the weighted mean for each radio source component of the time series. Systematic phenomena in the time series can thus be detected. Nevertheless, an inspection of rate estimation and weighted root-mean-square (WRMS) variations about the mean is also necessary. On the basis of the time series computed by the BKG group in the frame of the ICRF2 working group, 226 stable radio sources with an axis stability of 10 as could be identified. They include 100 ICRF2 axes-defining sources which are determined independently of the method applied in the ICRF2 working group. 29 stable radio sources with a source structure index of less than 3.0 can also be used to increase the number of 295 ICRF2 defining sources.

Engelhardt, Gerald↗
NASA NTRS1991

Comparison of VLBI radio-core and X-ray flux densities of extragalactic radio sources

The relation between compact radio core and X-ray emission in extragalactic radio sources, suggested by Worral et al. (1987) and Kembhavi et al. (1986), is investigated by comparing the X-ray flux densities observed in 56 extragalactic radio sources with the Einstein Observatory with the compact radio-core flux densities derived from published VLBI maps for these radio sources. It was found that the radio to X-ray spectral index distribution had a small dispersion, whereas the log-log plot of the flux densities showed no correlation. This implies that the basic level of X-ray emission is determined by the radio-core emission, but that the exact value depends on other parameters.

Bloom, Steven D.↗
NASA NTRS1989

The prevalence of supernova remnants among unidentified Galactic radio sources

Nine Galactic radio sources were mapped to identify new Crab-like and composite supernova remnants. The sources were selected on the basis of existing stringent upper limits on their hydrogen recombination line fluxes. One new Cracb-like remnant, one new composite remnant, at least one, and probably two, new shell-like remnants, and a compact H II region were found, along with the expected collection of extragalactic objects. The results suggest that there are several hundred SNRs in the Galaxy which are detectable with current instruments, but which have yet to be identified.

Helfand, David J.↗
NASA NTRS1994

Radio, millimeter-submillimeter, and infrared spectra of flat-spectrum extragalactic radio sources

We present radio to submillimeter-wave continuum spectra of 44 bright, compact extragalactic radio sources with flat spectra at centimeter wavelengths ('blazars'). Infrared J, H, and K flux densities are added to the spectra of six of these objects. These spectra are useful in comparisons of x-ray and gamma-ray measurements with the multiwaveband properties of blazars. A number of the objects have been detected as strong, hard gamma-ray sources by the Compton Gamma Ray Observatory (CGRO). The millimeter-wave spectra of the gamma-ray bright blazars we observe are flatter on average than for the sample as a whole.

Bloom, Steven D.↗
NASA NTRS2012

Chandra Observations of Dying Radio Sources in Galaxy Clusters

Context. The dying radio sources represent a very interesting and largely unexplored stage of the active galactic nucleus (AGN) evolution. They are considered to be very rare, and almost all of the few known ones were found in galaxy clusters. However, considering the small number detected so far, it has not been possible to draw any firm conclusions about their X-ray environment. Aims. We present X-ray observations performed with the Chandra satellite of the three galaxy clusters Abell 2276, ZwCl 1829.3+6912, and RX J1852.1+5711, which harbor at their center a dying radio source with an ultra-steep spectrum that we recently discovered. Methods. We analyzed the physical properties of the X-ray emitting gas surrounding these elusive radio sources. We determined the global X-ray properties of the clusters, derived the azimuthally averaged profiles of metal abundance, gas temperature, density, and pressure. Furthermore, we estimated the total mass profiles. Results. The large-scale X-ray emission is regular and spherical, suggesting a relaxed state for these systems. Indeed, we found that the three clusters are also characterized by significant enhancements in the metal abundance and declining temperature profiles toward the central region. For all these reasons, we classified RX J1852.1+5711, Abell 2276, and ZwCl 1829.3+6912 as cool-core galaxy clusters. Conclusions. We calculated the non-thermal pressure of the radio lobes assuming that the radio sources are in the minimum energy condition. For all dying sources we found that this is on average about one to two orders of magnitude lower than that of the external gas, as found for many other radio sources at the center of galaxy groups and clusters. We found marginal evidence for the presence of X-ray surface brightness depressions coincident with the fossil radio lobes of the dying sources in A2276 and ZwCl 1829.3+691. We estimated the outburst age and energy output for these two dying sources. The energy power from the AGN outburst is significantly higher than the X-ray luminosity in both clusters. Indeed, it is sufficient that a small fraction of this power is dissipated in the intra-cluster medium to reheat the cool cores.

Murgia, M.↗
NASA NTRS1986

VLBI observations of 416 extragalactic radio sources

Very long baseline interferometry (VLBI) observations of 416 radio sources with declinations north of -45 deg have been conducted at frequencies of 2.3 and 8.4 GHz. At 2.3 GHz, 323 of 391 radio sources observed were detected with a fringe spacing of 3 milliarcsec and a detection limit of about 0.1 Jy. At 8.4 GHz, 278 of 416 radio sources were detected with a fringe spacing of 1 milliarcsec and a detection limit of about 0.1 Jy. This survey was conducted primarily to determine the strength of compact components at 8.4 GHz for radio sources previously detected with VLBI at 2.3 GHz. Compact extragalactic radio sources with strong correlated flux densities at both frequencies are used to form a high-accuracy reference frame.

Morabito, D. D.↗
NASA NTRS1981

Very high-resolution observations of compact radio sources in the directions of supernova remnants

Compact radio sources whose positions lie within the outlines of supernova remnants may be the stellar remnants of supernova explosions and, if they are related to the supernova remnants, may be used to explore the nature of any morphological connection between the Galactic and extragalactic radio sources. Three such compact sources, G 127.11+0.54, CL 4, and 2051+433, have been observed at 10.65 GHz with an array of very long baseline interferometers having elements in the USA and West Germany. The radio source 2051+433 was also observed briefly at 5.01 GHz. The measured size of CL 4 at 10.65 GHz is about 0.0005 arcsec and seems to be dominated by the effects of interstellar scattering. No fringes were seen in 2051+433, and results indicate there is no compact component of 2051+433 smaller than 0.001 arcsec radiating at 10.65 GHz above a level of about 50 mJy. The possibility is presented that G 127.11+0.54 is a Galactic object. It is found to consist of two components separated by about 0.002 arcsec and oriented perpendicular to both the radio bridge of the supernova remnant G 127.1+0.5 and the underlying optical image. G 127.11+0.54, if Galactic, lies at the extreme low-luminosity end of an apparent continuum of Galactic and extragalactic compact radio source luminosities.

Geldzahler, B. J.↗
NASA NTRS1983

A search for X-ray emission from optically quiet, compact radio sources

A search for X-ray emission from optically quiet, compact radio sources reveals that such sources are significantly weaker X-ray emitters than are optically identified objects with comparable radio properties. For the radio samples examined here, the X-ray luminosity, relative to the radio, is typically at least 3 times lower in the optically quiet, compact radio sources than in the optically identified ones. Possible reasons for the distinction between optically quiet, compact radio sources and other compact radio sources are discussed. If strong nonthermal sources occur in the nuclei of dust-rich galaxies and do not eliminate the dust and gas of the interstellar medium, a few percent of such sources should be optically faint and red (due to extinction by dust) and X-ray weak (due to K-edge absorption). However, many optically quiet, compact radio sources are apparently 'blazars' with particularly sharp high-frequency breaks in their synchrotron spectra.

Ledden, J. E.↗
NASA NTRS1991

Multifrequency VLA observations of PKS 0745 - 191 - The archetypal 'cooling flow' radio source?

Ninety-, 20-, 6- and 2-cm VLA observations of the high-radio-luminosity cooling-flow radio source PKS 0745 - 191 are presented. The radio source was found to have a core with a very steep spectrum (alpha is approximately -1.5) and diffuse emission with an even steeper spectrum (alpha is approximately -1.5 to -2.3) without clear indications of the jets, hotspots, or double lobes found in the other radio sources of comparable luminosity. It is inferred that the energy to power the radio source comes from the central engine, but the source's structure may be heavily influenced by the past history of the galaxy and the inflowing intracluster medium. It is shown that, while the radio source is energetically unimportant for the cluster as a whole, it is important on the scale of the cooling flow. The mere existence of cosmic rays and magnetic fields within a substantial fraction of the volume inside the cooling radius has important consequences for cooling-flow models.

Baum, S. A.↗
NASA NTRS1987

Sub-mJy radio sources - A population of starburst galaxies at intermediate redshifts

Optical-infrared observations are used to show that the sub-mJy sources, discovered in deep radio surveys and responsible for the upturn in the radio source counts at sub-mJy levels, represent a population of star-forming galaxies at intermediate redshifts and with Mv between about -23 and -20. The very high frequency of line-emission, the very large incidence of galaxy mergers and interactions associated with these sub-mJy sources, and the good agreement between the sub-mJy radio source counts and the counts of starburst galaxies from deep IRAS surveys imply that these star-forming galaxies are in fact undergoing starbursts.

Thuan, Trinh X.↗