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At least 379 records · Page 21
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NASA NTRS1989

Overpressured cocoons in extragalactic radio sources

It is shown that the cocoons of shocked gas which surround powerful double radio sources can have significantly higher pressures than the surrounding intergalactic medium. The pressures can be high enough to confine the jets in these sources, obviating the need for magnetic confinement. The cocoon pressure and the age of a radio source may be estimated from observable quantities, as demonstrated here for the radio galaxy Cygnus A. It is suggested that overpressured cocoons in high-redshift radio galaxies engulf and compress circumgalactic clouds, driving them over the Jeans limit and triggering star formation. It is proposed that this process leads to the observed alignments of optical continuum emission with radio source axes.

Begelman, Mitchell C.↗
NASA NTRS1990

No evidence for radio-quiet BL Lacertae objects

Using a large, flux-limited sample of faint X-ray sources, a search has been conducted for radio-quiet BL Lacertae objects. None has been found. Thirty-two X-ray-selected BL Lac objects and BL Lac candidates have been found within the sources of the Einstein Medium Sensitivity Survey (EMSS). Thirty-one of these have been observed with the VLA and all have been detected at 5 GHz. While the optical magnitudes of the EMSS BL Lac objects range from 17 to 20.8, their radio-to-optical spectral indices occupy a very small range. The very bright X-ray-selected BL Lac objects like PKS 2155-304 and Markarian 501 have similar range values. Therefore, unlike the clear dichotomy between radio-loud quasars and radio-quiet QSOs, there is no evidence for two populations of Lacertids distinguished by radio loudness.

Stocke, John T.↗
NASA NTRS1990

Comparative morphological analysis of Puppis A at radio, infrared, optical, and X-ray wavelengths

This paper presents a new radio image of the supernova remnant Puppis A obtained with the Molonglo Observatory synthesis telescope (MOST) and the Parkes telescope, and infrared images of Puppis A and its surrounding medium obtained with the IRAS, and compares the remnant morphologies at radio, infrared, optical, and X-ray wavelengths. The radio image, which includes large-scale emission, shows a clumpy shell of emission with moderate central brightness. The infrared observations have the advantage of offering a detailed look at the medium into which the remnant is expanding. From the IR data, evidence is found of the remnant's expansion into a strong density gradient (distinct from the more gentle gradient of the Galactic plane). A strong isolated knot of enhanced X-ray and radio emission is a region of enhanced infrared emission as well. Detailed correlative analysis of the infrared, radio, and X-ray emission shows that the IR emission correlates very well with the X-ray emission, confirming the idea that the IR emission arises from dust that is collisionally heated by the shocked gas.

Arendt, Richard G.↗
NASA NTRS1990

Infrared and radio measurements of the density structure of compact H II regions

The density structure of compact H II regions is studied by comparing the ratio of the S III forbidden 33.47-micron and 18.71-micron emission lines and 5-GHz continuum maps. Electron densities are computed for DR 22, G29.9 - 0.0, G75.84 + 0.4, M8, M42, S158, W3, and W33. For DR 22, M42, and W3, the electron densities determined from the IR data are consistent with those derived from radio observations. The densities for S158 and M8 are quite low, implying that the S III emission is dominated by the diffuse ionized gas observed in the radio continuum. For G29.9 - 0.0, G75.84 + 0.4, and W33, the S III data give higher densities than predicted from radio observations. This implies that either these nebulae are filled with dense clumps of ionized gas which are not resolved in the radio maps, or that the S III zone is confined to the small, dense cores found in the radio maps of all three nebulae.

Megeath, S. T.↗
NASA NTRS1990

Low-frequency radio emissions at Neptune

The Voyager 2 plasma wave receiver detected weak radio emissions from Neptune's magnetosphere in the frequency range of 3 - 60 kHz. The emissions occurred in bursts lasting for typically 1.5 hours, often occurring twice per planetary rotation. Most of these radio bursts were detected within several degrees of the magnetic equatorial plane. During the passage through the magnetosphere, electrostatic upper hybrid resonance bands were observed close to the magnetic equator in conjunction with intensifications of the radio emissions at frequencies close to and above the upper hybrid bands. Further, near closest approach, the radio emissions were observed to cross the right-hand cutoff frequency with no apparent attenuation. It is concluded that the Neptunian radio emissions below about 60 kHz are produced by mode conversion from the upper hybrid waves and propagate in the ordinary mode into beams within about 12 deg of the magnetic equator. There is also evidence of an extraordinary mode emission at about 60 kHz which is apparently generated by an entirely different source from the escaping continuum radiation.

Kurth, W. S.↗
NASA NTRS1990

The 60 micron to 20 centimeter infrared-to-radio ratio within spiral galaxies

A detailed comparison is presented of the distribution of 60 micron IR and 20 cm radio continuum emission within 25 galaxies, mostly disk spirals. Local maxima in the thermal IR and nonthermal radio emission are found to be spatially coincident on scales of less than about 0.4 kpc in the nearest sample galaxies. The IR-red disk in normal spirals appears to be characterized by a shorter scale length than that of the radio continuum disk, suggesting that the IR-to-radio ratio should decrease as a function of radius. A model that successfully accounts for the observations is introduced which is based on the assumptions of steady-state star formation activity within the disk on kpc scales and a tight coupling between the origins of the dust-heating radiation and the radio-emitting cosmic-ray electrons. The underlying source is described as an exponential disk. The results also suggest that a random walk process cannot by itself describe the temporal evolution of cosmic rays.

Bicay, M. D.↗
NASA NTRS1990

4C 41.17 - A radio galaxy at a redshift of 3.8

Observations, comprising radio imaging at three frequencies, optical broadband and narrow-band imaging, optical spectroscopy, and K-band IR imaging, of a distant radio source, 4C 41.17, are presented. It is shown that 4C 41.17 is associated with an extended source of optical and IR emission at a redshift of 3.8. The morphological similarities between the radio source and the ionized gas and optical/IR continua suggest that the radio source is interacting vigorously with the galaxy. Various mechanisms for producing the optical/IR emission in 4C 41.17 are considered. It is concluded that a large fraction of the spectral energy distribution is from stars, and that the star formation induced by the radio source is the likely mechanism to account for the radio/optical alignment.

Chambers, K. C.↗
NASA NTRS1990

Field-independent source localization of Neptune's radio bursts

During the Voyager 2 encounter with Neptune, a narrowbanded bursty radio component was observed between 500 and 1326 kHz by the Planetary Radio Astronomy instrument. Based on the emission occurrence pattern, the radio source has been localized without the explicit use of the Neptunian offset-tilted dipole magnetic field model, which is accurate only at distances greater than 4 R(N) (Neptune radii) from the planet. Only assumptions based upon the general nature of radio wave propagation in planetary magnetospheres were used. A number of different candidate radial positions were sampled. For example, at 1.5 R(N), the derived source location was positioned only about 10 deg from the south magnetic pole. The radiation from this source was beamed into a cone of 77.5 + or - 6.3 deg half-angle that was tilted about 10 deg from the radial direction to the north-northeast. At other sampled radial positions, similar source locations were obtained. Due to its proximity to the south magnetic pole, the kilometric emission radio source is believed to be associated with an active auroral region, similar in nature to those found at earth and Saturn.

Farrell, W. M.↗
NASA NTRS1991

Astrometry of southern radio sources

An overview is presented of a number of astrometry and astrophysics programs based on radio sources from the Parkes 2.7 GHz catalogs. The programs cover the optical identification and spectroscopy of flat-spectrum Parkes sources and the determination of their milliarcsecond radio structures and positions. Work is also in progress to tie together the radio and Hipparcos positional reference frames. A parallel program of radio and optical astrometry of southern radio stars is also under way.

White, Graeme L.↗
NASA NTRS1991

Correlations between the far-infrared, radio, and blue luminosities of spiral galaxies

FIR-radio luminosity correlations are presently derived for two statistically complete samples spanning a wide luminosity range. While observed correlations are tight, they are not linear; the observed scattering is mostly intrinsic. An empirical correction to the radio luminosities, based on observed blue luminosities, makes the corrected FIR-radio correlation linear and eliminates most of the residual variance. A two-component model that is consistent with the improved correlation is proposed for the heating luminosity within galaxies. The first, radio-loud component contains young massive stars that heat the dust, while contributing to the cool cirrus dust-heating interstellar radiation field. The second, radio-quiet component is made up of less massive older stars.

Condon, J. J.↗
NASA NTRS1991

Radio telescopes as the detectors of super-high-energy neutrinos

The registration of super high energy neutrinos is a very difficult and also very important problem that requires construction of detectors with large effective target masses. Askaryan pointed out the possibility of registering cascades in dense media by the Cherenkov radio emission of an excess of negative charges in the cascades which arose in interaction between high energy particles and the atoms of medium. The telescopes for cosmic high energy neutrino detection by radioemission of cascades induced underground, but whose development continues in the atmosphere were proposed by others. The effective target masses of such detectors could be approx. 10(exp 9) tons and more. The properties of Cherenkov radio emission of cascades and the properties of ice in the Antarctic Region make it possible to propose Radio Antarctic Muon and Neutrino Detection (RAMAND): antennas should be placed on the ice surface of approx. 10 sq km to search for radio signals for neutrino (muon) cascades of energy. It is evident from data given that the largest radio telescopes gives the opportunity for registration of the cascades induced by neutrinos with the energies E is greater than or = 10(exp 20) eV.

Dagkesamansky, R. D.↗
NASA NTRS1992

Li/BCX (thionyl chloride) battery for the NASA AN/PRC-112 survival radio

As part of the NASA contingency planning related to aborting a launch after liftoff, an emergency radio is required for use by the crew when they return to Earth at some unplanned location. The power source for the radio must be able to satisfy the performance requirements for the radio's mission as well as be compatible with in-cabin storage in the space shuttle. The radio needs a base load power of about 1 W with capability to handle power spikes greater then 6.5 W. A slightly enlarged battery pack using the Li/BCX chemistry in C-size cells was developed that meets these power levels and extends the operational life of the radio by over a factor of four compared to its operation using a Li/SO2 cell battery pack. In addition, the cells meet the requirements for the Li/BCX cells used for extra-vehicular activities by the crew of the shuttle. One of the major qualifying tests is the ability of the cells to withstand exposure to high temperature (149 C) without leaking. Electrical performance and thermal abuse test data will be presented for the cells.

Ebel, Steven J.↗
NASA NTRS1991

The role of solar wind reconnection in driving the Neptune radio emission

The only remote diagnostic of conditions within the outer planets' magnetospheres is the highly variable flux of low-frequency radio waves. As at the other radio planets, Neptune radio emission also manifests, on a time scale of days, major intensity fluctuations that are indicative of a solar wind energy-coupling process of some kind. It is found that the merging of interplanetary magnetic field lines with Neptune's magnetosphere is the best predictor of emitted radio energy. By contrast, viscouslike energy coupling processes, such as might be caused by solar wind density or bulk speed fluctuations, are apparently ineffective in driving the radio emission.

Desch, M. D.↗
NASA NTRS1991

The association of solar millisecond radio spikes with hard X-ray emission

Conventional observational data regarding solar millisecond spikes are compared with data gathered simultaneously in the hard X-ray band by means of a statistical analysis. The analysis considers the association rate, correlation degree, and relative time delays between hard X-ray emissions (in the 25-438 keV range) and radio-spike events. About 95 percent of the radio-spike bursts occur during impulsive hard X-ray bursts, and approximately 43 percent of the compared events are characterized by hard X-ray time profiles that mimic the concentration of simultaneous radio spikes. The delay of the radio emission with respect to the hard X-ray bursts puts some constraints on the acceleration and propagation of particles. The time delays and the quantization into discrete radio events are theorized to be caused by the operation of the accelerator.

Guedel, M.↗
NASA NTRS1992

A model for the radio brightness of the supernova remnant 1987A

The recently observed rise in radio flux from SN1987A is earlier than predicted in models of the interaction with the observed line-emitting gas. The observed size of the radio source is roughly consistent with the expected position of the SN shock wave and the position of the termination shock of the blue supergiant progenitor wind. The radio increase is attributed to the interaction of the SN shock with this density jump. The required efficiency of electron acceleration and magnetic field amplification is larger than usually occurs in SNe and SNR. Possible reasons for the enhanced radio emission are particle acceleration by the weak reflected shock that travels through the interaction region, and the action of the Richtmyer-Meshkov instability near the initial termination-shock density jump. The action of these mechanisms is time-limited; the prediction of this model is that the radio flux should begin to decline within the next year.

Chevalier, Roger A.↗
NASA NTRS1992

Evolution of luminous IRAS sources - Radio imaging

Observations of the compact radio morphology of 111 luminous IRAS-selected active galaxies covering a wide range of IR and optical properties are presented and discussed. Of these sources, 72 are observed for the first time with the VLA A configuration. The circumnuclear radio sources are generally small and weak, with 15 nondetections to a limit of about 0.4 mJy. Comparison with the IR and optical properties of the objects indicates that the radio sources turn on within 10 exp 8 yr of the tidal encounter that is presumed to generate the IR activity. However, the radio sources do not all appear at the same time. The radio observations are consistent with the evolution scenario for luminous IRAS galaxies suggested by Hutchings and Neff (1991).

Neff, S. G.↗
NASA NTRS1992

Radio and infrared emission from Markarian starburst galaxies

Radio and infrared emission were compared for a sample of 58 Markarian starburst galaxies, chosen to cover a wide range of 60-micron luminosity density. New radio observations were from the VLA at 6 and 20 cm in the B and A configurations. IRAS data were reanalyzed for 25 of the starbursts that were previously undetected at either 25 or 100 microns. The correlation between the global radio and IR emission for the starbursts in the sample is strongest at 25 and 60 microns, wavelengths in which the warm dust dominates. The radio spectral index steepens away from the center. This indicates that nonthermal emission leaks out of the starburst region. The change in the spectral index implies that while nonthermal sources dominate in the entire region, the bulk of the interior emission at 6 cm is thermal. The radio spectral index does not appear to vary as a function of the infrared luminosity or the infrared colors, which indicates that the slope of the initial mass function does not appear to be a function of either the mass or temperature of the starburst.

Stine, Peter C.↗
NASA NTRS1992

Resolution of the southern radio lobe of 3C 33 at 660 nm

A strand of emission coincident with the peak in the radio emission at 2 and 6 cm was revealed by an HST planetary camera image of the southern radio lobe of 3C 33. The detailed morphological similarity between the optical and radio data conclusively confirms the common origin of the radio and optical emission. The question of whether or not the detailed correlations in emission seen in the jets can also be observed on the larger scales of radio lobes is addressed.

Crane, Phillipe↗