Search NASA⌕ Search

SEARCH · Search NASA

Results for “RADIO SOURCE”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Absorption models for low-frequency variability in compact radio sources

The consequences of the most plausible version of the absorption model for low-frequency variability in compact extragalactic radio sources are considered. The general restrictions placed on such a model are determined, and observational tests are suggested that can be used either to support the model or to discriminate among its various versions. It is shown that low-frequency variability in compact radio sources can be successfully explained by a class of models in which the flux is modulated by changes in free-free optical depth within an intervening ionized medium. Two versions of such a model are distinguished, one involving large changes in optical depth and the other, small changes. It is noted that while absorption effects are capable of causing rapid flux and structural variations at centimetric wavelengths, the models predict detailed behavior that is in direct conflict with observational data.

Marscher, A. P.↗
NASA NTRS1988

A large family of compact radio sources in the starburst nucleus of NGC 253

Deep VLA maps of NGC 253 show that it has at least 35 compact radio sources similar to those in the prototypical IR starburst galaxy M82. It is noted that NGC 253 is a twin of M82 in IR spectrum and luminosity as well as distance. Some of the sources in M82 which are rapidly expanding and fading are believed to be extremely young supernova remnants. Most of the sources detected in NGC 253 are unresolved or only marginally resolved, with typical diameters no greater than 2-5 pc. If they are young supernova remnants expanding at 12,000 km/s, the source ages are no more than 160-400 yr. This requires a supernova rate of about 1/0.1 yr. If the compact radio sources are indeed young supernova remnants, star formation must have ceased shortly after the progenitors of the current supernovae were produced.

Antonucci, Robert R. J.↗
NASA NTRS1973

Extended extragalactic radio sources.

The most important feature of the extended sources is the double structure characterized in an ideal model by two extended radio-emitting regions situated symmetrically about the nucleus of an optical galaxy or QSO, with a characteristic overall size of about 100 kpc. Not all extended radio sources show this double structure. It has been found that at moderate resolutions about 20% of the well-resolved sources have a core-halo structure. Theoretical proposals for the containment of extended double sources have so far relied either upon self-gravitation of the ejected material or upon assuming the existence of a gaseous intergalactic medium. The advantages and the problems of a model proposed by Burbidge (1967) are discussed, together with the nuclear event serving to release the energy required to make an extended radio source.

De Young, D. S.↗
NASA NTRS1984

Submilliarcsecond VLBI Using Compact Close Pairs of Radio Sources: Error Analysis

The potential accuracy attainable for (delta) VLBI positional measurements (submilliarc second level) is reached by simultaneously observing pairs of compact radio sources whose angular separation are smaller than the beamwidth of each antenna. Simultaneous (delta) VLBI (SVLBI) enables significant cancellation of measurement errors. Solar plasma is the dominant fluctuating error source in SVLBI positional measurements since there is enhanced cancellation of the troposphere and ionosphere, and complete cancellation of oscillator instabilities. Of the nonfluctuating error sources, errors due to universal time predominate. By performing SVLBI experiments over several years with many different close pairs of radio sources, limits can be placed on reference frame stability. Intrinsic properties of the sources, such as source structure and proper motion, will limit measurements. The SVLBI differential phase and corruptive noise sources will be discussed here along with estimated results.

Morabito, D. D.↗
NASA NTRS1992

Occultation of a compact radio source by Venus

An occultation of the compact radio source P 0507+17 by Venus on 19 Jul. 1988, was observed in Tidbinbilla, Australia at a frequency of 2.3 GHz. The purpose of this observation was to measure the position of Venus in the radio reference frame. When data from both ingress (Venus dayside) and egress (Venus nightside) were used to solve for the position of Venus in ecliptic longitude and latitude, the results were consistent with zero offsets from the nominal values, with an uncertainty of approximately 0.2 arcsec in both coordinates. By using the nightside data alone, a value of -0.026 +/- 0.04 arcsec was obtained for the linear combination delta(lambda) + 0.51delta(beta), where delta(lambda) and delta(beta) were the offsets from their nominal values of the ecliptic longitude and latitude of Venus. Distortion of a vacuum Fresnel fringe pattern by the Venus troposphere, and especially by the Venus ionosphere, was observed. The dayside ionosphere of Venus caused very large distortions; the amplitude of the first Fresnel fringe in the ingress data was eight times larger than was expected for an airless planet. The observed fringe patterns were modeled by using plausible ionospheres (i.e., consistent with spacecraft measurements of the Venus ionosphere and with solar extreme ultraviolet flux and solar wind pressure measurements at the occultation epoch). However, the range of Venus ionospheric profiles (electron density as a function of altitude) allowed by a priori constraints and by the occultation data was large (e.g., the ionopause height on the dayside was uncertain by a factor of two). This ionospheric uncertainty (particularly on the dayside) translated into a large position uncertainty (0.2 arcsec for the dayside and 0.04 arcsec for the nightside). If it was possible to calibrate the Venus ionosphere by some external means, the accuracy in delta(lambda) and delta(beta) would have been 0.01 arcsec or better.

Linfield, R.↗
NASA NTRS1985

Arcsecond positions for milliarcsecond VLBI nuclei of extragalactic radio sources. III - 74 sources

VLBI measurements at 2290 and 8420 MHz on baselines of 10,000 km have been used to determine the positions of the milliarcsecond nuclei in 74 extragalactic radio sources. Estimated accuracies range from 0.1 to 4.3 arcsec in both right ascension and declination with typical accuracies of about 0.3 arcsec. The observed sources are part of an all-sky VLBI catalog of milliarcsecond radio sources. Arcsecond positions have been determined for 819 sources. These positions are presently being used to identify optical counterparts in the southern hemisphere.

Morabito, D. D.↗
NASA NTRS1992

Blue galaxies identified with submilliJansky radio sources in the 1300 + 3034 field

We have obtained deep B and R CCD frames in order to optically identify sub-mJy radio sources discovered in a deep 1.49 GHz VLA survey field centered at alpha (1950) = 13h00m37s, delta(1950) = + 30 deg 34 arcmin. B and R photometry is presented for 37 optical identifications. Using spectral evolution models, we conclude that galaxies brighter than B about 25 identified with sub-mJy radio sources are much bluer than giant elliptical galaxies, but are similar to starburst galaxies with absolute magnitude M(v) = about -23-20 and redshifts in the range 2 = about 0.1-0.8. This population of blue galaxies is the same as that found by Thuan and Condon (1987) in the 0852 + 1716 field using different techniques (optical and near-infrared photometry), suggesting that this population of starburst galaxies at intermediate redshifts is universal.

Thuan, Trinh X.↗
NASA NTRS1991

Second-epoch VLA observations of compact radio sources in NGC 253

Results are presented of second-epoch 6-cm VLA observations of the starburst galaxy NGC 253 in early 1989, approximately 18 months after the first observations identified more than 30 compact radio sources in the galaxy. The galaxy was also mapped at 3.6 cm. No new radio sources were found at 6 cm after the 18-month interval. Assuming that at least two-thirds of new type II supernovae would have been detectable radio emitters stronger than 3 mJy, the 95-percent confidence upper limit to the SN rate is 3.0/yr. None of the 13 sources with flux densities above 1 mJy showed significant decreases in strength over 18 months; the strongest source showed an apparent increase of 6.5 percent in flux density, which differs from that of Kronberg and Sramek (1985) for M82. The 3.6 map identified a number of new sources because of the reduction of confusion. The available spectral data show that a few of the resolved sources may be H II regions.

Ulvestad, James S.↗
NASA NTRS1983

Arcsecond positions for milliarcsecond VLBI nuclei of extragalactic radio sources. II - 207 sources

Time delay and fringe frequency measurements at 2.29 GHz have been obtained with VLBI on baselines of about 10,000 km, in order to determine the positions of milliarcsec nuclei in 207 extragalactic radio sources. Accuracy ranges are estimated to lie between 0.1 and 1.0 arcsec in both right ascension and declination, with all sources having uncertainties of less than 4 arcsec in both coordinates. These sources constitute part of an all-sky VLBI catalog of milliarcsec radio sources, for 752 of which arcsec positions have thus far been determined.

Morabito, D. D.↗
NASA NTRS1985

Arcsecond Positions for Milliarcsecond VLBI Nuclei of Extragalactic Radio Sources. Part 3: 74 Sources

VLBI measurements at 2290 MHz and 8420 MHz on baselines of 10,000 km between Deep Space Network stations have been used to determine the positions of the milliarcsecond nuclei in 74 extragalactic radio sources. Estimated accuracies range from 0.1 sec. to 4, 3 sec. in both right ascension and declination with typical accuracies of approx. 0.3 sec. The observed sources are part of an all-sky VLBI catalog of milliarcsecond radio sources. Arcsecond positions have now been determined for 819 sources. These positions are presently being used to identify optical counterparts in the Southern Hemisphere.

Morabito, D. D.↗
NASA NTRS1986

Arcsecond positions for milliarcsecond VLBI nuclei of extragalactic radio sources. IV - Seventeen sources

VLBI measurements of time delay and delay rate at 2.29 and 8.42 GHz on baselines of 10,000 km have been used to determine the positions of the milliarcsecond nuclei in 17 extragalactic radio sources with estimated accuracies of 0.1 to 0.3 arcsec. The observed sources are part of an all-sky VLBI catalog of milliarcsecond radio sources. In addition, slightly improved positions are presented for 101 sources originally reported by Morabito et al. (1983). Arcsecond positions have now been determined for 836 sources.

Morabito, D. D.↗
NASA NTRS1988

Flat-spectrum, variable radio sources

The general behavior of flat-spectrum (compact) radio sources is examined in terms of adiabatic-jet models. Two puzzling properties - namely, (1) the broad, rather flat spectrum (over a large range of radio frequencies) and (2) the relatively slow decay of burst amplitude (with decreasing radio frequency) - are explained. Acceptable models are characterized by the following: (1) a nearly conical, adiabatic jet, with conserved magnetic flux transverse to the axis of the jet; (2) prolonged injection (for which the duration of an event exceeds the apparent transit time scale); and (3) a transparent spectral index which is not too steep. It is suggested that the acceleration mechanism in the core of compact jets may differ substantially from that far from the core, producing a flatter electron number index s = 1.4-2.6.

O'Dell, S. L.↗
NASA NTRS1983

The probable association of SN 1957d in NGC 5236 (M83) with an unresolved radio source

Astrometric measurements of the position of SN 1957d in the spiral galaxy NGC 5236 using archival plates indicate that it is within 2 arcsec of the position of a point radio source of approximately Cas A strength on a VLA map made by Cowan and Branch (1982). This coincidence strongly suggests that the radio source originated from the supernova. If further investigation shows this point source to indeed be a supernova remnant, then this would be the first reported detection of an intermediate-age supernova remnant.

Pennington, R. L.↗
NASA NTRS1990

A catalog of selected compact radio sources for the construction of an extragalactic radio/optical reference frame (Argue et al. 1984): Documentation for the machine-readable version

This document describes the machine readable version of the Selected Compact Radio Source Catalog as it is currently being distributed from the international network of astronomical data centers. It is intended to enable users to read and process the computerized catalog. The catalog contains 233 strong, compact extragalactic radio sources having identified optical counterparts. The machine version contains the same data as the published catalog and includes source identifications, equatorial positions at J2000.0 and their mean errors, object classifications, visual magnitudes, redshift, 5-GHz flux densities, and comments.

Source record↗