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At least 397 records · Page 22
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NASA NTRS1992

Detection of fundamental and harmonic type III radio emission and the associated Langmuir waves at the source region

Type III radio emission generated in the vicinity of the Ulysses spacecraft has been detected at both the fundamental and harmonic of the local plasma frequency. The observations represent the first clear evidence of locally generated type III radio emission. This local emission shows no evidence of frequency drift, exhibits a relatively short rise time, is less intense than the observed remotely generated radio emission, and is temporally correlated with observed in situ Langmuir waves. The observations were made with the unified radio astronomy and wave (URAP) experiment on the Ulysses spacecraft between 1990 November 4 and 1991 April 30, as it traveled from 1 to 3 AU from the sun. During this time period many thousands of bursts were observed. However, only three examples of local emission and associated Langmuir waves were identified. This supports previous suggestions that type III radio emission is generated in localized regions of the interplanetary medium, rather than uniformly along the extent of the electron exciter beam.

Reiner, M. J.↗
NASA NTRS1992

Radio observations of solar and stellar coronae

Recent results of radio observations of the solar and stellar coronae are reviewed. Attention is given to the results obtained on quiet-sun fine structures; the active region observations, with particular consideration given to the soft X-ray and radio comparisons of observations made during the Coronal Magnetic Structures Observing Campaign and the strength of the magnetic fields in the corona; the radio observations of solar flares; and the radio observations of stellar coronae of the RS Canum Venaticorum binaries, M dwarf stars, pre-main-sequence stars, and chemically-peculiar B stars. Also discussed are current issues in stellar radiophysics and perspectives in solar radio physics.

Kundu, M. R.↗
NASA NTRS1993

A high-sensitivity survey of radio continuum emission from Herbig Ae/Be stars

Results of a high-sensitivity VLA/Australia Telescope survey of radio continuum emission from the 57 Herbig Ae/Be stars and candidates in the 1984 catalog of Finkenzeller and Mundt are presented. Twelve stars were detected at the primary observing wavelength of 3.6 cm, on the basis that not less than 4 sigma radio sources lie within 1 arcsec of the optical positions. It is suggested that the radio emission is predominantly thermal and in many cases wind-related. The unusual eclipsing binary TY CrA is an exception and is classified as a nonthermal radio source on the basis of its decidedly negative spectral index (alpha = -1.2). A simple spherically symmetric free-fall accretion model is used to show that the predicted radio fluxes due to accretion at rates, estimated in the literature, of about 10 exp -6 to 10 exp -5 solar mass/yr are one to four orders of magnitude larger than observed.

Skinner, Stephen L.↗
NASA NTRS1993

Radio morphology and parent population of X-ray selected BL Lacertae objects

High-dynamic range (typically 1700:1) radio maps of 15 X-ray BL Lac (XBL) objects from the HEAO-1 Large Area Sky Survey are presented. Morphological characteristics of these sources are compared with Fanaroff-Riley (FR) class I radio galaxies in the context of unified schemes, with reference to one-sided kiloparsec-scale emission. Evidence that cluster membership of XBLs is significantly higher than previously thought is also presented. It is shown that the extended radio powers, X-ray emission, core-to-lobe ratios, and linear sizes of the radio selected BL Lac (RBL) and XBL populations are consistent with an FR I radio galaxy parent population. A source list and VLA observing log and map parameters are provided.

Laurent-Muehleisen, S. A.↗
NASA NTRS1992

Reduction and analysis of seasons 15 and 16 (1991 - 1992) Pioneer Venus radio occultation data and correlative studies with observations of the near-infrared emission of Venus

Radio occultation experiments, and radio astronomical observations have suggested that significant variations (both spatial and temporal) in the abundances of sulfur-bearing gases are occurring below the Venus cloud layers. In addition, recent Near Infra-Red images of the nightside of Venus revealed large-scale features which sustain their shape over multiple rotations (the rotation periods of the features are 6.0 +/- 0.5 days). Presumably, the contrast variations in the NIR images are caused by variations in the abundance of large particles in the cloud deck. If these particles are composed of liquid sulfuric acid, one would expect a strong anticorrelation between regions with a high abundance of sulfuric acid vapor, and regions where there are large particles. One technique for monitoring the abundance and distribution of sulfuric acid vapor (H2SO4) at and below the main Venus cloud layer (altitudes below 50 km) is to measure the 13-cm wavelength opacity using Pioneer Venus Orbiter Radio Occultation Studies (PV-ORO). We are working to characterize variations in the abundance and distribution of subcloud H2SO4(g) in the Venus atmosphere by using a number of 13-cm radio occultation measurements conducted with the Pioneer Venus Orbiter near the inferior conjunction of 1991. When retrieved, the vertical profiles of the abundance of H2SO4(g) will be compared and correlated with NIR images of the night side of Venus made during the same period of time. Hopefully, the combination of these two different types of data will make it possible to constrain or identify the composition of the large particles causing the features observed in the NIR images. Considered on their own, however, the parameters retrieved from the radio occultation experiments are valuable science products.

Jenkins, Jon M.↗
NASA NTRS1993

Voyager planetary radio astronomy studies

Analysis of nonthermal radio emission data obtained by the Planetary Radio Astronomy (PRA) spectrometers on the Voyager 1 and 2 spacecraft was performed. This PRA data provided unique insights into the radio emission characteristics of the outer planets because of PRA's unique spectral response below the terrestrial ionospheric plasma frequency and its unprecedented proximity to the source. Of those results which were documented or published, this final report surveys only the highlights and cites references for more complete discussions. Unpublished results for Uranus, Neptune, and theoretical Ionian current distributions are presented at greater length. The most important conclusion to be drawn from these observations is that banded spectral emission is common to the radio emission below 1-2 MHz observed from all four Jovian planets. In every case multiple spectral features evolve on time scales of seconds to minutes. To the extent these features drift in frequency, they appear never to cross one another. The Neptunian spectral features appear to drift little or not at all, their evolution consisting principally of waxing and waning. Since other evidence strongly suggests that most or all of this radio emission is occurring near the local magnetospheric electron cyclotron frequency, this implies that this emission preferentially occurs at certain continually changing planetary radii. It remains unknown why certain radii might be favored, unless radial electric field components or other means serve to differentiate radially the magnetospheric plasma density, particle energy vectors, or particle coherence. Calculation of the spatial distribution and intensity of the Io-generated magnetospheric currents are also presented; these currents may be limited principally by wave impedance and local field strengths.

Staelin, David H.↗
NASA NTRS1991

Phasing the Very Large Array on Galileo in the presence of Jupiter's strong radio emission

Work is in progress to determine the feasibility of using the Very Large Array (VLA) radio telescope to receive telemetry from Galileo during its close encounter with Io on 7 Dec. 1995. The VLA was used previously to receive telemetry from Voyager 2 at Neptune. However, Jupiter's strong radio emission is an additional complication in the case of the Galileo encounter. This article analyzes the effect of Jupiter's radio emission on the phase-adjustment procedure ('autophasing') used to maintain coherence among the 27 VLA antennas. Results of an experiment designed to mimic the Io encounter are presented. As expected, Jupiter's strong radio emission has a considerable effect on the autophasing procedure. A simple emission model is found to give a good approximation to the fringe-visibility plots derived from the VLA data, and that successful model is used to estimate the VLA's ability to autophase on Galileo during the Io encounter. The effect of Jupiter should be small for projected baselines longer than approximately 800 m and completely negligible for projected baselines longer than approximately 1.1 km. The most extended configuration of the VLA (the A configuration) probably can be used successfully for telemetry reception during the Io encounter. Further analysis and testing of the effect of correlated noise from Jupiter is necessary before a final decision can be made about the feasibility of using the second largest (B) configuration of the VLA for reception of Galileo telemetry. Use of the B configuration could simplify the upgrades needed to support the Io encounter. Tests to help choose the preferred VLA configuration could be performed by using the VLA to observe the Magellan spacecraft at Venus during Jul. and Oct. 1991. Examination of the effects of planet noise on the VLA have implications beyond the use of that telescope for supporting the Io encounter. The effects of planet radio emission on spacecraft data received by antenna arrays are relevant to choosing the exact locations of antennas that might be built by the Deep Space Network in coming years.

Ulvestad, J. S.↗
NASA NTRS1994

Deep HST imaging of distant weak radio and field galaxies

We present deep Hubble Space Telescope (HST) Wide-Field Camera (WFC) V- and I-band images of three distant weak radio galaxies with z = 0.311-2.390 and seven field galaxies with z = 0.131-0.58. The images were deconvolved with both the Lucy and multiresolution CLEAN methods, which yield a restoring Full Width at Half Maximum (FWHM) of less than or equal to 0.2 sec, (nearly) preserve photons and signal-to-noise ratio at low spatial frequencies, and produce consistent light profiles down to our 2 sigma surface brightness sensitivity limit of V approximately 27.2 and I approximately 25.9 mag/sq arcsec. Multi-component image modeling was used to provide deconvolution-independent estimates of structural parameters for symmetric galaxies. We present 12-band (m(sub 2750) UBVRIgriJHK) photometry for a subset of the galaxies and bootstrap the unknown FOC/48 zero point at 2750 A in three independent ways (yielding m(sub 2750) = 21.34 +/- 0.09 mag for 1.0 e(-)/s). Two radio galaxies with z = 0.311 and 0.528, as well as one field galaxy with z = 0.58, have the colors and spectra of early-type galaxies, and a(exp 1/4)-like light profiles in the HST images. The two at z greater than 0.5 have little or no color gradients in V - I and are likely giant ellipticals, while the z = 0.311 radio galaxy has a dim exponential disk and is likely an S0. Six of the seven field galaxies have light profiles that indicate (small) inner bulges following a(exp 1/4) laws and outer exponential disks, both with little or no color gradients. These are (early-type) spiral galaxies with z = 0.131-0.528. About half have faint companions or bars. One shows lumpy structure, possibly a merger. The compact narrow-line galaxy 53W002 at z = 2.390 has less than or = 30% +/- 10% of its HST V and I flux in the central kiloparsec (due to its weak Active Galactic Nucleus (AGN)). Most of its light (V approximately equal to 23.3) occurs in a symmetric envelope with a regular a(exp 1/4)-like profile of effective radius a approximately equal to 1.1 sec (approximately equal to 12 kpc for H(sub 0) = 50, q(sub 0) = 0.1. Its (HST) V - I color varies at most from approximately 0.3 mag at a approximately equal to 0.2 sec to approximately 1.2 mag at a approximately greater than 0.4 sec, and possibly to approximately greater than 2.2 mag at a approximately greater than 1.2 sec. Together with its I - K color (approximately equal to 2.5 mag for a approximately greater than 1.0 sec-2.0 sec), this is consistent with an aging stellar population approximately 0.3-0.5 Gyr old in the galaxy center (a approx. less than 2 kpc radius), and possibly approximately 0.5-1.0 Gyr old at a approximately greater than 10 kpc radius. While its outer part may thus have started to collapse at z = 2.5-4, its inner part still is aligned with its redshifted Ly(alpha) cloud and its radio axis, possibly caused by star formation associated with the radio jet, or by reflection from its AGN cone.

Windhorst, R. A.↗
NASA NTRS1995

Ulysses radio observations of the heliospheric activity in early June 1991

The radio receiver of the URAP (Unified Radio and Plasma Wave) experiment on Ulysses has recorded a heliospheric activity particularly intense between late May and early June 1991. Many solar radio emissions of types III and II were observed together with interplanetary (IP) shocks. In the same time, the radio spectrograph ARTEMIS at Nancay (France) observed several intense type II bursts. We investigate the association and/or interaction of these radio emissions, which are remotely observed, with some IP shocks detected in situ, in the context of a Coronal Mass Ejection (CME) induced scenario.

Hoang, S.↗
NASA NTRS1996

Simultaneous X-Ray and Radio Observations of the Unusual Binary LSI + 61 deg 303

We present simultaneous 0.5 - 10 keV X-ray and two-frequency radio observations at 2.25 and 8.3 GHz of the unusual binary system LSI + 61 deg. 303. This system was observed twice in a single binary orbit by the ASCA satellite, and monitored daily at two radio frequencies during the same orbital cycle with the Greenbank Interferometer. During the first ASCA observation the source was detected with a 1 - 10 keV luminosity 3.6 x 10(exp 33) (d/2.0 kpc)(exp 2) erg 1/s and during the second at a similar level with evidence for a decrease in average flux of 30%. During the first pointing the radio source was at a quiescent 8 GHz flux level of 30 mJy while during the second the radio flux was rising dramatically with an average value of 100 mJy. No variability is seen in the X-ray flux during the first pointing, but during the second the flux is variable by approx. 50% on timescales of approx. 30 minutes. No pulsations are seen in either X-ray observation with an upper limit on pulsed flux of 20%. The low X-ray luminosity and lack of observed pulsations indicate that accretion onto a neutron star surface is not the origin for the high-energy emission. Rather, the X-rays must result either from accreted matter which is stopped at the magnetosphere because the magnetospheric boundry is rotating at super-Keplerian rates or due to a shock formed in the interaction of the dense wind of the Be star companion and a moderately young pulsar. We derive a required pulsar spin down luminosity of approx. 10(exp 37) erg 1/s, and argue that the shock model more easily explains the observed X-ray radio observations.

Harrison, Fiona A.↗
NASA NTRS1995

Recent observations and analyses of heliospheric 2-3 kHz radio emissions

Since the discovery of the 2-3 kHz heliospheric radio emissions in the Voyager data over twelve years ago, there have been two major events, the first in 1983-84 and the second in 1992-93, as well as several minor events. Strong evidence now exists that these radio emissions are generated by interactions that take place in the outer regions of the heliosphere, most likely in response to strong interplanetary shocks and associated disturbances propagating outward from the Sun. Just where these interactions take place is still a subject of controversy. The three main possibilities are at or near the termination shock, between the termination shock and the heliopause and at or near the heliopause. In this paper we will review the present state of knowledge concerning the 2-3 kHz radio emissions, including the most recent data from Voyagers 1 and 2. These observations will then be compared to models that have been proposed to explain the origin of the radio emissions. Depending on the model, various estimates can be made concerning the plasma densities in the region where the radio emission is produced, and the distance to the source. The constraints provided by these models on the size and structure of the heliosphere are discussed.

Gurnett, D. A.↗
NASA NTRS1995

Radio maps of the solar wind transonic region

Extensive radio astronomical exploration of near-solar plasmas was carried out in 1987-1993 at radial distances nowadays inaccessible for direct spacecraft experiments. Radio wave sounding experiments were carried out using two types of natural sources-water vapor masers at 1.35 cm and quasars at 2.9 m wavelength. Russian Academy of Sciences radio telescopes RT-22 and DCR-1000 were used. The results of daily observations formed the radial dependence of the radio wave scattering from whence structural peculiarities of the solar wind flow were derived. Simultaneous observations of several sources makes it possible to realize radio maps visualizing the plasma flow stream structure. The 1987 to 1993 maps allow to analyse the transonic region structural changes in relation to the 11-year cycle phase.

Lotova, N. A.↗
NASA NTRS1995

NASA and CD Radio's TDRSS industrial test program

The National Aeronautics and Space Administration (NASA) has embarked on a joint test program with CD Radio Inc. The program will demonstrate spatial diversity techniques in support of industrial development of a new satellite direct broadcast national radio service called Satellite Radio. Satellite Radio will operate in the FCC approved frequency band 2310-2360 MHz which is close to NASA's Tracking and Data Relay System (TDRSS) satellites' high power transmit frequency near 2110 MHz. The cooperative test program in which NASA provides use of a TDRSS satellite and CD Radio provides the measurement equipped vehicle is described as well as its current status. Some initial measurement data are presented.

Briskman, Robert D.↗
NASA NTRS1996

ROSAT results for radio galaxies

Previous, pre-Rosat conclusions concerning the origin of X-ray emission in radio galaxies derived from correlations of X-ray versus core radio luminosity are considered to be unreliable. In relation to this, the following questions that are intended to be solved by Rosat observations are discussed: are the X-rays nuclear, thermal or a combination; what constraints are placed on relativistic beaming models which unify BL Lac objects and Fanaroff and Riley type radio galaxies through orientation; is there sufficient external pressure to give static support to the radio jets and lobes, and if hot gaseous atmospheres are seen, are they shaped by the radio jets?

Worrall, D. M.↗
NASA NTRS1997

Radio and X-ray observations of GX 339-4

Observations of the Galactic black hole candidate binary system GX 339-4 at radio, optical and X-ray wavelengths are reviewed. The radio observations reveal a compact, persistent and variable source. On the average, the radio intensity is less than 10 mJy. These data, when compared with previous data, indicate no set correlation between the radio and X-ray emission. The X-ray intensity suggests a 14.5 month cycle. The latest radio observational data are presented together with X-ray data and are discussed in the context of similar correlated observations from other X-ray binaries.

Sood, R.↗
NASA NTRS1997

Discovery in Cygnus X-3 of correlated behavior between the hard X-ray and radio

Using the Compton Gamma Ray Observatory (CGRO)/burst and transient search experiment (BATSE) hard X-ray data together with GHz radio monitoring data, a long term study was performed on the unusual X-ray binary Cyg X-3. This study resulted in the discovery of a relationship between the two wavebands. The combined data show that the radio emission is linked to the hard X-ray production. Radio flares, preflare low radio states and quiescence radio emission can be associated with changes in the hard X-ray intensity. Jet production is directly related to changes in the hard X-ray emission.

McCollough, M. L.↗
NASA NTRS1995

Chromospheric Evaporation and Decimetric Radio Emission in Solar Flares

We have discovered decimetric signatures of the chromospheric evaporation process. Evidence for the radio detection of chromospheric evaporation is based on the radio-inferred values of (1) the electron density, (2) the propagation speed, and (3) the timing, which are found to be in good agreement with statistical values inferred from the blueshifted Ca xix soft X-ray line. The physical basis of our model is that free-free absorption of plasma emission is strongly modified by the steep density gradient and the large temperature increase in the upflowing flare plasma. The steplike density increase at the chromospheric evaporation front causes a local discontinuity in the plasma frequency, manifested as almost infinite drift rate in decimetric type III bursts. The large temperature increase of the upflowing plasma considerably reduces the local free-free opacity (due to the T-(exp -3/2) dependence) and thus enhances the brightness of radio bursts emitted at the local plasma frequency near the chromospheric evaporation front, while a high-frequency cutoff is expected in the high-density regions behind the front, which can be used to infer the velocity of the upflowing plasma. From model calculations we find strong evidence that decimetric bursts with a slowly drifting high-frequency cutoff are produced by fundamental plasma emission, contrary to the widespread belief that decimetric bursts are preferentially emitted at the harmonic plasma level. We analyzed 21 flare episodes from 1991-1993 for which broadband (100-3000 MHz) radio dynamic spectra from Phoenix, hard X-ray data from BATSE/CGRO, and soft X-ray data from GOES were available. We detected slowly drifting high-frequency cutoffs between 1.1 and 3.0 GHz, with drift rates of -41 +/- 32 MHz/s, extending over time intervals of 24 +/- 23 s. Developing a density model for type III-emitting flare loops based on the statistically observed drift rate of type III bursts by Alvarez & Haddock, we infer velocities of up to 360 km/s for the upflowing plasma, with an average of v(sub CE) = 236 +/- 130 km /s for episodes with 5-15 s duration. The mean electron density of the upflowing plasma is n(sub e) = 5.2(+/-3.1) x 10(exp 10) /cu cm when it is first detected in radio, at coronal altitudes of h(sub 0) = 9.2 +/- 2.3 Mm.

Aschwanden, Markus J.↗
NASA NTRS1998

Discovery of Correlated Behavior Between the Hard X-Ray and the Radio Bands in Cygnus X-3

Using CGRO/BATSE hard X-ray data and GHz radio monitoring data from the Green Bank Interferometer (GBI), we have performed a long term study (approximately 1800 days) of the unusual X-ray binary Cygnus X-3 resulting in the discovery of a remarkable relationship between the two wavelength bands. We find that quiescent radio states are strongly anticorrelated with the intensity of the hard X-ray emission. With the onset of a flaring state the relationship switches to a correlation. This is most easily seen in the larger (> or = 1 Jy) flares where the preflare quenched radio emission is accompanied by a very low hard X-ray intensity, and recovery of the hard X-ray flux during the radio flare. The injection of plasma into the radio jets, associated with Cyg X-3, can be directly related to changes in the hard X-ray emission, and suggests the possibility of accretion-related and jet-related components of the high energy emission.

McCollough, M. L.↗