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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 505 records · Page 28
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NASA NTRS1983

Venus - Global surface radio emissivity

Observations of thermal radio emission from the surface of Venus, made by the Pioneer Venus radar mapper at a wavelength of 17 cm, show variations that are dominated by changes in surface emissivity. The regions of lowest emissivity (0.54 + or - 0.05 for the highland areas of Aphrodite Terra and Theia Mons) correspond closely to regions of high radar reflectivity reported earlier. These results support the inference of inclusions of material with high electrical conductivity in the surface rock of these areas.

Ford, P. G.↗
NASA NTRS1994

The early ultraviolet, optical, and radio evolution of the soft X-ray transient GRO J0422+32

We have monitored the evolution of the transient X-ray source GRO J0422+32 from approximately 2 weeks postdiscovery into its early decline phase at ultraviolet, optical, and radio wavelengths. Optical and ultraviolet spectra exhibit numerous, but relatively weak, high-excitation emission lines such as those arising from He II, N III, N V, and C IV superposed on an intrinsically blue continuum. High-resolution optical spectroscopy reveals line profiles which are double peaked, and in the case of the higher order Balmer lines, superposed on a broad absorption profile. The early outburst optical-ultraviolet continuum energy distribution is well represented by a two power-law fit with a break at approximately equal 4000 A. Radio observations with the Very Large Array (VLA) reveal a flat-spectrum source, slowly increasing in intensity at the earliest epochs observed, followed by an approximate power-law decay light curve with an index of -1. Light curves for each wavelength domain are presented and discussed. Notable are the multiple secondary outbursts seen in the optical more than 1 year postdiscovery, and spectral changes associated with secondary rises seen in the radio and UV. We find that the ultraviolet and optical characteristics of GRO J0422+32 as well as its radio evolution, are similar to other recent well-observed soft X-ray transients (also called X-ray novae) such as Cen X-4, A0620-00 (V616 Mon), and Nova Muscae 1991 (GS 1124-683), suggesting that GRO J0422+32 is also a member of that subclass of low-mass X-ray binaries. We present definitive astrometric determination of the source position, and place an upper limit of R approximately equals 20 from our analysis of the Palomar Observatory Sky Survey (POSS). Additionally, we derive distinct values for color excess from analysis of the optical (E(B-V) = 0.23) and ultraviolet (E(B-V) = 0.4) data, suggesting an intrinsic magnitude of 19-19.5 for the progenitor if it is mid-K dwarf. This leads to a likely range of 2.4-3.0 kpc for the source distance, which is consistent with our separate estimate of 2.4 +/- 0.4 kpc based on measurement of the NaD interstellar line profile. Adopting 2.4 kpc and E(B-V) = 0.23, the outburst absolute magnitude was M approximately equals 0.0, which is a typical value for this class of objects.

Shrader, C. R.↗
NASA NTRS1976

A search of the SAS-2 data for pulsed gamma-ray emission from radio pulsars

Data from the SAS-2 high-energy (above 35 MeV) gamma-ray experiment have been examined for pulsed emission from each of 75 radio pulsars which were viewed by the instrument and which have sufficiently well-defined period and period-derivative information from radio observations to allow for gamma-ray periodicity searches. When gamma-ray arrival times were converted to pulsar phase using the radio reference timing information, two pulsars, PSR 1747-46 and PSR 1818-04, showed positive effects, each with a probability of less than 1 part in 10,000 of being a random fluctuation in the data for that pulsar. These are in addition to PSR 0531+21 and PSR 0833-45, previously reported. The results of this study suggest that gamma-ray astronomy has reached the detection threshold for gamma-ray pulsars and that work in the near future should give important new information on the nature of pulsars.

Ogelman, H.↗
NASA NTRS1979

Soft X-ray spectrum and structure of the supernova remnant G65.2+5.7

The HEAO 1 low energy detectors have observed the supernova remnant G65.2+5.7 in the energy range 0.2-2.5 keV. A strong, extended source of X-ray emission is observed coincident with the eastern side of the optical filamentary structure. The characteristic temperature of the emission is in the range 2-3.8 x 10 to the 6th K, and evidence is found for line emission at about 0.8 keV in the X-ray spectrum. These observations imply a shock velocity (425 km/s) and age (about 20,000 yr) that markedly disagree with the estimates from optical and radio observations.

Mason, K. O.↗
NASA NTRS1995

The difference between radio-loud and radio-quiet active galaxies

The recent development of unified theories of active galactic nuclei (AGNs) has indicated that there are two physically distinct classes of these objects--radio-loud and radio-quiet. Despite differences, the (probable) thermal emissions from the AGNs (continua and lines from X-ray to infrared wavelengths) are quite similar to the two classes of object. We argue that this last result suggests that the black hole masses and mass accretion rates in the two classes are not greatly different, and that the difference between the classes is associated with the spin of the black hole. We assume that the normal process of accretion through a disk does not lead to rapidly spinning holes and propose that galaxies (e.g., spirals) which have not suffered a recent major merger event contain nonrotating or only slowly rotating black holes. When two such galaxies merge, the two black holes are known to form a binary and we assume that they eventually coalesce. The ratio of the number of radio-loud to radio-quiet AGNs at a given thermal (e.g., optical) luminosity is determined by the galaxy merger rate. Comparisons between the predicted and observed radio luminosity functions constrain the efficiencies with which jet power is extracted from the spinning hole and radio emission is produced by the jet.

Wilson, A. S.↗
NASA NTRS2007

A Spectacular Radio Flare from XRF 050416a at 40 Days and Implications for the Nature of X-Ray Flashes

We present detailed optical, near-infrared, and radio observations of the X-ray flash 050416a obtained with Palomar and Siding Springs Observatories as well as HST and the VLA, placing this event among the best-studied X-ray flashes to date. In addition, we present an optical spectrum from Keck LRIS from which we measure the redshift of the burst, Z=0.6528. At this redshift the isotropic-equivalent prompt energy release was about 10(exp 51) erg, and using a standard afterglow synchrotron model we find that the blastwave kinetic energy is a factor of 10 larger, E-K,iso approximately equals 10 (exp 52) erg. The lack of an observed jet break to t - 20 days indicates that the opening angle is larger than 7 deg and the total beaming-corrected relativistic energy is larger than 10 exp (50) erg. We further show that the burst produced a strong radio flare at t is similar to 40 days accompanied by an observed flattening in the X-ray band which we attribute to an abrupt circumburst density jump or an episode of energy injection (either from a refreshed shock or off-axis ejecta). Late-time observations with HST show evidence for an associated supernova with peak optical luminosity roughly comparable to that of SN 1998bw. Next, we show that the host galaxy of XRF 050416a is actively forming stars at a rate of at least 2 M-solar per year with a luminosity of L-B is similar to 0.5L* and metallicity of Z is similar to 0.2-0.8 Z-solar. Finally, we discuss the nature of XRF 050416a in the context of short-hard gamma-ray bursts and under the framework of off-axis and dirty fireball models for X-ray flashes.

Soderberg, A. M.↗
NASA NTRS1980

WQ 2059-247: An unusual high redshift X-ray cluster

X-ray, optical, and radio observations of a high redshift, Bautz-Morgan type I cluster of galaxies are reported. The cD galaxy contains a powerful, flat spectrum radio source coincident with the possibly stellar nucleus. The cluster is an extremely luminous X-ray source; however, unlike nearby luminous X-ray clusters the X-ray spectrum appears to be rather soft. Two possible interpretations of the soruces are suggested: either the intracluster gas is much cooler in high redshift clusters because they are less relaxed, or the X-ray and radio emissions from WQ 2059-247 are the result of a non thermal QSO/BL Lac type object in the nucleus of the cD.

White, R. A.↗
NASA NTRS1980

Collapsing cloud models for Bok globules

The dynamic collapse of rotating gas clouds is calculated for a wide range of initial conditions. Properties of cloud models are compared with observed radio and optical properties of Bok globules, to test the hypothesis that globules undergo collapse and to determine parameters which are not easily observed. Five of the six globules studied are consistent with collapse models. It is inferred that these objects have masses of about 100 solar masses and ages smaller than their free-fall times. Inferred initial densities are much larger than minimum densities for gravitational collapse, suggesting that collapse is initiated by strong external compression or that globules are fragments of larger condensed clouds. Values inferred for the (C-13)O/H2 ratio are smaller than previous estimates and depend strongly on cloud density.

Villere, K. R.↗
NASA NTRS1987

Circumstellar matter and the nature of the SN1987A progenitor star

It is argued that radio observations of the supernova 1987A can be interpreted in terms of its interaction with circumstellar matter. The early turn-on of the radio emission implies a relatively low density circumstellar medium. The optical properties of the supernova imply that the progenitor star had a smaller radius than that of a typical type II supernova progenitor. The mass loss properties are consistent with this hypothesis. The thermal X-ray luminosity of the supernova is predicted and noted to be below the current upper limit. A bright infrared dust echo is not expected, although a weak echo from an earlier mass loss phase is possible. Weak ultraviolet emission lines from cicumstellar gas may be visible. Although the circumstellar density is low, it is possible that the progenitor star did lose a substantial fraction of its mass prior to the supernova explosion.

Chevalier, R. A.↗
NASA NTRS1977

Interplanetary baseline observations of type 3 solar radio bursts

Simultaneous observations of type III radio bursts using spacecraft separated by several tenths of an AU were made using the solar orbiters HELIOS-A and -B. The burst beginning at 1922 UT on March 28, 1976, was located from the intersection of the source directions measured at each spacecraft, and from the burst arrival time differences. Wide baseline observations give the radial distance of the source at each observing frequency. Consequently, coronal electron densities and exciter velocity were determined directly, without the need to assume a density model as is done with single spacecraft observations. The separation of HELIOS-A and -B also provided the first measurements of burst directivity at low frequencies. For the March 28 burst, the intensity observed from near the source longitude (HELIOS-B) was significantly greater than from 60 W of the source (HELIOS-A).

Weber, R. R.↗
NASA NTRS1985

Radio and optical observations of the jets from L1555 IRS 5

New high-sensitivity radio continuum and optical observations of jets associated with the recently formed star IRS 5 are presented. The observations suggest that IRS 5 is the source of a collimated high-velocity outflow of partially ionized gas that produces the observed bipolar molecular outflow in L1551. The emission from the jets can be traced for over 10 arcsec from IRS 5; the jets are aligned with the axis of the molecular bipolar outflow observed by Snell, Loren, and Plambeck (1981). The integrated radio spectrum of the jets is flat, indicating emission by an optically thin plasma. The total mass of ionized gas is estimated to be 0.4-8 x 10 to the -6th solar mass. The absence of optical emission from the northeast jet is best explained by the presence of a dense disk of gas that lies perpendicular to the outflow axis which obscures the northeast jet and may also be responsible for collimating the outflow into two initially opposed jets.

Snell, R. L.↗
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 NTRS1990

Giotto IMS measurements of the production rate of hydrogen cyanide in the coma of Comet Halley

The ion composition measurements in the ionosphere of Comet Halley by the ion mass spectrometer (IMS) experiment on the Giotto spacecraft are used to estimate the relative abundance of HCN. From a comparison of the normalized number density of ions with mass-to-charge (M/q) ratio of 28 AMU/e with steady-state photochemical models, it can be determined that the production rate of HCN directly from the central nucleus is Q(HCN) is less than about 0.0002 Q(H2O) at the time of Giotto encounter. The related photochemical- model calculations also indicate that Q(NH3)/Q(H2O) at the time of Giotto encounter. The related photo-chemical model calculations also indicate that Q(HN3)/Q(H2O) equals about 0.005, in agreement with recent determination from ground-based observations. The estimated value of Q(HCN) is lower than the relative abundance of Q(HCN)/Q(H2O) of about 0.001, as derived from radio observations of the 88.6 GHz emission of the J = 1 - 0 transition of HCN. The difference may be the result of time variations of the coma composition and dynamics, as well as other model-dependent effects.

Ip, W.-H.↗
NASA NTRS1992

Electron acceleration by young supernova remnant blast waves

Some general considerations regarding relativistic particle acceleration by young supernova remnants are reviewed. Recent radio observations of supernova remnants apparently locate the bounding shock and exhibit large electron density gradients which verify the presence of strong particle scattering. The radio 'rim' in Tycho's remnant has been found to contain a predominantly radial magnetic field. This may be attributable to an instability of the shock surface and a progress report on an investigation of the stability of strong shocks in partially ionized media is presented.

Blandford, R. D.↗
NASA NTRS2003

Obscured Star-Formation in Merging Galaxies: High Resolution Radio Imaging of a Time-Ordered Sequence

We present new, deep, high resolution 6cm and 4cm radio continuum images of the central regions of a time-ordered sequence of seven large galaxy mergers. The radio observations are able to detect star-forming re- gions that are completely obscured at optical wavelengths. In all systems, we detect numerous compact radio sources embedded in more diffuse ra- dio emission, with limiting luminosities of approx. 1-5 x 10(exp l8) W Hz or approx. 1-5 times the luminosity of Cas A. Many of the compact radio sources are loosely associated with active starforming regions but not with specific optical or W emission sources. Several of the compact radio sources are coincident with Ultra-luminous X-ray objects (ULX's). In most systems, we are able to measure reliable spectral indices for the stronger sources. We find that the fraction of compact radio cources with nominally flat radio spectral indices (indicating they ae dominated by thermal radio emission from HII regions) decreases with merger age, while the fraction of sources with nonimally steep spectral indices (indicating they are dominated by nonthermal emission from supernova remnants) increases. For the flat-spectrum sources, we estimate the numbers of young massive stars, associated ionized gas masses, we estimate supernova rates and required star-formation rates, We compare these results with those from other well-studied merging galaxy systems and from other determinations of star-formation rates. We gratefully acknowledge use of the NRAO Very Large Array (VLA) and the VLA Archive. NRAO is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Neff, S. G.↗
NASA NTRS1977

Interplanetary baseline observations of type III solar radio bursts

Simultaneous observations of type III radio bursts from spacecraft separated by 0.43 AU have been made using the solar orbiters Helios-A and Helios-B. The burst beginning at 19:22 UT on March 28, 1976, has been located from the intersection of the source directions measured at each spacecraft and from burst arrival-time differences. The source positions range from 0.03 AU from the sun at 3000 kHz to 0.08 AU at 585 kHz. The electron density along the burst trajectory and the exciter velocity (0.13c) were determined directly without the need to assume a density model, as has been done with single-spacecraft observations. The separation of Helios-A and -B has also provided measurements of burst directivity at low frequencies. For the March 28 burst the intensity observed from near the source longitude (Helios-B) was 3-10dB greater than that from 60 deg west of the source (Helios-A)

Weber, R. R.↗
NASA NTRS1990

Observations of Cygnus X-2 with IUE - Ultraviolet results from a multiwavelength campaign

IUE observations of Cyg X-2 obtained during June and October 1988 are reported and analyzed, with reference to data from simultaneous X-ray and radio observations and the predictions of theoretical models. The results are presented in extensive tables and graphs and characterized in detail. The UV flux is shown to increase monotonically as the X-ray emission moves along the Z-shaped trajectory from the horizontal to the flaring branch and the estimated mass accretion rate increases from about 6 x 10 to the -9th to 1.6 x 10 to the -8th solar mass/yr. Model calculations indicate that X-ray illumination of the accretion disk accounts for at least 80 percent of the observed UV emission, with 20 percent or less from the X-ray-heated surface of the companion star and less than 1 percent from the cool surface.

Vrtilek, S. D.↗
NASA NTRS1992

The structure of the inner arcsecond of R Aquarii observed with the Hubble Space Telescope

The inner arcsec of R Aquarii has been observed with the Faint Object Camera on the Hubble Space Telescope. A simple and reliable linear deconvolution method is used to resolve the two features, designated C1 and C2 from radio observations, into several condensations. C1 is composed of four objects, designated C1a, C1b located at 0.099 arcsec from C1a, C3 at 0.162 arcsec from C1a, and C4 at 0.137 arcsec from C1a. The source C3, detected at 2 cm in the radio and in H-alpha, might be the V = 6-11 Mira variable. The nature of feature C4 is still unknown. Features C1a and C1b have not been resolved by another instrument, and it might be possible that the hot star is one of the two or a nearby nondetected object.

Burgarella, Denis↗