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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Radio emission, cosmic ray electrons, and the production of gamma-rays in the Galaxy

The paper examines the traditional derivation of the interstellar electron spectrum using the galactic nonthermal radio spectrum and new radio data. The radio spectrum derived in the polar directions is now used as a base for this derivation instead of the anticenter spectrum. The exponent is determined for the interstellar electron spectrum between 70 and 1200 MeV, and radio maps are used to predict the gamma-ray fluxes produced by the bremsstrahlung process expected from these electrons. The differences in the predicted and observed gamma-ray intensities in the galactic plane are small, but in the polar direction the predicted gamma-ray flux using radio data is six times larger than that actually observed.

Webber, W. R.↗
NASA NTRS1988

A solar-wind 'trigger' for the outer heliosphere radio emissions and the distance to the terminal shock

The solar wind data from the plasma science experiment on the Voyager spacecraft are examined to search for the source of the 2 to 3 kHz radio noise detected in the outer heliosphere. It is found that two anomalous high speed streams passed Voyager 2 before the noise was initially observed. It is suggested that the interaction of these streams with the terminal shock is responsible for the more intense emission. Using a time-of-flight argument, the distance to the shock is estimated at about 70 AU-140 AU. The larger value is consistent with an estimate using Voyager data and a standard pressure-balance argument.

Mcnutt, Ralph L., Jr.↗
NASA NTRS1989

U. radio emission from quiescent filaments

Full-disk Very Large Array (VLA) synthesis maps of the quiet Sun indicate that filaments can be seen in emission at 91.6 cm wavelength; they are detected in absorption at shorter microwave wavelengths. The 91.6 cm emission has a brightness temperature of T sub B = 3 x 10(exp 5) K. It is hotter, wider and longer than the underlying filament detected at H alpha wavelengths, but the similarity between the shape, position, elongation and orientation of the radio and optical features suggests their close association. The 91.6 cm emission is attributed to the thermal-bremsstrahlung of a hot transition sheath that envelopes the H alpha filament and acts as an interface between the cool, dense H alpha filament and the hotter, rarefied corona. The transition sheath is seen in emission because of the lower optical depth of the corona at 90 cm wavelength, and the width of this sheet is 10(exp 9) cm. A power law gradient in pressure provides a better match to the observations than a constant pressure model; definitive tests of theoretical models await simultaneous multi-wavelength studies of filaments at different observing angles. When the thermal bremsstrahlung is optically thin, the magnetic field strength in the transition sheath can be inferred from the observed circular polarization. Variable physical parameters of the sheath, such as width, electron density, and electron temperature, can explain controversial reports of the detection of, or the failure to detect, the meter-wavelength counterpart of H alpha filaments.

Lang, Kenneth R.↗
DOE OSTI2022

Connecting radio emission to AGN wind properties with broad absorption line quasars

ABSTRACT Broad absorption line quasars (BALQSOs) show strong signatures of powerful outflows, with the potential to alter the cosmic history of their host galaxies. These signatures are only seen in ∼10 per cent of optically selected quasars, although the fraction significantly increases in IR and radio selected samples. A proven physical explanation for this observed fraction has yet to be found, along with a determination of why this fraction increases at radio wavelengths. We present the largest sample of radio matched BALQSOs using the LOFAR Two-metre Sky Survey Data Release 2 and employ it to investigate radio properties of BALQSOs. Within the DR2 footprint, there are 3537 BALQSOs from Sloan Digital Sky Survey DR12 with continuum signal-to-noise ≥5. We find radio-detections for 1108 BALQSOs, with an important subpopulation of 120 LoBALs, an unprecedented sample size for radio matched BALQSOs given the sky coverage to date. BALQSOs are a radio-quiet population that show an increase of ×1.50 radio-detection fraction compared to non-BALQSOs. LoBALs show an increase of ×2.22 that of non-BALQSO quasars. We show that this detection fraction correlates with wind-strength, reddening, and C iv emission properties of BALQSOs and that these features may be connected, although no single property can fully explain the enhanced radio detection fraction. We create composite spectra for subclasses of BALQSOs based on wind strength and colour, finding differences in the absorption profiles of radio-detected and radio-undetected sources, particularly for LoBALs. Overall, we favour a wind-ISM interaction explanation for the increased radio-detection fraction of BALQSOs.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1972

Very long baseline interferometry observations of radio emissions from geostationary satellites.

Review of the results of very long baseline interferometry (VLBI) observations of L-band emissions of the ATS-5 geostationary satellite, performed over a transcontinental baseline in 1970, as part of a more comprehensive program using radio stars as sources. Although the data were not sufficient to demonstrate fully the capability of the method for determining the orbital elements of a satellite and its inertial position, the analysis of the ATS observations show the presence of interference fringes related to the modulation waveform emitted by the satellite and to the various motions of the satellite. The measured VLBI time delays are in agreement with the geometry of the observations and with the instrumental delay, both determined independently.

Michelini, R. D.↗
NASA NTRS1977

Cosmic-ray electrons and galactic radio emission - A conflict

An analysis which takes into account the observed energy spectrum of cosmic-ray electrons above 5 GeV and calculated mean magnetic field data shows that the observed spectral index of the radio continuum in the Galaxy is in conflict with some of the cosmic-ray electron measurements. It is found that the absolute intensities of cosmic-ray electrons measured by some of the experimenters are so low that they cannot be reconciled either with the interstellar magnetic field limits or with the extent of the galactic disk toward the anticenter.

Badhwar, G. D.↗
NASA NTRS1986

Analysis of Jovian decametric data: Study of radio emission mechanisms

Catalogues of approx. 200 decametric arcs and approx. 200 gaps between arcs were studied, in an effort to reconcile the data with predictions for the model wherein reflections of Io-induced currents each emit in a conical pattern and yield a distinct radio arc. The most recent interpretations of this data suggest that these Io-produced Alfven waves persist for at least one or two passages of Io, and that the emission cone half angles are approx. 40 to 90 deg., varying from arc to arc. Below 1.2 MHz it was discovered that Jupiter emits radiation strongly modulated in frequency with periods of approx. 200 kHz; this quasi-sinusoidal emission (MSA) can shift more than 180 deg. in phase over periods of 6 seconds, although these shifts are usually much smaller. MSA is not strongly correlated with the longitudes of Io or Jupiter, and typically occurs in patches covering approx. 500 kHz or more for periods of a few minutes. Furthermore, this modulation sometimes resembles a train of impulses in frequency with exponential decays toward high frequencies. Comparison of these results with the previous studies of V-shaped S-bursts is suggestive of an emission mechanism.

Staelin, D. H.↗
NASA NTRS1989

Radio emission and the hot interstellar medium of early-type galaxies

The results of an analysis of a sample of 84 elliptical and S0 galaxies, observed in X-rays with the Einstein Observatory and in radio continuum at 5 GHz, are reported. Radio flux densities result in some of the lowest radio powers yet reported for early-type galaxies. Radio structures extending beyond the optical radius are found only in galaxies with 5 GHz radio power greater than about 10 exp 29.5 ergs/s/Hz. Radio and X-ray luminosities are correlated, although with large intrinsic scatter, suggesting that more than one mechanism may be involved. A correlation between core radio power and the X-ray-to-optical ratio suggests a connection between the hot ISM and nuclear radio sources and points to accreting cooling flows as the fuel for the radio sources. For the same radio core power, extended radio lobes tend to be associated with galaxies with relatively smaller X-ray-to-optical ratios, pointing to the importance of the hot ISM in disrupting the radio jets and confining extended radio structures.

Fabbiano, G.↗
NASA NTRS1997

Collisional quenching of OH radio emission from comet Hale-Bopp

Observations of comets in the 18-cm OH transitions offer a means to probe gas production, kinematics, and OH excitation in comets. We present initial results of OH observations of comet Hale-Bopp obtained with the NRAO 43 m antenna located in Greenbank, WV. Maps of the emission provide strong constraints on the amount of quenching of the inversion of the OH ground state A-doublet in the coma. Analysis of the total radio OH flux and maps of its radial brightness distribution indicate a quenched region on the order of approximately 500,000 km during March and April 1997. This large value is generally consistent with previous observations of radio OH quenching in lower production rate comets when the high production rate of comet Hale-Bopp is considered.

NASA Discipline Exobiology↗
NASA NTRS2018

Coordinated Satellite Observations of the Very Low Frequency Transmission Through the Ionospheric D Layer at Low Latitudes, Using Broadband Radio Emissions from Lightning

Both ray theory and full-wave models of very low frequency transmission through the ionospheric D layer predict that the transmission is greatly suppressed near the geomagnetic equator. We use data from the low-inclination Communication/Navigation Outage Forecast System satellite to test this semiquantitatively, for broadband very low frequency emissions from lightning. Approximate ground-truthing of the incident wavefields in the Earth-ionosphere waveguide is provided by the World Wide Lightning Location Network. Observations of the wavefields at the satellite are provided by the Vector Electric Field Instrument aboard the satellite. The data set comprises whistler observations with the satellite at magnetic latitudes<26deg. Thus, our conclusions, too, must be limited to the near-equatorial region and are not necessarily predictive of midlatitude whistler properties. We find that in most broadband recordings of radio waves at the satellite, very few of the lightning strokes result in a detectable radio pulse at the satellite. However, in a minority of the recordings, there is enhanced transmission of very low frequency lightning emissions through the D layer, at a level exceeding model predictions by at least an order of magnitude. We show that kilometric-scale D-layer irregularities may be implicated in the enhanced transmission. This observation of sporadic enhancements at low magnetic latitude, made with broadband lightning emissions, is consistent with an earlier review of D-layer transmission for transmission from powerful man-made radio beacons.

Jacobson, Abram R.↗
DOE OSTI2021

Resolving the Radio Emission from the Quasar P172+18 at z = 6.82

We present high angular resolution imaging of the quasar PSO J172.3556+18.7734 at z = 6.82 with the Very Long Baseline Array (VLBA). This source currently holds the record of being the highest redshift radio-loud quasar. These observations reveal a dominant radio source with a flux density of 398.4 ± 61.4 μJy at 1.53 GHz, a deconvolved size of 9.9 × 3.5 mas (52.5 × 18.6 pc), and an intrinsic brightness temperature of (4.7 ± 0.7) × 10{sup 7} K. A weak unresolved radio extension from the main source is also detected at the ∼3.1σ level. The total flux density recovered with the VLBA at 1.53 GHz is consistent with that measured with the Very Large Array (VLA) at a similar frequency. The quasar is not detected at 4.67 GHz with the VLBA, suggesting a steep spectral index with a limit of α{sub 4.67}{sup 1.53}<−1.55. The quasar is also not detected with the VLBA at 7.67 GHz. The overall characteristics of the quasar suggest that it is a very young radio source similar to lower redshift gigahertz peaked spectrum radio sources, with an estimated kinematic age of ∼10{sup 3} yr. The VLA observations of this quasar revealed a second radio source in the field 23.″1 away. This radio source, which does not have an optical or IR counterpart, is not detected with the VLBA at any of the observed frequencies. Its nondetection at the lowest observed VLBA frequency suggests that it is resolved out, implying a size larger than ∼0.″17. It is thus likely situated at lower redshift than the quasar.

47 OTHER INSTRUMENTATION↗
NASA NTRS1978

Frequency and time dependence of the Jovian decametric radio emissions - A nineteen-year high-resolution study

A nineteen-year catalog of observations has permitted detailed comparison of Jovian source characteristics at 10 frequencies from 5.6 to 30 MHz. The spectral morphology of Jupiter's decameter-wavelength radio sources was studied by means of high-resolution histograms of occurrence probability for intervals as small as 2 deg in Jovian system III longitude and the departure of Io from superior geocentric conjunction. Longitude histograms clearly exhibit a substantial shift in the source A and source B positions between 15 and 10 MHz. Similar diagrams isolate the dependence of each source on the position of Io, illustrating changes with frequency in the structures and sizes of the main probability maxima and in the positions of surrounding secondary peaks. Individual frequency probability histograms for both longitude and Io phase reveal new phenomena, such as a bridge of radiation through source B longitudes connecting sources A and C at 15 MHz and spectral modification of portions of the Io-related source B peak.

Thieman, J. R.↗
NASA NTRS1975

Saturn radio emission near 1 MHz

Emission from the direction of the planet Saturn was observed by the IMP-6 spacecraft at 15 frequencies between 375 and 2200 kHz during the period April 1971 to October 1972. The Saturnian radio bursts were identified in the IMP-6 data through an analysis of the phase of the observed modulated signal detected from the spinning dipole antenna. Initial data reduction has isolated approximately 12 storms whose occurrence corresponded to times in which the spacecraft had an unobstructed view in the direction of Saturn. These events persisted over periods between one and ten minutes. Over the span of 500 days of data another 10 to 20 Saturnian events may exist, but positive identification is confused by the presence of terrestrial noise as well as a geometric ambiguity with Jupiter. A power spectral analysis of the storm occurrence times indicates a weak periodicity at 10h 30m + or - 5m at some of the observing frequencies. The spectral character of the radiation was found analogous to that of Jupiter.

Brown, L. W.↗
NASA NTRS1985

Radio emission from AM Herculis

Observations of the quiescent microwave emission of the magnetic cataclysmic variable AM Herculis are presented. The emission, which declined from a mean value of 0.58 mJy at 4.9 GHz to about 0.3 mJy, in rough coincidence with the entry of AM Herculis into an optical low state (mid-1983), is explained in terms of optically thick gyrosynchrotron emission. It is noted that the observation of a coherent outburst at 4.9 GHz, interpreted as the result of a cyclotron maser on the red dwarf secondary, indicates that the secondary is magnetized. Possible implications are briefly explored. Comparisons between this system and other stellar continuum radio sources are made.

Bastian, T. S.↗
NASA NTRS1978

X-ray and radio emission from the compact galaxy III Zw 2

X-rays from the compact galaxy III Zw 2 have been detected with the rotating modulation collimator on board SAS 3. The (2-10-keV) flux is (4.3 + or - 0.5) by 10 to the -11th power erg/sq cm per sec, which corresponds to a luminosity of 1.4 by 10 to the 45th power erg/s at 530 Mpc. Subsequent radio observations between 1.46 and 90 GHz were made at the NRAO Very Large Array and 36-foot (11-m) installations. The radio spectrum rises sharply toward higher frequencies and, below about 100 GHz, is indicative of a self-absorbed synchrotron spectrum from an extremely compact, rapidly evolving object.

Schnopper, H. W.↗
NASA NTRS1999

The Contribution of Ionizing Starts to the Far-Infrared and Radio Emission in the Milky Way: Evidence for a Swept-up Shell and Diffuse Ionized Halo Around the W4 Chimney/Supershell

Normandeau have proposed that W4 is a galactic chimney, the only chimney to-date identified in our Galaxy. Using the recent approx. 1 min resolution IGA (Infrared Galaxy Atlas) and DRAO (Dominion Royal Astrophysical Observatory) CGPS (Canadian Galactic Plane Survey) galactic plane surveys we analyze the far-infrared and radio structure of the W 4 chimney/supershell. We show W4 has a swept-up partially ionized shell of gas and dust which is powered by the OCl 352 star cluster. Analysis of the dust column density establishes there is dense interstellar material below the shell, directly showing the dense material which caused the lower shell expansion to stall. Due to much lower densities above the Galactic plane, the upper W4 shell achieved 'breakout' to form a Galactic chimney. Although the shell appears ionization bounded, it is very inhomogenous and an ionized halo provides evidence of significant Lyman continuum leakage. A large fraction of the OCl 352 cluster photons escape to large distances and are available to ionize the WIM (warm ionized medium) component of the interstellar medium.

Terebey, Susan↗
NASA NTRS1980

Radio emission evolution of nonstationary sources in the Hedgehog model

Correlations are obtained for numerical calculation of flux F sub v and polarized radiation intensity of a cloud of arbitrary geometry, consisting of ultrarelativistic electrons that dissipate in a radial magnetic field of the nucleus at a random angle to the observer. It is possible that some of the variable extragalactic objects that were previously described by the Shklovskiy model are young formations in the examined model. Radio astronomical observations would permit a determination of their distance, age, and lifetime.

Kovalev, Y. A.↗